Child Line - 1098 is a 24 HoursChild Line - 1098 is a 24 HoursChild Line - 1098 is a 24 HoursChild Line - 1098 is a 24 HoursChild Line - 1098 is a 24 HoursNational Emergency Service toNational Emergency Service toNational Emergency Service toNational Emergency Service toNational Emergency Service to

save the children who needsave the children who needsave the children who needsave the children who needsave the children who needprotection.protection.protection.protection.protection.

Please make a call - save a life.Please make a call - save a life.Please make a call - save a life.Please make a call - save a life.Please make a call - save a life.

He is…

The philosopher of present who thinks about future

A tireless traveler travels to reveal the secrets of the nature

His hands…His hands…His hands…His hands…His hands…

Reaches to hug beyond the horizon

Tries to reach even the other side of the endless sky

His looks…His looks…His looks…His looks…His looks…

Penetrate through the deep oceans

And breaks through the invisible nucleus of an atom

His foot…His foot…His foot…His foot…His foot…

Thought to spread over nautical miles just within a short period

It shrinks to a nanometer and travels as fast as light

His heart…His heart…His heart…His heart…His heart…

While vibrates rhythmically on violin strings one side

The other side explores the wonders of virus

Leading to discover the biodiversity

His soul…His soul…His soul…His soul…His soul…

Wondering with you and me at the same time

It creates the novelty in relationship

He is an ideal servant of nature since ancient ages

And inspires of all the time

Sacrifice the life for the welfare of the human kind

Through his inventions and discoveries

Enlightens the lives through science

His is nothing but…

The Kepler… The Jenner… The Raman…

The ScientistINSPIRE AWARDS

Inspire is a National level programme to strengthen the roots ofour traditional and technological development.

The major aims of Innovations in Science Pursuit for InspiredResearch (INSPIRE) programme are...

• Attract intelligent students towards sciences

• Identifying intelligent students and encourage them to study science from early age

• Develop complex human resources to promote scientific, technological developmentand research

Inspire is a competitive examination. It is an innovative programme to make youngergeneration learn science interestingly. In 11th five year plan nearly Ten Lakhs of studentswere selected during 12th five year plan (2012-17) Twenty Lakhs of students will be selectedunder this programme.

Two students from each high school (One student from 6 - 8 classes and one from 9 -10 classes) and one student from each upper primary school are selected for this award.

Each selected student is awarded with Rs. 5000/-. One should utilize 50% of amountfor making project or model remaining for display at district level Inspire programme.Selected students will be sent to State level as well as National level.

Participate in Inspire programme - Develop our country.

iTelangana Government Free Distribution

Co-ordinators

Published by Government of Telangana, Hyderabad.

Respect the LawGet the Rights

Grow by EducationBehave Humbly

Editors

Academic Support

PHYSICAL PHYSICAL PHYSICAL PHYSICAL PHYSICAL SCIENCESCIENCESCIENCESCIENCESCIENCECLASS VIIICLASS VIIICLASS VIIICLASS VIIICLASS VIII

Sri M. Ramabrahmam, Lecturer,Govt. IASE, Masabtank, Hyderabad.

Dr. P. Shankar, Asst. Professor,IASE, O.U., Hyderabad.

Prof. Kamal Mahendroo,Vidya Bhavan Educational Resource Centre,

Udaipur, Rajastan.

Dr. TVS Ramesh,Co-ordinator, C&T Dept.,

SCERT, Hyderabad.

Miss. Preeti Misra,Vidya Bhavan Educational Resource Centre,

Udaipur, Rajastan.

Mr Kishore Darak,Vidya Bhavan Educational Resource Centre,

Udaipur, Rajastan.

Dr.B. Krishna rajulu Naidu,Retd., Professor of Physics

Osmania University, Hyderabad.

Dr. M. Adinarayana,Retd., Professor of Chemistry

Osmania University, Hyderabad.

Dr. Nannuru Upendar Reddy,Professor & Head C&T Dept.,

SCERT., Hyderabad.

Prof. V. SudhakarDept of Education, EFLU, Hyderabad.

ii

© Government of Telangana, Hyderabad.

First Published 2013New Impressions 2014, 2015

All rights reserved.

No part of this publication may be reproduced, storedin a retrieval system, or transmitted, in any form or byany means without the prior permission in writing of thepublisher, nor be otherwise circulated in any form ofbinding or cover other than that in which it is publishedand without a similar condition including this conditionbeing imposed on the subsequent purchaser.

The copy right holder of this book is the Directorof School Education, Hyderabad, Telangana.We have used some photographs which are undercreative common licence. They are acknowledge atthe end of the book.

This Book has been printed on 70 G.S.M. S.S. Map litho,Title Page 200 G.S.M. White Art Card

Free Distribution by Government of Telangana

Printed in Indiaat the Telangana Govt. Text Book Press,

Mint Compound, Hyderabad,Telangana.

iiiTelangana Government Free Distribution

Sri K. Sudhakara Chary, SGT,UPS Neelikurthy, Warangal.

Sri Kishan Thatoju, Computer Operator,C&T Dept., SCERT, Hyderabad.

Sri R. Ananda Kumar, SA,ZPHS Laxmipuram, Visakhapatnam.

Sri K.V.K. Srikanth, SA,GTWAHS S.L.Puram, Srikakulam.

Sri M. Eswara Rao, SA,GHS Sompeta, Srikakulam.

Sri Y. Guru Prasad, SA,ZPHS Chinnacherukuru, Nellore.

Sri K.L. Ganesh, SA,ZPHS M.D.Mangalam, Chittoor.

Dr. P. Shankar, Asst. Professor,IASE, O.U., Hyderabad.

Dr. K. Suresh, SA,ZPHS Pasaragonda, Warangal.

Sri Y. Venkat Reddy, SA,ZPHS Kudakuda, Nalgonda.

Sri Dr. S. Anjaneyulu, SA,ZPHS Veeraballi, YSR Kadapa.

Sri D. Madhusudhana Reddy, SA,ZPHS Munagala, Nalgonda.

Sri A. Satyanarayana Reddy, Director,S.C.E.R.T., Hyderabad

Sri B. Sudhakar, Director,Govt. Textbook printing press,

Hyderabad.

Dr. Nannuru Upendar Reddy,Professor & Head C&T Dept.,

S.C.E.R.T., Hyderabad.

Text Book Development Committee

Writers

Graphics & Designing

Sri M. Ramabrahmam, Lecturer,Govt. IASE, Masabtank, Hyderabad.

Sri Kurra Suresh Babu, B.Tech., MA., MPhillMana Media Graphics, Hyderabad.

iv

Intro ...

The nature is life source for all living organisms. Rocks, water, hills and

valleys, trees, animals etc. embedded in it… each of them are unique by

themselves. Everything has its own prominence. Human being is only a part of the

nature. The aspect which distinguishes the humans from all other organisms and

exclusive for them is their extraordinary thinking power. Thinking transforms a

person as a unique entity from rest of the nature. Though it usually appears

simple and normal, the intricacies of the very nature often challenges us to untie

the tough knots of its hidden secrets, day in and day out.

The human being intuitionally contemplates and searches solutions for all the

critical challenges, all around,relentlessly. Curiously, the questions and answers are

concealed in the nature itself. The role of science, in fact, is to find them out. For

this sake, some questions, some more thoughts, and some other investigations

are quite necessary. Scientific study is to move on systematically in different ways,

until discovering concrete solutions. Essence of the investigations lies in inquiring i.e.

identifying questions, asking them and deriving adequate and apt answers. That is

why, Galileo Galilei, the Italian astronomer,emphasized that scientific learning is

nothing but improving the ability of questioning.

The teaching of science has to encourage children to think and work scientifically.

Also, it must enhance their love towards the nature. Even it should enable them to

comprehend and appreciate the laws governing the nature in designing tremendous

diversity found around here and everywhere. Scientific learning is not just disclosing

new things. It is also essential to go ahead with deep understanding of the nature’s

intrinsic principles;without interrupting the harmony of interrelation and

interdependence in the nature.

It is also necessary to step forward without interrupting the interrelationshipand interdependency along with understanding of the nature’s intrinsic principles.HighSchool children possess cognitive capacity of comprehending the nature andcharacteristics of the transforming world surrounding them. And they are able toanalyze abstract concepts.

vTelangana Government Free Distribution

At this level, we cannot quench their sharp thinking capability with the dryteaching of mere equations and theoretic principles. For that, we should create alearning environment in the classroom which provides an opportunity for them toapply the scientific knowledge, explore multiple alternatives in solving problems andestablish new relations.

Scientific learning is not just confined to the four walls of classroom. It has adefinite connection to lab and field as well. Therefore, there is a lot of importance

to field experience/ experiments in science teaching.

There is a great need for compulsory implementation of instructions of the

National Curriculum Framework- 2005 which emphasizes linking of the science teaching

with local environment. The Right to Education Act- 2009 also suggested that

priority should be given to the achievement of learning competencies among children.

Likewise, science teaching should be in such a way that it would help cultivate a

new generation with scientific thinking.The key aspect of science teaching is to

make the children understand the thinking process of scientists and their efforts

behind each and every discovery. The State Curriculum Framework- 2011 stated

that children should be able to express their own ideas and opinions on various

aspects.All the genuine concepts should culminate into efficacious science teaching,

make the teaching-learning interactions in the classroom, laboratory and field very

effective and really become useful for the children to face the life challenges

efficiently.

We thank the VidyaBhavan Society, Rajasthan, Dr. Desh Panday Rtd Prof.

College of Engineering Osmania University and Sri D.R. Varaprasad former Lecturer

ELTC Hyderabad for their cooperation in developing these new text books,the

writers for preparing the lessons, the editors for checking the textual matters and

the DTP group for cutely composing the text book.

Teachers play a pivotal role in children’s comprehensive use of the text book.

We hope, teachers will exert their consistent efforts in proper utilization of the text

book so as to inculcate scientific thinking process and inspire scientific approach in

the children.

Director,

SCERT, Hyderabad

vi

Dear teachers...New Science Text Books are prepared in such a way that they develop children’s observation

power and research enthusiasm. It is a primary duty of teachers to devise teaching- learningprocesses which arouse children’s natural interest of learning things. The official documents ofNational& State Curriculum Frameworks and Right to Education Act are aspiring to bring grassroot changes in science teaching. These textbooks are adopted in accordance with such anaspiration. Hence, science teachers need to adapt to the new approach in their teaching. In viewof this, let us observe certain Dos and Don’ts:

• Read the whole text book and analyze each and every concept in it in depth.

• In the text book, at the beginning and ending of an activity, a few questions are given.Teacher need to initiate discussion while dealing with them in the classroom, attempt toderive answers; irrespective of right or wrong responses, and so try to explain concept.

• Develop/Plan activities for children which help them to understand concepts presented intext.

• Textual concepts are presented in two ways: one as the classroom teaching and the otheras the laboratory performance.

• Lab activities are part and parcel of a lesson. So, teachers must make the children conductall such activities during the lesson itself, but not separately.

• Children have to be instructed to follow scientific steps while performing lab activities andrelevant reports can be prepared and displayed.

• In the text some special activities as boxed items- ‘think and discuss, let us do, conductinterview, prepare report, display in wall magazine, participate in Theatre Day, do fieldobservation, organize special days’ are presented. To perform all of them is compulsory.

• ‘Ask your teacher, collect information from library or internet’- such items must also beconsidered as compulsory.

• If any concept from any other subject got into this text, the concerned subject teacher hasto be invited into the classroom to elucidate it.

• Collect info of relevant website addresses and pass on to students so that they can utilizeinternet services for learning science.

• Let there be science magazines and science books in the school library.• Motivate every student to go through each lesson before it is being actually taught and

encourage everyone to understand and learn independently, with the help of activities suchas Mind Mapping and exciting discussions.

• Plan and execute activities like science club, elocution, drawing, writing poetry on science,making models etc.to develop positive attitude among children environment, biodiversity,ecological balance etc.

• As a part of continuous comprehensive evaluation, observe and record children’s learningabilities during various activities conducted in classroom, laboratory and field.

viiTelangana Government Free Distribution

We believe, you must have realizedthat the learning of science and scientific thinking arenot mere drilling of the lessons but, in fact, a valuable exercise in motivating the children toexplore solutions to problems all around by themselves systematically and preparing them tomeet life challenges properly.

Dear Students...

Learning science does not mean scoring good marks in the subject. Competencies likethinking logically and working systematically, learned through it,have to be practiced in dailylife. To achieve this, instead of memorizing the scientific theories by rote, one must be able tostudy them analytically. That means, in order to understand the concepts of science, you needto proceed by discussing, describing, conducting experiments to verify, making observations,confirming with your own ideas and drawing conclusions. This text helps you to learn in thatway.

What you need to do to achieve such things:• Thoroughly go through each lesson before the teacher actually deals with it.• Note down the points you came across so that you can grasp the lesson better.• Think of the principles in the lesson. Identify the concepts you need to know further,

to understand the lesson in depth.• Do not hesitate to discuss analytically about the questions given under the sub-heading

‘Think and Discuss’ with your friends or teachers.• You may get some doubts while conducting an experiment or discussing about a lesson.

Express them freely and clearly.• Plan to implement experiment/lab periods together with teachers, to understand the

concepts clearly. While learning through the experiments you may come to knowmany more things.

• Find out alternatives based on your own thoughts.• Relate each lesson to daily life situations.• Observe how each lesson is helpful to conserve nature. Try to do so.• Work as a group during interviews and field trips. Preparing reports and displaying

them is a must.• List out the observations regarding each lesson to be carried through internet, school

library and laboratory.• Whether in note book or exams, write analytically,expressing your own opinions.• Read books related to your text book, as many as you can.• You organize yourself the Science Club programs in your school.• Observe problems faced by the people in your locality and find out what solutions you

can suggest through your science classroom.• Discuss the things you learned in your science class with farmers, artisans etc.

viii

ACADEMIC STANDARDS

ACADEMIC STANDARDS

S.No. Academic Standard Explanation

1.

2.

3.

4.

5.

6.

7.

Conceptual understanding

Asking questions and makinghypothesis

Expermentation and fieldinvestigation.

Inforamtion skills and Projects

Communication throughdrawing, model making

Appriciation and aestheticsence, values

Application to daily life,concern to bio diversity.

Children are able to explain, cite examples, give reasons,and give comparison and differences, explain the processof given concepts in the textbook. Children are able todevelop their own brain mappings.

Children are able to ask questions to understand, toclarify the concepts and to participate in discussions.They are able to make hypothesis on given issues.

To understand given concepts in the textbook childrenare able to do experiments on their own. They are ableto participate in field investigation and making reportson them.

Children are able to collect information (by usinginterviews, internet etc.) and analyses systematically.They are able to conduct their own project works.

Children are able to explain their conceptualunderstanding by drawing figures and making models.Able to platting graphs by using given information orcollected data.

Children are able to appreciate man power and nature,and have aesthetic sense towards nature. They are alsoable to follow constitutional values.

Children are able to utilize scientific concept to facetheir daily life situations. They are able to show concerntowards bio diversity.

ixTelangana Government Free Distribution

1

INDEXINDEXINDEXINDEXINDEX

2

3

4

5

6

7

8

9

10

Page No.Periods Month

Force

Friction

Synthetic fibres and plastics

Metals and non-metals

Coal and Petroleum

Combustion, fuels and Flame

Electric conductivity of liquids

Some natural phenomena

Stars and solar system

Sound

12 June 1

12 July 19

11 August 31

12 September 50

12 October 63

12 November 81

11 December 96

12 January 108

12 February 120

14 March 136

x

OUR NATIONAL ANTHEM

- Rabindranath Tagore

Jana gana mana adhinayaka Jaya heBharatha bhagya-vidhata

Punjab Sindhu Gujaratha MarathaDravida Utkala Banga.

Vindhya Himachala Jamuna GangaUchchala Jaladhi taranga,

Tava shubha name jageTava shubha asisha mage

Gahe tava jaya gatha

Jana gana mangala-dayaka jaya he,Bharatha bhagya –vidhatha,

Jaya he, jaha he, jaya he,Jaya jaya jaya jaya he

PLEDGE

- Pydimarri Venkata Subba Rao

“India is my country; all Indians are my brothers and sisters.I love my country, and I am proud of its rich and varied heritage.

I shall always strive to be worthy of it.

I shall give my parents, teachers and all elders respect,and treat everyone with courtesy. I shall be kind to animals.

To my country and my people, I pledge my devotion.

In their well-being and prosperity alone lies my happiness.”

1Free distribution by T.S. Government

FORCEChapter

1

We can observe many changes aroundus, like changes in seasons, change duringsun rise and sun set, changes in tides of seaetc. Have you ever thought about the causebehind these changes? In ancient days,people thought that an invisible force wasresponsible for the changes occurring innature. Even now many people believe thatan invisible force exists that causeswhatever is happening in the world.

Later, the concept of force wasdeveloped, but it was limited to explainingour efforts and actions. The words force,effort, strength and power had almost thesame meaning at that time. Have you everwondered what forces are? What are thedifferent types of forces and how do theyact? Let’s find out the answers to suchquestions in this chapter.

For instance, when you ride a bicycle,most of the time your legs are pushingdown on the pedals. To push you have tomake an effort. When you pick up yourschool bag you have to make an effort inorder to lift or pull the bag upwards and offthe ground. When you open a door you

make an effort on the door knob with yourhands either to push the door forward orpull it backward.

What is force?Have you ever picked the hail stones

falling down in a hail storm? A paper boythrows a newspaper. Have you everwondered about this action. Actions likestretching a rubber band, pulling a rickshaw,rowing a boat etc., are some more exampleswhere our efforts help to change of theobjects. Such actions like picking,squeezing, twisting, stretching, loweringand lifting etc., cause a change in the stateof an object. Now let us try to group thesetasks as a pull or a push.

Activity-1

Identifying push or pullTable-1 gives some examples involving

the actions like digging, sucking, erasing,falling, attracting, raising etc. Classify theseactions in terms of a push or a pull or both.Write pull or push in the blank boxes. Ifyou feel that the action involves both pushand pull, write ''both'' in the box.

2 Force

Table 1: Identify tasks as Push or Pull or Both

S. No Action Diagram Push/Pull/Both

1 Diggingbore well

2 Sipping Juicewith a straw

3 Erasing blackboardwith duster

4 A magnetattracting nails

5 Fruits fallingfrom tree

6 Hoisting a flag

List three more activities where weexert force which appears as a push.List three more activities where weexert a force as a pull.State three actions which involve bothpush and pull.

Based on this activity, can youexpalin what is a force?

Shall we call the effort done on anobject by means of pushing or pulling as aforce exerted on the object?

We cannot directly see the forcesacting on a body, but we can see the effectscaused due to the forces.

3Free distribution by T.S. Government

which occurs without any physical contactbetween two objects is known as a forceat a distance or field force.

Contact Forces1. Muscular Force

In all the actions that we perform in ourdaily life like brushing, bathing, eating,writing, driving and walking; we have toexert a force. Do you know from wherethe force comes? The force which we exertby using our body muscles is known asmuscular force. Even when we smile ourmuscles exert force to bring changes in ourface. Human beings and animals usemuscular force to carry out their regularphysical activities. Muscular forces can beexerted only through contact.

Activity-2

Preparing a list of examples formuscular force

List at least ten activities where weapply muscular force to perform varioustasks, in table - 2.

When an object slips off your hand, italways falls down. What pulls it down? Ifyou roll a ball on a level ground, it slowsdown and after sometime it will come to astop. What makes the ball stop? Whatforces acting on objects, change their stateor state of motion?

Types of forcesContact forces and forces at adistance (Field Forces)Observe the following figures.

Fig.1 (a), Fig. 1(b)

Why does the toothpaste come outwhen we press the tube? Why does theneedle of a magnetic compass move whenwe place a bar magnet near it? Have youobserved the difference between the forceyou applied on the tube and the forceapplied by a magnet on the needle of acompass?

In Fig.1 (a) you observe that there is directphysical contact (or interaction) betweenyour hand and the tube. Force, whichresults when there is a direct physicalcontact between two interacting objects, isknown as contact force.In Fig.1 (b) the needle of the compasschanges its direction without any physicalcontact with the bar magnet. But a forcemust be acting on the needle. The force

Fig.2

4 Force

Sl.No

Table-2List of activitieswhere we exert force

1 Lowering a basket

2

3

4

5

6Usually we are unaware of the muscular

forces that are responsible for the variousactions taking place inside our body, likeblood circulation, expansion andcontraction of lungs during breathing, heartbeat etc.

Do you feel your muscles get tightenedwhile performing any physical activity?What could be the reason for it?

Activity-3

Observing the changes in anymuscle while working

Take a dumbbell and lift it in differentways. Observe while doing this exercisewhich muscle is going to be shortened.

Ask your friends to do the same andobserve the movement of their muscles.

The term muscle refers to multiplebundles of muscle cells held together.Muscles are normally arranged in such away that as one group of muscles contractor shortens, another group relaxes orexpands for example, if you throw a ball

the muscles in the chest and front side ofthe shoulder lengthen to pull the armforward, while the muscles in the back andrear of the shoulder undergo contractionto slow down the motion.

2. Force of FrictionWhen you roll a ball on a level ground

it invariably stops after sometime.Why does the ball stop?Is there any hidden force which bringsit to stop?

If you stop peddling your bicycle on alevel road you observe that its speeddecreases gradually.

Why does the speed of the bicycledecrease gradually?Is there any force acting on it which

tends to reduce its speed?Does the change in speed of the ball

and bicycle depend on roughness andsmoothness of the surface on which theymove? Let us find out.

Activity-4

Observing the motion of a ballon different surfaces

Try to roll a ball on different surfaceslike carpet, rough roads, smooth floor etc.

On which surface does the ball rollfarther?

The motion of the ball is different ineach case. The force of resistance to themotion seems to be more on the roughsurface than on the smooth surface. Therolling ball moves farther on a smoothmarble floor than on a rough sandy surface.

5Free distribution by T.S. Government

Fig.4

Activity-5Observing the motion of objectson an inclined plane

Take a tray. Place a small ice cube,eraser and a rupee coin on a line at one endof the tray. Now slowly lift this end of thetray as shown in the figure-3.

Fig-3: Motion of objects on an inclined plane.

What do you observe?Which of these three objects slidesdown first? Why?Do all the objects experience the

same resistance to motion? If not why?Which of the objects experiencesmore resistance to motion? Why?Why is there a change in resistanceexperienced by the objects thoughthey all slide down on the sameplane?

Do this activity with different objectslike a book, a ball, a pen, a stone etc., andrecord your observations.

Friction is the resistance to themovement of a body over the surface ofanother body.

Did you ever experience slipping on afloor? What conditions caused you to slip?Did you experience slipping while you arewalking on wet mud? Why do most roadaccidents happen during rainy days?

Would it be possible to drive a car ifthere were no friction between thetyres and the road?

The direction of friction is alwaysopposite to the direction of motion relativeto the surface. Let’s imagine a worldwithout friction. Can we walk on a roadwithout friction? Will it be possible towrite with a pen on a paper or with a pieceof chalk on the black board? You will learnmore about friction in the next chapter.

Think and discuss

A book placed on a table is at rest. Isthe force of friction acting on it or not?Explain?

6 Force

3. Normal force

Fig-5: Force of gravitation and normalforce acting on the book.

Place your science book on the table.Is it in a state of rest? Is there any forceacting on that book? Imagine that the tablehas disappeared suddenly by magic. Whatwill happen then? The book will fall downdue to the gravitational pull of the Earth.Even when a book is lying on the table, thegravity pulls the book down all the time butit does not fall down because it issupported by the table. Therefore, thereexists a force which supports the bookagainst gravity by pushing it upward. Whatdo we call this force?

Look at the fig.5 A direction which isperpendicular to the plane of a surface issaid to be normal. The force that a solidsurface exerts on any object in the normaldirection is called the normal force.

In above example the downwardgravitational force is balanced by theupward normal force. Since these twoforces are of equal magnitude and actingin opposite directions, we say that the netforce acting on the book is zero and thebook is in equilibrium.

4. TensionAs shown in the figure-6 a stone is

suspended with the help of a string and itsfree end is tied to the ceiling

What is the state ofthe stone?What forces areacting on it?What will happen ifthe string is broken?We know that the

stone would fall downdue to gravitational pull (weight) of theearth if the string is broken.

For a stone tied to the string, gravitypulls down the stone all the time but it doesnot fall down because it issupported by the string. Thus,there exists a force whichsupports the stone againstgravity by pulling it upward.

What do we call this force?It is called 'tension' which

always pulls the body along thestring. Tension is a contactforce. When you try to stretch a rope or astring the tightness of rope or string is calledtension. In the above example, thegravitational force is balanced by theupward tension force as shown in thefigure-7.

Aim: To find the limiting force that can beborne by a string.Material used: Spring balance, weights,light strings, weight hanger.Procedure:1. Arrange the system as shown in figure-8. Put some small weights like 50 gm on

Fig-6

Fig-7

Lab Activity

7Free distribution by T.S. Government

Forces acting at a distance (fieldforces)1. Magnetic force

You must have done some experimentswith magnets in class VI. Let us recall someof your experiences.

Activity-6Observing the magnetic force.

Take a sewing needle. Rub it with a barmagnet several times always moving themagnet in the same direction. Does theneedle get magnetised ? You may find thatthe needle always acts like a magnet. Withthe help of a magnetic compass you canidentify the north and south poles of theneedle. Pin a red coloured foam ball toSouth Pole and white ball to North Pole ofthe needle; then drop it in a bowl of water,it floats. (Fig-9)

Make another needle in the same way.Float both the needles side by side such thatlike ends facing each other (either red orwhite balls). What happens to the needles?How do they move? Now place the needlesin such a way that unlike ends (White ballof first needles and red ball of secondneedle) face each other. Now observe whathappens this time. How do they push or pulleach other?

Fig-9: Making needle magnets andfloating them in a bowl of water.

the weight hanger and note thereadings of the spring balance.Now, add some more weights tothe hanger and note the readingsof spring balance. Do the sametill the string is broken. Note thereading of the balance in thefollowing table when the string is broken.Find out the limiting force of differenttypes of strings and mention the values inthe given table.

2. Separate the whole system from theceiling, and tie the string to weight hangerand now slowly pull up the whole systemwith your hand when there is a small weighton the hanger. Note down the readings. Dothey same when you bring it down.

What do you observe from the readingswhen it is pulled up and released tomove down?Is the string broken when the wholesystem is pulled quickly up?

Think and discussA system of two bodiesA and B are placed asshown in figure. Howmany forces are actingon A and B respectively?Why is it necessary to separatecontact force into a normal force andfrictional force? Give at least tworeasons.

Sl.No. Type of String Length Force Limiting

Fig-8

8 Force

We can say that when the balloon isrubbed with a paper, it acquires anelectrostatic charge on its surface. Theballoon is now said to be a charged body.When it is brought near the bits of paper,the pieces acquire opposite charge and willrise and cling to the balloon.

The force exerted by a charged bodyon another charged body is known aselectrostatic force.

This force comes into play even whenthe bodies are not in contact. It is anexample of a force at a distance.

3. Gravitational forceIt is our common experience that if a

pen slips off from our hands it falls downto the floor.

Why does the pen fall down?What is the force which pulls thepen down?

If we keep the same pen on a table, itdoes not fall down. Why?

Generally our answer would be that thetable supports the pen. If the table does notsupport the pen it would fall down until itis supported by another object, like thefloor.

Why does a stone thrown up into thesky fall back to the earth?Why do rivers flow down to thesea?How does the earth hold theatmosphere?Is there any force pulling the objectstowards earth?

You have learnt in class VI that likepoles of two magnets repel each other andunlike poles attract each other. You canobserve the red end of one needle and whiteend of another needle attract each other,and ends with same colour repel.

Now, you know that like poles repel orpush each other and unlike poles attract orpull each other. This action of pull or pusharises due to a magnetic force. A magnetcan attract or repel another magnet withoutcontact. So magnetic force is a field force.

2. Electrostatic force

Activity-7

Observing electrostatic forcesTake a balloon. Inflate it and tie up the

open end. Now cut a paper into small piecesand place them on the floor. Rub the balloonwith a paper and bring the balloon near thepieces of papers. What happens now? Arethe bits of paper pulled towards theballoon? (Fig-10) Why does the balloonpull or attract the pieces of paper? Try touse pepper and salt in the place of piecesof paper. What do you observe?

Fig-10 Charged balloon

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bodies, when the bodies are not directlytouching each other is called force at adistance. We can explain the forces at adistance by using the concept of field.

Activity-8

Visualizing magnetic field.

Fig-11 : Magnetic field

Take a bar magnet and place it on atable. Place a thick white paper overit (White drawing sheet).On the paper, sprinkle fine powderof iron (iron filings) as shown in thefig -11.Tab the table or the paper gently withpen/pencil.What do you observe? Do you findany pattern of iron filings there?Rotate the magnet in differentdirections and do the same. Howhas the pattern changed?

You can see that in a small space aroundthe magnet, iron filings set themselves in apattern because they are affected by themagnetic force of the field created by thebar magnet. The pattern represents themagnetic field. The space around themagnet where its influence can be detectedis called the magnetic field.

If an object is thrown upwards, thereexists a force which pulls it down towardsthe earth, because of this it falls down tothe ground. We call this force as agravitational force.

Every object on the Earth or close toEarth, will experience a gravitational pull.The force of gravity is not just due to theattraction of the Earth. It is a force ofattraction that exists between any two bodies(or masses) everywhere in the universe.

As the earth is so massive and huge,all the other objects close to the earth areattracted or pulled towards it. When yousit in your class room, there will be agravitational force between you and yourteacher, and a similar force exists betweenyou and the black board.

You cannot experience the gravitationalforce that exists between you and yourteacher or between you and the block boardbecause it is very small when compared tothe gravitational force exerted by the earthon these objects. You will learn more aboutthis in the lesson ''Gravitation''.

Think and discuss

A cricket ball of mass ‘m’ is thrownupward with some initial speed. If the airresistance is neglected, what forces areacting on the ball when it reaches (a) halfits maximum height and (b) its maximumheight?

Explaining of force acting at adistance: concept of field

The force which acts between two

10 Force

Fig - 12(a)

Technically, we say that the net force onthis object is zero.

Imagine that the same object is kept ina lift which is accelerating.

How many forces are acting on thisobject?Is the net force acting on the objectzero? Why?

Note: When an object is in non uniformmotion, it is said to be in acceleration.

The net force acting on an object keptin a lift which is accelerating is not zero,as the object is in non-uniform motion.

When two forces act on a body, as inthe above case, one of the forces shouldbe greater than the other to set the body inmotion.

The strength of a force is usuallyrepresented by its magnitude. The directionof a force is as important as its magnitude.We represent the direction of force using‘arrows’ ( ).

Activity-9

Effects of net force acting on atable

Try to push a heavy wooden table (Fig-12a). Is it hard to push ?

Thus, a field is a region in which a forcecan be experienced by another (magnetic)object placed at any point in that region.

A body creates a field and another bodyexperiences the force by the field when itis placed in that field.

A magnetic field surrounds a magnet,an electric field surrounds electriccharges and a gravitational field surroundsmasses.

The strength of a field in a particularregion can be represented by field lines;the greater the density of lines, the strongerthe forces in that part of the field.

Think and discuss

Two identical bars, one which is steeland the other a magnet, are painted withthe same colour. How can you tell whichone is the magnet using only these twobars? (don’t break the bars)

Net forceIn reality, many forces can act

simultaneously on a body. For example,there exists two forces on an object placedon a horizontal floor. One is gravitationalforce (vertically down) and other is normalforce (vertically up).

Do you observe any change in the stateof rest of that object because of theseforces? Obviously your answer is ‘No’.

In this case two forces acting on theobject are equal and opposite in direction.Hence, there is no change in its state.

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Fig-12(c)

Fig - 12 (b)

Ask your friend to help you in pushingthe table in the same direction, asshown in the fig.12 (b). Do you find iteasier to move the table now? Why?

You may notice that it is easier to pushthe table when you take the help of yourfriend. The force applied by your friendadded to the force exerted by you, results inboth forces being applied on the table in thesame direction. The total force applied byboth of you made it easy to move the table.

Now ask your friend to push the tablefrom the opposite side as shown infig.12(c). Does it move? If it moves, thenin which direction does it move?

When you and your friend push the tablefrom opposite sides, the table doesn’t moveif both of you apply force with equalmagnitude. Let us assume that one of youexerts a larger force, what will happen?Why?

F2F1

All forces have both magnitude anddirection. While adding forces, thedirections of forces have to be taken intoaccount. When forces act on a body alonga straight line and they are in the samedirection the net force is taken as the sumof all forces acting on the body. To addforces, sign convention must be used.

As shown in the above figure the forceF1 directed towards right could be taken aspositive and the force F2 acting towards leftcould be taken as negative. Let the forcesF1 and F2 act on the table in oppositedirections as shown in the figure and F1>F2,

Then Fnet = F1 + (-F2) = F1 - F2

When the forces on a body in astraight line are in opposite directions, thenet force is equal to difference between thetwo forces. The object at rest moves in thedirection of the net force acting on it.

Activity-10

Effects of stretched rubber bandson fingers

Take a rubber band, stretch it using yourfingers. When you stretch the rubber bandit exerts force on your fingers and you feelthe force of pull on your fingers. Whathappens if you add one more similar rubberband around your fingers and stretch bothtogether to the same length? Do you feelthe combination of two bands exerts alarger force than that of one? Increase the

Fig - 13

12 Force

number of rubber bands around yourfingers and observe the force exerted onyour fingers by the rubber bands.

Fig-14 Stretching rubber bands

Let us say the force exerted by onerubber band is F units and the force exertedby the second rubber band is also F units.Then what will be the net force of tworubber bands? We can express it as:

Fnet = F + F = 2F unitsThe unit of force in SI system is

newton(N).What is the net force acting on your

finger when three, four etc. rubber bandsare used?

How to calculate net force fromfree body diagrams

The diagram showing all the forcesacting on an object at a particular instant iscalled Free Body Diagram. It is denoted asFBD.Example:

Let a car be moving with a non uniformspeed along a road. What are the forcesacting on the car? What is the net forceacting along the vertical direction? Whatis the net force acting along horizontaldirection?

Draw all the relevant forces acting ona body (we called it a free body diagram,FBD)

Choose a coordinate system with x andy axis as shown in figure-15.

Sign convention is to be taken along xand y axis directions.

Add forces algebraically with signconventions along X and Y axis separately.Then those values give net forces along Xand Y directions respectively.

Solution:

The forces acting on the car are shownin the FBD (Free body diagram): They are

Force applied by the engine = FFriction applied by road = fNormal forces are N1and N2

Gravitational force (Fg) = WNet force along x-axis or direction

Fnet, x = f - FNet force along y-axis or direction

Fnet, y = N1 + N2 - W

Think and discuss

Play arm wrestling with your friend.How can you explain the winning ofthe game by using the concept of netforce?Name forces acting on arm and theirdirection while playing the game.Try to draw FBD for this situation.What Forces can do ?

F

N1 N2

Wf

Fig-15: Free Body Diagram(FBD)

y-axis

x-axis

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Fig.17

Activity-11

Effect of force on change of thestate of motion of an object andit's direction.

Place a football on the ground. Theball will remain in a state of rest unlesssomeone kicks the ball. Now kick the ball(Fig-16a). What happens? Does the ballstart moving? Kick the moving ball againin the same direction (Fig-16b). What willbe the result? Place your hand or leg againstthe ball. Does the ball stop? Or does itchange its direction? Note yourobservations.

Fig-16: (a) Force applied on a ball at rest. (b) Force applied on a moving ball

in the direction of motion.

We can move the ball from its positionof rest by applying a force on it. We canstop the moving ball and bring it back torest by catching it. Give few more exampleswhere the state of motion of an objectchanges due to the application of force.

You might have seen children playingwith a rubber tyre by pushing it with a stick.

They push the tyre again and again with thestick to increase its speed. Do youunderstand why the speed of the tyreincreases whenever it is pushed by the stick?

With every push they are applying a littlemore force on the moving tyre in thedirection of motion. Hence the speed ofthe tyre increases continuously.

If the net force acts in the direction ofmotion, the speed of an object moving withconstant speed also increase. If the netforce acts in a direction opposite to themotion, then it either slows down theobject or brings it to a rest or it may changethe direction of motion.

Give some more examples where theobject speeds up or slows down or a changemay occur in its direction of motion. Whenwe exert a force on it?

Activity-12

Effects of net force on directionof moving object

Hit a carrom coin with the striker. Ask yourfriends to do the same. Does the coin movein the same direction in each case? If not why?

You can observe that in each case thedirection of the coin changes.

When you hit the coin with the striker,

16(a)

16(b)

14 Force

not only does the coin change its direction,but the striker changes its direction too.What might be the cause for that?

From these observations, we can saythat a net force stops a moving object ormakes a stationary object move and alsochanges the speed and direction of amoving object. Thus, a force can changethe state of motion of an object. Does theforce change only the state of motion? Arethere any other effects of force?

Other effects of force

Activity-13

Effects of force on the shape ofan object

In table-3 some situations are given inthe first column showing how the force isapplied on an object. Observe the shape ofthe objects carefully before and afterapplying the force. Mark 'T' for temporarychange and mark 'P' for permanent changein the second column.

Table 3

Give some more examples where forcethat can change the shape of an object.

From the above table we can understandthat a force not only changes the state ofmotion of an object but can also change theshape of an object. It may change the shapetemporarily or permanently, based on thenature of the object and the force appliedon it.

Pressure

Activity-14

Change in effect of force witharea of contact

Take a pencil. Just push its rounded endon your palm. Now push from the otherside of the pencil gently so that the sharpend is on your palm. What difference didyou experience? Why?

Fig-18

Why do the porters place turbans(talapaga) on their head when they have tocarry heavy loads? Why do School bags andshopping bags have broad straps as handles?Have you ever wondered why lorriescarrying heavy loads have a larger number

Stretchingrubber bandSqueezing sponge

Tearing paper

Crushing plastic bottle

Making chapathi

Breaking glass

Change in shape(temporary/permanent)

Actionof force

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Since the masses of both bricks aresimilar the force applied on lime powderby them is the same on both the trays. Thedifference lies in the surface area of thebrick in contact with the lime powder andthis is responsible for the change in theextent to which the brick sinks in the limepowder.

In above activity, the contact area onwhich force is acting is different in eachcase. The depth to which the brick sinks inthe first tray (Fig 19 a) is deeper than thatin the second tray (Fig 19 b).This is becausein Fig 19(a), the contact area or the surfacearea on which force is acting is smaller andhence, the pressure exerted by the brick ismore. In Fig 19(b), the contact area or thesurface area on which force acting islarger. Hence the pressure exerted by thebrick is less.

Why does the sharper side of a knifecuts more easily than the blunt side of it? Asharp side of knife has a smaller contactarea. Therefore, for the same amount offorce applied on it, the sharp side of knifeexerts more pressure than the blunt side andhence cuts more easily. Can you give somemore examples of pressure?

From the above examples, you cansay that for a given force, if the surface areais smaller, the pressure will be greater. Ifyou use a larger area, you are spreading outthe force, and the pressure (or force perunit area) becomes smaller.

Think and discuss

Does pressure have direction? Explain.

of broader tyres? In these examples youmight have noticed that the effect of forcedepends on the area of contact on whichthe force is acting. When there is adecrease in the area of contact of the forceor load then the effect of force increasesand vice versa.

The force acting perpendicularly on unitarea of a surface is called pressure.

Pressure = Force/AreaThe unit of pressure in S.I. system is

Newton/meter2 or N/m2.

Activity 15

Identifying effects of forceTake two trays. Fill both the trays

with lime powder or fine sand. Now taketwo rectangular bricks of equal mass andsimilar shape. Keep one brick vertically inone tray and the other brick horizontally inthe second tray. What do you notice? Doboth bricks sink to the same depth in limepowder? If not why?

Fig-19

You may notice that the brick standingvertically sinks deeper in lime powder thanthe brick standing horizontally.

16 Force

What we have learnt

Improve your learning

Key words

Force, Push, Pull, Contact force, Force at a distance, Field, Friction, Muscularforce, Gravitational force, Magnetic force, Electrostatic force, Net force, Magnitude,Equilibrium, Normal force, Tension, State of motion, Pressure, Freebody diagram

• Force is a push or a pull.

• A force can act on an object with or without being in contact with it. A force actingon body is either a contact force or force acting at a distance.

• Field is an region influenced. If an object is kept at any point in the region, isinfluenced by it.

• Friction is the force that opposes relative motion of surfaces in contact.

• The force which we exerts by using our body muscles is known as muscular force.

• The attractive force between any two massive objects is called gravitational force.

• The magnetic force attracts a magnetic material such as iron. But it either attractsor repels another magnet.

• The force exerted by a charged body on other charged body is known as electrostaticforce.

• Force has magnitude as well as direction.

• The algebraic sum of all the forces acting on a body is known as net force, and isdenoted by Fnet.

• A force can change the state of motion of an object.

• Force may cause a change in the shape of an object.

• The force acting perpendecularly on a unit area of a surface is called pressure.

1. What is a force? What changes can be produced by a force? (AS1)

17Free distribution by T.S. Government

a) b) c) d)

2. State an example for a situation mentioned below in which a force: (AS1)a) changes the speed of an objectb) changes the shape of an objectc) changes the direction of an object.

3. How can you differentiate between a contact force and a force at a distance? (AS1)

4. Give two examples each for a contact force and a force at a distance. (AS1)

5. Detect the errors in the following statement and rewrite it making necessarycorrections.

“Because the car is at rest, no forces are acting on it” (AS1)

6. Why do tools meant for cutting always have sharp edges? (AS1)

7. Objects change their state of motion due to the net force acting on them. Do youagree with this statement? Discuss with examples. (AS1)

8. When you push a heavy object, it doesn’t move. Explain the reason in terms of netforce.(AS1)

9. If you push a heavy box which is at rest, you must exert some force to start itsmotion. However, once the box is sliding, you can apply a lesser force to maintainthat motion. Why? (AS1)

10. Find net forces from the following diagrams (AS1)

11. How do you increase the pressure by keeping (a) area unchanged and (b) forceunchanged? (AS1)

12. Imagine that friction has disappeared from the earth. What will happen? Explain.(AS2)13. Karthik is observing the live telecast of a one day cricket match. He noticed motion

of a roller on the pitch during lunch break. He thought about various forces actingon the roller and the net force when it is in motion. Many questions arose in hismind regarding the direction of the net force. Can you guess what would be thosequestions?(AS2)

14. Design and conduct experiments to test few ways how friction may be reduced.(AS3)

15. a) Take two identical straws and suspend one of them freely. Rub the other strawwith a piece of paper. Bring the rubbed end of the straw near the suspended one.

8N

10N12N 8N 8N

8N6N

9N

8N

18 Force

What do you observe from this activity? Can you tell which type of force is it ? (AS3)

b) Comb your dry hair. Bring the comb near small pieces of paper. What do youobserve? Explain.(AS3)

16. Collect pictures from various sources like internet, magazines, news papers, etc, toillustrate contact forces, forces and prepare a scrap book.(AS4)

17. A stick is placed on steps as shown in the figure. Draw normal forces on the stick (AS5)

18. Observe the figure below and find in which direction the force of frictionacts. Also indicate the normal forces and their directions.(AS5)

19. A man is standing still on a level floor. What forces act on him? Draw a free bodydiagram (FBD) to show all forces acting on him. (AS5)

20. How do you appreciate the role of friction in facilitating our various activities?(AS6)

21. A monkey hangs stationary at the end of the vertical vine. What forces act on themonkey? (AS7)

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FRICTIONChapter

2

However, the speed of the book graduallydecreases and after some time it stops.

Why does the book stop after coveringsome distance?Is the book moving with uniform speed?Why does the speed of the book changegradually?

You know that the book is in non-uniform motion with respect to thefloor. In the "Force" chapter we studiedthat non-uniform motion of a body takesplace only when a net force acts on it.How many forces act on the book whenit is pushed on the floor?Let us examine the forces acting on the

book. Two forces act on the book in thevertical direction as shown in fig-3.

We have learnt about the various typesof forces in the chapter 'Force'. We alsohave learnt about the 'force of friction'which plays an important role in daily life.Let us learn in detail about this force in thischapter.

Force of friction and its Types

Activity -1

Identifying forces acting on abody and effect of frictional force

Fig-1: Pushing the book

Gently push a book on a horizontal flooras shown in fig.1.

What do you observe?You may observe that the book acquires

a certain speed in the direction of push.

Direction of motion

Fig-2 : The book acquires a speed

20 Friction

Fig-3: Forces acting on the book

They are(i) Weight of the book (W) or gravitational

force acting vertically down.(ii) Normal force (N) or reaction force

applied by the floor vertically up.As there is no change in motion of the

book along the vertical direction, the netforce acting on the book in the verticaldirection is zero.

That is, 0W N W N− = ⇒ =

In the horizontal direction, the speedof the book is changing continuously. Itsspeed is decreasing gradually in thehorizontal direction i.e., the book hasacceleration opposite to the direction ofmotion (which we call deceleration).

What are the forces acting on the bookin the horizontal direction?What is the net force acting in thehorizontal direction?When the speed of the body moving in

a straight line changes continuously, we saythat the body has acquired an acceleration.

By close observation of this activity, wecan understand that the floor applies a forceon the book against its motion. Similarly thebook also applies the same amount of forceon the floor in the opposite direction. Hereit is clear that the floor is at rest. Hence thenet force acts in the direction of the forceapplied by the floor on the book.

The force applied by the floor on thebook is called “frictional force” orfriction.

Aim: Understanding the nature of frictionand the concept of static friction.Material required: Toy Trolley, smallwooden block, inextensible string, weighthanger, weights, pulley and table.

Fig-4:The trolley accelerating towards left

Procedure: Take a small toy trolleyand keep a small wooden block on it asshown in fig-4. Tie an inextensible stringto the trolley and pass it over a pulley. Andother end of the string is fixed to weight

Lab Activity

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We can increase the trolley’sacceleration by increasing the weight onthe hanger. If we increase the accelerationof trolley gradually, at certain limitingacceleration or limiting weight, the blockcomes into motion in the reverse direction.This means that now there exists relativemotion between the surface of the trolleyand the block.

What happens when theexperiment is repeated by usingrock and iron blocks of the samemass and different masses?

Does the limiting weight change?If so, why?

Apply some grease to the bottomof the wooden block and keep it onthe trolley’s surface and do thesame experiment.

What happens to the limitingweight?

What should we do to increase thelimiting weight?

From the above activities we maydefine friction as follows,

The force which opposes the relativemotion of two surfaces of bodies incontact, is called 'frictional force'.

In activity-1, the book moves withrespect to the floor. So, this friction iscalled sliding friction.

Sliding friction is the friction whichcomes into play when the surface of oneobject moves relative to the surface ofanother object.

hanger. Take a small weight and keep it onweight hanger and observe the changes inmotions of block and trolley.

What happens to the position of theblock kept on the trolley?Does it fall or move along with thetrolley?What changes occur in the motion oftrolley and block?You will notice that the trolley with the

block on it moves towards left with anacceleration. The block is at rest withrespect to the surface of the trolley, but itis in motion with respect to the surface ofthe table.

Now keep on increasing the weight onthe hanger. Observe the motions of bothtrolley and block.

The surface of the trolley tries to keepthe block at rest here with respect to itssurface.

Thus, the force of friction by thesurface of the trolley acts on the block inthe direction of motion. At the same timethe block also applies a force on the trolleyin opposite direction and tries to movetowards the right.

Fig-5: The direction of friction on the block.

Direction of motion

22 Friction

F

f (maximum)

F

f

From this experience we conclude thatstatic friction is a self adjusting force

Fig-7 : Pushing a heavy box with increasingforce.

But there is a limit to this static friction.As you keep on increasing the appliedforce, at some point the box starts moving.That is, when the applied force is more thanthe limit of the static friction, the bodystarts to move as shown in fig-8.

Fig-8: The heavy box starts moving

Think and Discuss

Does friction oppose motion orrelative motion of surfaces incontact? Disscuss.What observations and experimentscan you cite to show the existenceof friction?When do we speak of slidingfriction?

In lab activity, the block is at restrelative to the surface of the trolley up to acertain limiting acceleration. The frictionexists at this stage is a static friction.

So, static friction is the friction whichcomes into play when surfaces of theobjects are at rest relative to each other.

In the above activity we observe thatthere exists two types of frictional forceat a time. One is sliding friction betweensurface of the table and trolley, and theother is the static friction that existsbetween the surface of trolley and woodenblock kept on it.

Activity-2

Observing the variation offriction

Push a heavy box which is kept on afloor with a small force to movehorizontally as shown in fig-6. The boxdoes not move because there is a frictionalforce which is equal and opposite to theapplied force on the box.

Now gradually increase the appliedforce, the box still does not move, becausethe frictional force also increasesaccordingly and thus balances the increasedapplied force.

Fig-6: Pushing a heavy box with small force

F

f

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Factors affecting friciton

Activity-3

Effect of roughness on frictionalforce

Fig-9: Motion of a Ball on an inclined plane.

Set up an inclined plane on thehorizontal floor. Use a wooden board asinclined plane. Put a mark at any point “A”on the inclined plane. Now let a pencil cellor ball move down from this point. Notethe distance covered by the pencil cell fromthe bottom of the inclined plane to pointwhere it comes to a stop.

Now, spread a cloth over the floor.Make sure that there are no wrinkles in thecloth. Try again with pencil cell. Now notedown the distance.

What are your observations from theseexperiments?

In which case is the distance coveredmaximum?

In which case is the distance coveredminimum?

Why is the distance covered by thepencil cell different on differentsurfaces? Discuss the result.

If you do the above activity by replacing

the cloth with white marble surface or glasssurface, can you predict the distancecovered by the pencil cell?

You can conclude that smoothness /roughness of the surfaces of both the floorand the pencil cell could affect the distancetravelled by it.

Though many surfaces look likeperfect planes, there exists many ups anddowns (irregularities of surface) on them.See Fig-10.

Fig-10: Irregularities of two surfacesin contact

Friction is caused by the irregularitieson the two surfaces which are in contact.Irregularities on the two surfaces lock intoone another, when we attempt to move onany surface. We have to apply a force toovercome interlocking. On rough surfaces,there exist a large number of irregularities(ups and downs). Hence, the force of frictionis greater if a rough surface is involved.

F

24 Friction

Activity-4

Effect of area of contact onfrictional force

Fig -11: Pulling a brick with spring balance

Tie a string around a brick and pull thebrick by using a spring balance as shown infig-11. We use spring balance as a deviceto measure the applied force.

In a spring balance the spring isstretched by the applied force. The changein the length of the spring is proportionalto the applied force. So the scale of thespring balance directly gives the appliedforce in Newtons and in some springbalances the force is given in kilogram-weight also.

Pull it to move the brick. Note downthe reading of spring balance when the brickjust begins to move.

How many forces act on the brick inthe horizontal direction?

Two forces act on the brick in thehorizontal direction as shown in fig-12 .

Fig-12: Horizontal forces on the brick

One is force of friction (f) and the otheris the force applied (F) by you. The appliedforce is equal to the maximum limit of thefrictional force at the instant when the brickjust begins to move. But they act in oppositdirection. You can note down its value byobserving the reading of the spring balance.In this way we can measure the maximumfrictional force offered by the surface.

Now turn the brick upright as shown infig-13 so that the contact area with thefloor becomes small. Repeat the sameexperiment and measure the friction usingthe spring balance.

Fig-13: Pulling same brick with anotherorientation

How does the frictional force vary withthe change in the area of contact?The frictional force appears to be the

same in both cases irrespective of area ofcontact of the surfaces. So, the frictionalforce is independent of the area of contact.

Activity-5

Effect of normal force onfriction

As in the activity-4, keep a brick on thehorizontal floor and pull it with the springbalance attached to it and measure thefrictional force.

F

f

25Free distribution by T.S. Government

Now put another brick over the bricktied to the spring balance or press itvertically with your hand and then measurethe force of friction as described above.

Is there any difference betweenfrictional forces in two cases? If yes,why?From the above activity you can

understand that when we add a brick to theexisting brick or apply a force by pressingit vertically, the normal force increases andhence, we find there is an increase in thefrictional force.

So, Friction is proportional to theNormal force i.e.,

Friction ∝Normal Forcewhere ∝ is the symbol representing

“proportional to”

Think and Discuss

Does friction act on a table resting onthe floor?

If normal force is doubled, whathappens to friction? Discuss.

Your friend says, “Friction depends onthe area of contact”. How do youcorrect your friend through someexperiments?

“Friction is independent of weight, butdepends on normal force betweensurfaces of contact where frictionexists”. Do you agree with thisstatement? Discuss.

Is friction necessary?Try to walk on muddy or slippery

surface. Why do you find it difficult to walkon slippery surfaces?

We cannot walk or run without friction.Let us see the things that will not happen inthe world if friction were not present. Wewill not have any cars, bicycles or scooters.All of them move only because of friction.

Even if somebody pushes a car, we willnot be able to stop it by applying brakes.Carpenters will not be able to smoothensurfaces. You will not be able to hold anyappliances such as hammer, soap etc.

It will not possible to write with pen orpencil if there is no friction. You would notbe able to fix a nail on the wall. No buildingcould be constructed if there were no friction.

All the above examples tells theimportance of friction.

On the other hand friction is undesirabletoo in the machinery. For instance, frictionis responsible for overheating and wearingout of moving parts. You need to apply oil orgrease to your bicycle parts in order to makeit move smoothly.

Make a list of few more examples ofthe situations where we need to reducefriction for efficient functioning of tools.

Activity-6Friction produces heat

Fig-14: Rubbing the hands

26 Friction

Increasing and decreasingfriction

Activity-7

How to reduce friction ?Take a spoon and hold its head (broader

portion) in the left hand and hold the midportion of the spoon by the right hand andpull it towards the other end of the spoon. What do you notice?

Now dip your right hand fingers inwater, do it again as said above.

In which case it is easy to pull? Why?Repeat the activity with other liquids

such as coconut oil, grease etc and observethe difference.

Friction can be advantageous in somecases and disadvantageous in other cases.In the former case of activity 7 you willfind that the friction is more, in the lattercase, friction is reduced.

Let us try some examples.

Fig-16: Bottom of the shoe

Have you ever thought why the sole ofyour shoes is grooved as shown infig-16?

Fig-15: Striking a matchstick againstthe surface of matchbox

Rub your palms against each otherfor a few minutes. How do you feel?Strike a match stick against the roughsurface of match box. What happens?

In both the activities we observebecause of the friction, temperature of thesurfaces increases. Matchstick catches firebecause of increase in temperature byfriction.

Thus, we can conclude that friction canproduce heat.

Give some more examples wherefriction produces heat.

You have probably heard that space craftreturning to the earth has to be protectedby a heat shield covering it. Find out why?What is the material used as the heatshield?

Think and Discuss

What important role does friction play in the life of human being and

animals?Why is friction important fortransport?

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It is done to provide the shoes bettergrip on the floor, so that you can movesafely. Similarly, the tires of cars, trucksand bulldozers are threaded (fig-17). Why?

Why do you need to change the tyreswhen it’s threading is worn out?

Fig-17: Pattern of tyreGymnasts apply some coarse substance

on their hands to increase friction forbetter grip.

In some situations, however, friction isundesirable and we would like to minimizeit.

Fig-18: The carom board

Play carom board without powder andthen play with fine powder sprinkled on theboard.

In which case is the movement of thestriker and the coins easy? Why?Why do we pour a few drops of oil onthe hinges of a door?

Why do we use grease between themoving parts of motor vehicles?In all the above cases, we want to reduce

friction in order to increase efficiency.When oil, grease or any other lubricants

are applied between the moving parts of amachine, a thin layer is formed between themoving surfaces and hence they do notdirectly rub against each other. Interlockingof irregularities is avoided to a great extentby the application of lubricants. Hencemovement becomes smooth.

The substances which reduce frictionare called “Lubricants”

Activity -8

Effect of rollers on friction

Fig-19: Pulling suitcase with rollers

Pull a suitcase without rollers and pulla suit case which has rollers.

In which case is pulling easy? Why?

28 Friction

F F

Fig-20: Pushing a book on pencils

Try to push a book lying on the table.Now place the book on two to three pencilsor pens without caps. Push the book again.

What do you notice? Why?In which case is it easy to pull the book?Why?It is always easier to roll a body than to

slide it over a surface. So it is convenientto pull the luggage fitted with rollers.

When one body rolls over the surfaceof another body, the friction offered iscalled rolling friction.

Activity-9

Understanding the principle ofball bearings

Fig-21: Rotating the lids

Take two lids and rotate them by puttingone on the top of the other. What do youobserve? Now place four to five marbleson one lid and place the other lid on thetop of the marbles. Then try to rotate thetop lid. What do you observe?

This is the principle of ball bearings.To reduce friction between rotating

shafts of machine tools, we use “ballbearings”.

Think and Discuss

Can we reduce friction to zero?Explain.

What purposes are served bybearings in machines? Explain withdaily life situations.

Fluid friction

Activity -10

Observing fluid friction

Fig-22: Stirring water

Take a glass of water and stir it with aspoon. You know that water whirls aroundan axis. Stop stirring and see what happens.You may notice that whirling speed of watergradually decreases and after some time,the water stops whirling and comes to astable state.

Which force is responsible forstopping the rotation of the water?The frictional forces between the liquid

layers and between the liquid surfaces thatis in contact with glass surface areresponsible for stopping the rotation of thewater.

Similarly water and other liquids exertforce of friction when objects move

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Key words

depends on its speed with respect to fluid,on the shape of the object and on the natureof the fluid.

Fig-23: Bird and Aeroplane

It is obvious that when objects movethrough fluids, they have to overcomefriction acting on them. So efforts are madeto minimize friction. Objects are givenspecial shapes. Where do you thinkscientists get hints for these specialshapes? From nature, of course.

Birds and fishes have to move about influids all the time. Their bodies must haveevolved to shapes which would make themnot to lose much energy in overcomingfriction.

Do you find any similarity in the shapeof an aeroplane and a bird? In fact allvehicles are designed to have shapes whichreduce fluid friction.

Friction, Static Friction, Sliding friction, Lubricants,Ball bearings, Drag, Fluid Friction.

Friction opposes the relative motion between two surfaces in contact. It acts on boththe surfaces.

Static friction comes into play when we try to move an object at rest relative to anothersurface or object.

through them. You can observe fluidfriction when you travel in a boat.

Not only liquids, gasses and air alsooffer friction to bodies like aeroplanes,jets when they move through air.

In science the common name for gasesand liquids is 'fluids'. So, we can say thatfluids exert force of friction on objects inmotion through them.

The frictional force exerted by fluidsis also called "drag".

Activity-11

Identifying factors influencingthe fluid friction

Take a tub of water. Try to move yourhand in water in the direction of stretchedfingers (up and down).Now try to moveyour hand in the direction perpendicular tothe plane of the hand (to and fro).

In which orientation of your hand, doyou experience more drag? Why?Frictional force on an object in a fluid

What we have learnt

30 Friction

Sliding friction comes into play when an object is sliding over another.Friction depends on the nature of surface and the normal force with which the bodypresses the other surface.Friction is independent of area of contact.Friction can be reduced by using lubricants and ball bearings in many machines.When bodies move through fluids, fluids exert a frictional force called drag.

1. Do you agree with the statement, "friction is both good and an evil"? Explain with examples.(AS1)

2. Explain why sportsmen use shoes with spikes? (AS1)3. Would it be easier or more difficult for you to walk on soapy water on the marble

floor? Why? (AS1)4. What ways do you suggest to reduce friction? (AS1)5. What conditions are needed for static friction to come into play? (AS1)6. Give examples of practical application of static friction. (AS1)7. Give examples showing the existence of sliding friction. (AS1)8. Explain how can you measure frictional force? (AS1)9. Explain how does lubrication reduces friction? (AS1)10. What kinds of friction do you know? (AS1)11. Explain why sliding friction is less than static friction? (AS1)12. Give examples of how is friction responsible for energy wastages? Give suggestions to

reduce energy wastages by friction. (AS1)13. Seetha is observing a moving bus with the luggage on its top. As the bus is moving

slowly there is change in the state of luggage on its top. But when the bus speeds up andstarts moving fast, she noticed that the luggage on the top of the bus fell to the back ofthe bus. This raised many doubts in her mind regarding the effect of frictional forceacting on the luggage as well as on the tyres of the bus. Can you guess the questionsraised in her mind? Write them. (AS2)

14. Collect information either from internet or from books in library, about various newtechniques being adopted by human beings to reduce energy losses due to friction.Prepare a note on that. (AS4)

15. Draw a free body diagram (FBD) to show various forces acting on a body which issliding on an inclined plane. (AS5)

16. “Reducing friction to the lowest possible level in machine tools solves the problem ofenergy crisis and conserve biodiversity”. How do you support the statement? Explain?(AS7)

Improve your learning

31Free distribution by T.S. Government

In previous classes, you werefamiliar with different types of fibres.

You know that clothes can be made fromnatural fibres such as cotton, silk and wool(figure 1). You also learnt how they areprepared.

Clothes made of natural fibres:We wear a wide variety fabrics in our

daily life. Are all of our clothes made of

natural fibres?

Fig-1: Clothes from Natural Resources

You might have heard about fabrics suchas polyester, nylon and acrylic etc. They areexamples of synthetic fibres. Why do we call them synthetic?

Synthetic fibres are not obtained fromplant or animal source. They are made bythe raw materials obtained from petroleum.Synthetic fibres are not only used forfabrics but also used in preparing manyhouse hold articles.

Chapter

3SYNTHETIC FIBRES AND

PLASTICS

Activity-1

Identify household articles made up of natural and synthetic fibres.Identify the articles around you in your sarroundings and write their names against relevantcategory in table-1.

Table -1Source ArticlesNatural fibres from plants cotton saree, ......Natural fibres from animals silk clothes,.....Synthetic fibres ..........................

32 Synthetic Fibres and Plastics

What do you observe from the listmade by you? You notice that the list ofhousehold articles made of synthetic fibresis very long. It encompasses all our day today activities.

What do you know about syntheticfibres? How are they manufactured? Letus discuss about synthetic fibres in detail.

What is synthetic fibre?Have you seen a bead neckalce ? Do

you observe any pattern in arrangement ofbeads ? see (Figure 2 a) Can you describethe pattern?

Fig-2(a):bead necklace Fig-2(c):

paper clips chain

Fig-2(b):single paper clip

Activity-2

Beads and paper clips patternTake a few paper clips (Figure 2b)

and join them together as shown in figure2(c). Observe the pattern of clips. Do youfind any similarity between the pattern ofbeads in necklace and the pattern of paperclips in the chain?

Each unit of paper clips or beads canbe joined together to form a long chain likestructure. Each bead (or clip) is a separate

unit but when many such units are joinedtogether, they form a new, differentstructure.

Similarly, synthetic fibres are made ofchain like structure which are themselvesformed by joining small units together.Each small unit in the chain is a is knownas monomer. Many small, identical unitsmonomers combine to form a large unitcalled polymer. Synthetic fibres are madeof polymers.

Unlike natural fibres, synthetic fibresare made from petroleum based chemicalsor petro chemicals. Petrochemicals aresubjected to various chemical processes toobtain synthetic fibres. Due to this, theyare called Synthetic fibres or artificialfibres.

Do you know?What does Polymer mean?‘Poly’ means many and ‘mer’ means part/unit. The word is derived from Greek. Sowe can say polymer is a structure madeup of many small repeating units.

Think and Discuss

What made the human beings tosearch for the alternative fornatural fibres?Which fibre source is notexhaustible? Why?

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popular during the Second World War nowa days Nylon replaced silk in most hosieryarticles such as stockings.How is nylon made?

Nylon is a polymer made of chemicalunits called polyamides. Polyamides aremade with monomers, hexamethylenediamine and adipic acid. Solid chips ofthese polyamides are melted and forcedthrough a heated spinneret which has very,very tiny holes.

Have you ever heard a crackling soundwhen you take off certain types of clothes?Or did you see sparks in them when it isdark? This is due to static electricity. Learnfrom your teacher about it.

The size and shape of the holes change,the characteristics of the resulting fibre.The fibre solidifies as it cools and can bespun or woven.

Nylon fibre is strong, elastic and lightin weight. Clothes made of Nylon arelustrous and easy to wash. So it has becomevery popular. Nylon does not absorb water- this fact explains many of its uses. Butstatic electricity is easily created in nylonfabrics. It also easily catches fire. So weshould not wear nylon clothes whilecooking, welding, working near a fire orusing heavy machineries. Why do we wearapron during cooking or working near thefire?

Many articles that we use in our dailylife are made up of nylon.

Can you name a few things made ofnylon? Tooth brush bristles, ropes, fishingnets, tents, sarees, stockings and socks, carseat belts, sleeping bags, curtains etc, aremade of nylon. Can you add some more tothis list?

Some synthetic fibresHow do you find whether a fabric is

synthetic or not ? Take a piece of cloth andpull out a thread. Unravel this thread (yarn).How long is the fibre? Is it lustrous or not?Do these characterstics help in identifyinga fibre?

You can’t identify the fibres by simplevisual inspection alone in the absence ofbrand label. Then how to identify them? Letus see.

Activity–3Identifying fibres – burning test

You can do a fibre burning test. Unravelseveral warp and weft yarns. Using tweezershold the yarn to the edge of a flame.Observe the changes.

If it smells like burning hair, theyarn is wool or silk.

If it smells like burning paper, theyarn may be cotton, or rayon.

If the yarn melts in the flame, it is asynthetic fibre such as nylon and acrylic.

You learnt how natural fibres areobtained and fabrics are made from themin the previous classes. Do you know howvarious synthetic fibres are obtained? Letus learn about it.

Think and Discuss

How synthetic fibres evolved to thepresent position?

NylonCan you name few synthetic fabrics that

you wear or know? Have you heard ofnylon?

Nylon is a synthetic fibre. It is preparedfrom coal, water and air. It was the first fullyprocessed synthetic fibre. It became

34 Synthetic Fibres and Plastics

Fig-3: Articles made of nylonNylon has replaced wool as the fibre

used in making carpets. Nylon is also usedin making parachutes. Rock climbers usenylon ropes to climb mountains. It is usedin making of swim suits, sheer hosiery,sails, umbrella cloth, dress materials, cartyres, etc,

Fig-5: An iron stand with a thread hanging from the clamp

Fig-4: Use of Nylon Fibers

(why nylon is prefered to make a goodparachute or a climbing rope? Let us findout!)

Table 2Type of Total weight

S.No. thread/ in gr / kgfibre required to

break the thread1 Cotton2 Wool3 Silk4 Nylon

nylon. Fill the data in table 2. (Precaution:Note that all the threads should be of thesame length and almost of the samethickness.)

Arrange the threads in order of theirincreasing strength. What do you observefrom the above activity?

Do you know that if you compare anylon thread with a steel wire of samethickness, nylon will be as strong as steelwire?

Activity -4

How strong is nylon?Take an iron stand with a clamp. Take

cotton, wool, nylon and silk threads about50cm in length. Tie cotton thread to standso that it hangs freely from it. At the freeend, attach a pan so that a weight can beplaced on it (Figure 5). Add weight startingfrom 10 grams one by one, till the threadbreaks down. Note down the total weightrequired to break the thread. Repeat thesame activity with threads of wool, silk and

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synthetic fibres. But the source materialfor rayon is wood pulp. It is the onlysynthetic fibre obtained from plant’scellulose and so it is called cellulose fibre.

If rayon is obtained from plantcellulose then why don’t we call it as anatural fibre? Let us find out the processof preparing rayon to decide whether rayonis natural or artificial.

The cellulose that was collected fromwood or bamboo pulp, is treated withseveral chemicals. First sodium hydroxideis added and then carbon disulphide to thecellulose. The cellulose dissolves inchemicals, added to it and gives a syrupcalled viscose. Viscose is forced througha Spinneret (Metal plates with very, verytiny holes) into a solution of dilute sulfuricacid. This gives us silk like threads. Thethreads are cleaned with soap and dried.This new fibre is called rayon. What do yousay now is rayon natural fibre or artificial?Some kinds of rayons are made from theshort cotton fibres left on cotton seedsafter ginning.

Rayon is cheaper than silk and can bewoven like silk fibre. It can be dyed in awide variety of colours.

Is rayon used only for clothes? Whatare the other things that are made fromrayon fabric?

Rayon is mixed with cotton to make bedsheets. Rayon is mixed with wool inmaking of carpets. Rayon is often used infashion and home furnishings. Rayon is alsofound in sanitary products, diapers andbandages and lints for dressing wounds(Fig-6).

Think and discuss

If we use cotton cloth and cottonropes in preparing a parachute, whatwill happen?Traditionally fishermen used cottonnets. Now they are using nylon nets.What is the advantage of usingnylon nets?Nylon sarees are better than thecotton sarees. It is better to useonly nylon sarees. Do you agreewith this ? Why?

RayonWe know about natural silk which is

obtained from silkworms. Fabrics made ofsilk are very costly. It is not even accessibleto all. Production and maintenance of silkclothes are also very difficult and timetaking process. But it’s texture fascinateseveryone.

For many years scientists attempted tomake silk artificially and their efforts weresuccessful towards the end of the nineteenthcentury. The first commercial production ofartificial silk was achived in USA in 1911.But this fibre was named as rayon only in1924. The first rayon factory in India wasestablished in Kerala in 1946.

Where do we find rayon factories in ourstate? Discuss with your teacher.

Let us now discuss how rayon isprepared.How rayon is prepared?

We noted that petrochemicals are thesource material for many polymers and

36 Synthetic Fibres and Plastics

wool. Even Nylon is mixed with polyester.Why do we combine fibres? What is thisprocess called?

Any synthetic fibre can be combinedwith two or more other fibres. This is calledblending. Natural and synthetic fibres areoften blended for preparing better fabric.

A blend does not simply meanalternating threads of cotton and polyester.Unique and different textures can becreated through blending. When a fibre iscombined with other fibre, certain qualitiesof first fibre are combined with thequalities of other fibers and give us ablended fabric which possess the bestqualities of both. Blending helps us toreduce the limitations of both fibres. Forexample, if you consider cotton andpolyester blend, cotton is comfortable towear but it forms wrinkles. But polyesteris wrinkle free. Therefore when these twoare combined, the end result is acomfortable and wrinkle free fabric. Cotton

Rayon is not a perfect fibre to prepareall fabrics because it is made up from plantcellulose, it absorbs water easily.Absorbing water makes rayon weak andcauses the fibre to break easily.

Think and Discuss

What characteristics make rayonbetter than natural silk?If you want to purchase a door matmade of synthetic fibre, whichsynthetic fibre door mat will youselect? Why?If sanitary diapers and bandages aremade of nylon, what will happen?

Activity-5

Why do we combine fibres?Whenever you buy clothes or

readymade garments observe the brandlabel. What do you notice? Or, visit a nearbygarment shop and look at the labels on thecloth (Figure 7).

Record the percentage of differentfibres mentioned on the labels.

You may find rayon mixed with wooland cotton, polyester mixed with cotton and

Fig-7:Labels showing different percentages of blend

Fig-6: Articles made from rayon.

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shrinks in wash while polyester generallydoes not. So when these two are combinedin a proper ratio, the resultant material doesnot shrink to a large extent when comparedwith a pure cotton fabric.

The more the percentage of naturalfibre in blending of fibres, the more wouldbe the comfort to the skin.

When natural fibres contribute to afabric, it allows the skin to breatheeasily. Also natural fibres are generallyfree from irritating chemicals.

AcrylicWhat type of clothes do you wear in

winter? All of us wear clothes which keepour body warm. We use sweaters, shawlsor blankets. People feel that all these aremade up of natural wool. Wool is veryexpensive and generally is not affordableand accessible to everyone. Think! Howmany sheep would be needed to obtain thewool to prepare sweaters for everyone inIndia?

All the winter wear nowadays is madeof a synthetic fibre called acrylic. Itbecame commercially available in 1941. Itlooks like natural wool. It can beconsidered as artificial wool. It is generallycalled ‘fake fur’.

It is made from a combination of coal,air, water, oil and limestone. It is spun byeither dry spinning, or wet spinning. In dryspinning the dissolved polymers areextruded into warm air. The fibres solidifyby evaporation. In wet spinning, the

polymers are dissolved and extruded into abath and then driedout. The wool obtainedfrom natural sources is quite expensive,whereas clothes made from acrylic arerelatively cheap.

Can you list out the things made up ofacrylic? It is used in knitted apparels suchas fleece, socks, sportswear and sweaters.It is also used in craft yarns, upholsteryfabric, carpets, luggage awnings and vehiclecovers. Do you have any of them in yourhome?

(Here, we just introduced a new word‘polyester’. You will learn about this indetail in later part of this chapter).

Think and Discuss

Which type of blended fabrics doyou find more comfortable inwinter? Why?The fabrics namely natural, syntheticand blended are available forgarments. Which fabrics will youprefer to wear for rare occasions likefunctions and in routine? Why?

Why synthetic fibres?What kind of umbrella would you use

on a rainy day? Would you use an umbrellamade of cotton or wool?

We have learnt about three differentsynthetic fibres. How are they differentfrom natural fibres? Synthetic fibres absorbless water and dry at a faster rate. Some areeven water proof. Most of them possess

38 Synthetic Fibres and Plastics

several unique characteristics. They aredurable, less expensive, readily available,affordable and are easy for maintenance.

In activity 3, when you burn syntheticfibre you find that their behaviour isdifferent from that of natural fibres.Synthetic fibres melt on heating. This is thedisadvantage of synthetic fibres. If thesynthetic fabric catches fire, it can bedisastrous. The fabric melts and sticks tothe body of the person who wears it. So weshouldn’t wear the synthetic clothes whileworking in the kitchen or in a laboratory.Washing and ironing also different forsynthetic clothes. Have you ever observedthe labels given in the figure-8 on thecollars of your shirts or inside the frocksand trousers? What does it indicate? collectsome labels and discuss with your teacherabout the symbols they represent.

Think and Discuss

Which fabrics do you prefer?Natural or synthetic? Why?Discuss comparatively.What is the difference betweenwashing of clothes at home andwashing by dry cleaning at laundry?

PolyestersYou might have heard about polyester

clothes. You might have also heard aboutterylene. What type of fabrics are they?

Polyester is the most commonlyused synthetic fibre. Polyester fibresrevolutionalized the fabric industry andchanged the costume culture. Fabric madefrom polyester fibre doesn’t get wrinkledeasily. It remains crisp and is easilywashable. So it is quite suitable for makingdress material. Terylene is a popularpolyester. It can be drawn into very finefabric fibres. That can be woven like anyother fibres. This fibre blends well withnatural fibres. Terylene is often mixed withcotton to make terricot and with wool togive terriwool. Like nylon, pure polyesteror terylene easily catches fire.

Polyester is made by reacting, terephtharic acid to di-methyl ether, then withdihydric alcohol. Polyester can be meltedand spun. This property allows the fibre toconvert into different shapes and sizes.Polyester fibres of these days are ultra thinmicrofibers which gives them a smoother,softer feel than the polyester of twentyyears ago. This base material can be usedto make not just fibres for fabric but manyother things: from soda bottles to boats.

Have you seen or heard of PETbottles? PET is a very familiar form ofpolyester. It is used for making bottles,utensils, films, wires and many other usefulthings.

Fig-8: Laundry label codes

39Free distribution by T.S. Government

Activity-6

How can you say a bottle is PETbottle?

Collect different kinds of waterbottles of your classmates and look at themcarefully. Do you observe any triangleshaped symbol at the bottom of the bottleor on the brand label sticker? What numberis marked in the center of the triangle?Look at figure 9. You will find that manyof the bottles will have 1 in the center ofthe triangle. If it is 1, then it is a PET bottle.

Code Numbers:

1. Polyethylene Terephthalate (PET,PETE)

2. High Density Polyethylene (HDPE)3. Vinyl (Polyvinyl Chloride or PVC)4. Low Density Polyethylene (LDPE)5. Polypropylene (PP)6. Polystyrene (PS)7. Other (The category of “Other” includesany resin not specifically numbered 1, 2,3, 4, 5, or 6, or combinations of one ormore of these resins.)

Fig-9 : Resin identification codes

Activity -7Identification of various articleswith recycling codes

Collect bottles of soft drinks (500 ml ormore), bottles of juice, and containers of fruit

jam, Ketchup, shampoo, coffee powder andtry to look for the triangle. What do younotice? What are these codes? What is theuse of these codes? Are all bottles found withcodes?

First look at the soft drinks and juices.You may observe that irrespective of thebrand name, the number 1 is marked in themiddle of the triangle. It indicates that it isa PET bottle. What about other bottles?

There are other numbers as shownin figure-9.

Where do you find these codes?Explore from various sources and try

to know more about coded articles.

Do you know?Why are soft drinks stored in these

PET bottles? Soft drinks are carbonatedso they should be stored in containerswith which they will not react.

Plastics around usObserve various articles around you, in

your house, kitchen, rooms, andbathrooms. What is the most commonmaterial used in making these objects? Milkand oil pouches, containers to store picklesand rice, buckets to store water, chairs,water pipes, electric appliances, television,radio and computers, mobile phones–everything seems to be made of plastic.

Talk to the elders in your family aboutthe materials they used in the past.Particularly, what do they think about buying

40 Synthetic Fibres and Plastics

Fig-11(a): Linear arrangement ofmonomers

Fig-11(b): Cross-linked arrangement

water in polythene pouches? What did theyuse to bring milk, oil, other liquids fromshop to home in the past? What were thecontainers, buckets, mugs, chairs and tablesused in the past made of? What do we useto make these articles?

Plastic has taken over the place thatoccupied by metals and wood earlier.Plastics have also replaced glass items. Ifwe continue to write the list, it will beendless. Plastics completely occupied ourlife because of their characteristicproperties.

Fig-10: Articles made of plastics

What is plastic?

Plastic is also a polymer like manysynthetic fibers we have studied so far. Butthe monomers in plastic can be arranged intwo ways. Some are in linear chains (Figure11 a) and others cross linked (Figure 11b).

Plastics are available in differentshapes and sizes. They have a wide varietyof uses. We also observed that they havedifferent code numbers as mentioned inactivity-6. Does the arrangement ofmonomer units in plastics has any relationfor this diversity in plastics?

Do you know?

Alexander Parkes(1813-1890)- Creatorof first Plastic -“Parkesine”

One of the manytriumphs of modern

science which is completelyencompassed the day to day life acrossthe world is the invention of plastics.The synthetically produced materialwas first presented in 1862 in Londonby Alexander Parkes. To prepare thismaterial Parkes heated nitrated cottonwhich previously soaked in sulfuric acidand made fabric soft and elastic with oiland camphor. The end product was anivory – colored material that becamedistorted when subjected to heat. Thisis named as Parkesine. Based on theseinitial efforts plastic evolved as mainbase for many modern materials. Butat that time people were not interestedon this alternative to natural substances.Now it is the basis for most of themodern materials.

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Activity-8

Types of PlasticsLet’s take two bottles made up of plastic.

One is PP bottle and another ordinary. (PET)Pour some hot water in both. What changesdo you notice ? Have you experienced suchinstances in your daily life (Figure 12). Seethe code of the bottle that gets deformed.

Plastics which getdeformed easily onheating and can be bentare known asthermoplastics. Someof the thermoplasticsare polythene and PVC.These are used inmanufacturing toys,combs and varioustypes of containers.There are someplastics which molded once can’t besoftened by heating. They are calledthermosetting plastics. Bakelite andmelamine are examples for this type. Nowcan you tell whether Tupperware isthermosetting plastic or thermoplastic?

Thus we can conclude that differenttypes of plastics have different properties.Plastics are easily mouldable, can beconverted into any shape, can be recycled,reused, coloured, melted and rolled intosheets and wires. Do all kinds of plasticsshow the above properties? Let us see.

Fig-12deformed bottle

Aim: Identifying thermoplastic andthermosetting plastics by flame test.

Materials required: pair of Tongs,spirit lamp, samples of plastics (Collectsmall pieces of plastics from the objects likecomb, tooth brush handle, plastic bucket,handle of utensil, electric switch, piece ofmelamine of meals plate and coffee mug)Procedure:

Take a spirit lamp and light it.Clamp one piece of plastic samplesay piece of tooth brush with tongs.Place the sample on spirit lampflame. See fig. 13.Observe the changes during theburning of sample.Note your observations like,whether sample is being softened orburnt with smell or become hard etc.Repeat the procedure with othersamples.Record your observations samplewise in the following table - 3.

Note: While doing this activity, if needed,cover your nose and mouth with a mask toavoid breathing of fumes. And also keepyourself away from spirit lamp flame whenyou put sample on the flame by stretchingyour hand.

Lab Activity

Fig-13: Conducting flame test

42 Synthetic Fibres and Plastics

In the above activity, we have taken theknown samples and tested. If unknownsamples are given, how do you decide whetherit is thermoplastic or thermosetting plastic?

If you burn a piece of wood, paper,cloth, steel rod, do you notice similarobservations like plastic material? Howare they different from plastics?

Based on the observations can you writea note on the properties of thermoplasticsand thermosetting plastics?

Thermoplastics This is a plastic which will soften when

heated and harden when cooled. Athermoplastic is a polymer that turns intoa liquid when heated and freezes to a glassystate when cooled sufficiently.

Table-3Sl.no. Name of the Softened/burnt with Thermoplastic/

plastic sample burning smell and Thermosetting plasticbecome hard

1. Tooth brush handle2. Comb3. Piece of bucket4. Handle of utensil5. Electric Switch6. Meals plate7. Coffee mug

Do you know?Hermann Staudinger (1881-1965) :

Hermann Staudinger, aGerman Chemist , in 1920made an important stepforward in the processing ofsynthetic materials when hedeveloped thermoplastics which are stablein themselves. Hermann Staudinger wonNobel Prize in 1953 for demonstrating thatPolymers are long–chain molecules.

Thermosetting PlasticsThermosetting plastics are simply

plastics, when moulded into a shape andallowed to cool down, will remain inmoulded form and will not change theirshape. When heated again they will char orburn. Thermosetting plastics are syntheticmaterials which gain strength duringmoulding by heating, but cannot be remoulded or reheated after their initial heat,moulding. Thermoseting plastics are notre-mouldable. Strong cross links areformed during the initial moulding processthat gives the material a stable structure.They are more likely to be used in situationswhere thermal stability is required. Wheredo we use this type of plastics?

Observe the kitchenware in your houseor visit any home appliances shop. Whatdo you notice? Is all kitchenware totallymade up of metal? With what materials arethe handles of utensils made? Utensils aremade of metals like aluminum, copper orsteel. Then what is the additional material?Bakelite is used for making handles ofvarious utensils due to its poorconductivity of heat and electricity. It isalso used for making electrical appliancesincluding switch boards. Bakelite is used

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Fig-15: Articles of Melamine

as an alternative for pearl and jade. You cansee the Bakelite made articles in figure 14.

The other thermosetting plastic,Melamine used for making kitchenware.Utensils and other items of kitchen aremade by melamine. It is also used formaking of floor and dress material for theirnature of fire resistance. Computer and TVcabinets are made of melamine. Now Listout the melamine products in your house.You can see few melamine articles infigure15.

Why do we prefer plastics?Plastics are non- reactive: You mighthave experienced that iron gates and nailsexposed to atmosphere, soon get rusted.But does this happen to plastic articles?Plastic material will not corrode easily.That’s why they are used to store variouskinds of materials including chemicals.

Plastic is very light, strong, durable andcan be moulded into different shapes andsizes. Plastics are generally cheaper thanmetals

Father of Plastic IndustryA Belgian scientist

named Dr. Leo HendrikBaekeland wasresponsible for theinvention of Bakelite.In 1907 he wasworking as chemist, byaccident he discoveredthe compound of carbolic acid andformaldehyde. When he tried to reheatthe solidified compound, he discovered.It would not melt-no matter how highthe temperature was. Dr. Baekeland isconsidered as the father of the presentplastics industry.Hot Pin Test

To test if a piece is Bakelite, get avery very hot pin from an open flamesource, then touch the pin to the item. Ifit is Bakelite it will not penetrate. Itmay give off the acid smell and it mayleave a purple burn mark. If the pinpenetrates or melts the plastic, then itis not genuine Bakelite.

Do you know?

Fig-14: Articles made of Bakelite

Why do electrical wires have plasticcovering? You might have seen pans,pressure cookers and screw driver handles.Why are they made of plastic? Plastics arepoor conductors of heat and electricity.

Think and Discuss

Certain fry pans are said to be non-stick. What made them non-stick?Firemen wear dress which does notcatch fire. What type of fabric isit made of ?

44 Synthetic Fibres and Plastics

Fig-16: Plastic bags found in the stomach of acow after a postmortem

Plastics and environmentYou must have seen garbage (waste)

dumps. Some articles there seem to remainfor a long time while some other disappear.You may notice that most of the materialthat does not disappear are the polythenebags. Polythene is a plastic. It is mainlyused for making carry bags.

The polythene bags thrown around areresponsible for clogging drains. Animals inurban areas, particularly cows, eatpolythene bags containing food material.Can you imagine the consequences? Seethe Fig-16 and read the comments ofSupreme Court of India about the effect ofplastic on environment.

With the direction of the Supreme Courtof India, very thin polythene bags have beenbanned to stop indiscriminate usage ofpolythene bags. What is the differencebetween the banned polythene bags and thebags using presently in place of previousones? What do you notice on that bags?

Some efforts are taking place to reducenegative consequences of plastics on theenvironment.

If we burn plastic, it creates a lot of airpollution.

“Plastic bags are more danger thanatom bomb for future generations”:Supreme Court of India.

“Excessive use of plastic bagsand their unregulated disposal hasbeen choking lakes, ponds and urbansewerage systems, the Supreme Courtsaid while warning that it posed athreat more serious than the atom bombfor the next generation .AndhraPradesh-based NGOs drawing thecourt’s attention to 30-60 kg of plasticbags recovered from the stomachs ofcows because of irresponsible disposalof plastic bags and defunct municipalwaste collection system. But the benchwanted to address the larger questionsarising from indiscriminate use ofplastic bags, which not only posed agrave threat to nature and environmentbut also to the human race itself. “Allof us are watching how our lakes, pondsand urban sewerage systems aregetting choked by plastic bags. Unlesswe examine a total ban on plastic bagsor put in place a system formanufacturers mandating them tocollect back all plastic bags, the nextgeneration will be threatened withsomething more serious than the atombomb”, Justices Singhvi andMukhopadhaya said. Large quantitiesof water packed in plastic pouches,which were thrown around inundisciplined and uncivil manneracross the country every day. “A roughestimate shows more than 100 millionwater pouches are thrown all over thecities and towns in a day” the benchsaid.

45Free distribution by T.S. Government

Plastics take several years todecompose as compared to other materialslike peels of fruits, vegetables and wastefood stuff which decompose within a shortperiod. Slow decomposition causesenvironmental pollution. The burningprocess of synthetic material is also veryslow and can’t be burnt completely. Theprocess of burning, releases a lot ofpoisonous fumes into air causing airpollution. So it is better to avoid orminimize the use of plastics. Make a listof some occasions where you can avoid theplastics and use alternatives.

Reduce, Recycle, Reuse andRecover – 4R principle

Can we avoid using plastics altogether?The entire civilization is enveloped withthem. Every walk of life is linked withplastics. Now you can understand that youcannot avoid use of plastics completely butwe can reduce, recycle, and reuse plasticsand avoid indiscriminate use of plastics. Wecan also adopt recovery mechanisms toconvert waste into a usable resource. Letus discus the 4R principle for creating aneco friendly environment.Reduce

What do you observe when you attenda marriage or function? What plates theyuse to serve food? By what type of glassesfor the drinking water is supplied? Whatcontainers do they use to serve sweets andice cream? What are the spoons made upof? Everything is made of plastic. Imaginehow much plastic garbage will beaccumulated in this single function. Isthere a way to stop this ‘Use and throw’

Activity-9Biodegradable – Non-biodegradable

Let’s take peels of fruits andvegetables, left over food stuff, waste paper,cotton cloth and plastic bag. Keep thismaterial in a pit. Open the pit after somedays and list the material which remain along time and those that disappear quickly.

Table-4Type of waste Approximate Change

time to disappear

Peels of fruitsand vegetablesLeft over foodstuff

Waste paper

Cotton cloth

Plastic bag

You observe that certain materials canbreak down into smaller fragments in thepresence of water, sunlight and oxygen.These fragments get further broken downby bacteria. This is called decomposition.A material which is easily decomposed bynatural process is called bio-degradable,and one which is not decomposed bynatural processes, is called non-biodegradable.

The time taken for decompositiondepends upon whether a material is bio-degradable or not. Explore various sourcesof material to know the time required forthem to decompose.

46 Synthetic Fibres and Plastics

culture? Discuss and write the ways andmeans where we can reduce the usage ofplastic, to avoid its negative consequenceson environment. Land filling of plastics andburning of plastics in incinerators is theother way of disposal of plastics. They toohave negative consequences. Therefore, weshould reduce it’s usage whenever it ispossible.Reuse

Have you observed that when we go tomarket to buy something we go without anybag? With the result that we come backwith lot of plastic bags. Every time we goto market, we accumulate lot of plasticbags. Some of these can be used again andagain. Are we doing so? Articles made ofplastics may be used again and again for itsoptimum utilization. This should be everybody’s concern for an eco friendlyenvironment. If anything you feel now outof fashion and you want to buy a new one,give it to others instead of putting it idle athome or throwing it out .Can you givecertain instances where we reuse plastics?

RecycleYou might have noticed your mother

selling old plastic articles which are brokenand not useful, to the local vendor. Whatwill they do with that? They collect allplastics from the households and send itfor recycling. From this recycled plasticnew products are prepared, after giving it aproper treatment.

Are all types of plastics recyclable?There are 60,000 types of plastics

available in the world as on today (thesynthetic fibers that we discussed are alsoplastics). There are only 6 from these60,000 that we use regularly. We have seenthat each of these six plastics is given acode. These codes help us in recycling.

PET (code 1) and HDPE ( code -2) arecommonly recycled. But LDPE (code – 4used for carrier bags) is not recycledcommonly. Similarly PVC (code-3) whichis used for pipes are currently not recycled.The PS (code -6), used for making coffeecups, egg boxes, packing peanuts and ‘takeout’ food packing can be recycled.Recycling can be used to obtain materialsfrom which the original products weremade.Recycling code

In activity-7 we raised certain questionsregarding recycling symbol code and itsuse. Now let us discuss them in this section.

The Society of the Plastics Industries,Inc. (SPI) introduced its voluntary resinidentification coding system in 1988 toassist recycling programs. The SPI codingsystem offered a way to identify and sortthe resin content of containers commonlyfound in the household waste stream.Plastic household containers are markedwith a number that indicates the type ofresin, or plastic as shown in fig-9.

To identify the plastic, look at therecycling icon, the chasing arrows. Insidethe arrows, there will be a number that

47Free distribution by T.S. Government

Key words

identifies the polymer. When the numberis omitted as seen in the figure 17, thesymbol is known as the UniversalRecycling Symbol, indicating genericrecyclable materials.

What is role of codes in recyclingprocess?

Each plastic is manufactured withdifferent processes. During recyclingprocess, if same codes are not sorted outseparately the whole lot meant for recyclewill spoil the process. Therefore, it isessential to recycle the same codes in onelot. If you add a simple PET bottle in the lotof other plastics during recycling process,the entire lot will be spoiled. Think why ?

Uncoded plasticsDo all plastics have resin

indentification code on them? Plastic tarps,toys, computer key boards and many other

products simply do not fit in the codingsystem adopted for recycling. Wide varietyof plastic materials are made to suit theneeds of consumer by using differentvarieties of plastic resins or mixtures ofresins. Is it possible to code thousands ofplastic varieties? Recycling of varioustypes of plastics is not commercially viablebecause their production when comparedto coded plastics is less. Code 1 and 2plastics occupy major share in recycledplastics.Recover

Supreme Court gave a judgment onways and means of solid waste managementand gave order to implement this in all thecities of India by 2003. In the solid wastegarbage of municipalities, plastic occupiesmajor share. The principle of recover playsmajor role in this solid waste management.The solid waste should be converted intoresources such as electricity, heat,compost and fuel through thermal andbiological means. Is any such effort forsolid waste management taking place inyour village/town.? How do you appreciate4R principle?

Fig-17: Universal recycling symbol

Acrylic, synthetic fibre, Bakelite, biodegradable, blend, cellulose, incineratorsmelamine, natural fibres, non-biodegradable , nylon, petro chemicals, plastics, polymer,polyamide, polythene, polyester, rayon, recycling, spinneret, terricot, terylene,terriwool, thermoplastics, thermosetting plastics, universal recycling symbol.

48 Synthetic Fibres and Plastics

Synthetic fibres are made of very large units called polymers.Rayon is an artificial silk made up of cellulose fibre.Nylon is made artificially by using raw material like coal, water and airPolyester is a synthetic fibre.Acrylic is artificial wool made from coal, air, water, oil and limestone.Petro chemicals are used to manufacture synthetic fibres.Commonly used synthetic fibres are rayon, nylon, polyester and acrylic.The different types of fibres differ from one another in their strength, water absorbingcapacity, nature of burning, cost, and durability.Synthetic fibres and plastics enveloped our life.The waste created by plastics is not environment friendly.Plastics take years to decompose.Enjoy the good qualities of synthetic fibres and plastics and reduce the indiscriminateuse of plastics to minimize environmental hazards.Synthetic fibres find uses ranging from house hold articles to healthcare.Synthetic fibres blend with natural and artificial fibres.

1. Explain why some fibres are called synthetic? (AS1)2. Give reasons why plastic containers are used as storage devices? (AS1)3. Classify following materials which can be, can not be recycled. (AS1)

Plastic toys, electrical switches, plastic chairs, ball point pens, plastic containers,cooker handles, plastic bottles, carry bags, tumblers, tooth brush, plastic chapels.plastic plates, plastic buckets (AS1)

4. If electric switches are made by thermoplastics what would happen? (AS1)5. Thermoplastics are eco friendly than thermosetting plastics. What do you say? Why?

(AS1)6. Explain following. (AS1)

a) Blending b) Biodegradable c) Recycling d) Decomposition7. Match the following. (AS1)

i) Polyester ( ) (a) kitchenwareii) PET ( ) (b) artificial silk

What we have learnt?

Improve your learning

49Free distribution by T.S. Government

iii) Rayon ( ) c) many monomersiv) Nylon ( ) d) electric switchesv) Melamine ( ) e) Code 1vi) Polythene ( ) f) popular dress materialvii) Bakelite ( ) g) stronger than any fiber

8. Fill in the blanks. (AS1)i) Synthetic fibres are also called as ———— fibres.ii) Synthetic fibres are synthesized from raw material called ——iii) Like synthetic fibre, plastic also ———iv) Clothing labels are————a) Required by law b) Identify fabric contentc) Both a and b d) None of the abovev) Rayon is made ofa) Coal b) Oxygen c) Flaxd) Cellulosevi) A silk fiber’s smooth surface absorbs lighta) yes b) no c) can’t say

9. Where do we use the process of recycling? How is it useful? Give examples.(AS1)10. Take hair, wool, silk, paper, cotton thread, piece of plastic, thread of sweater, piece

of rope and carefully conduct a flame test. Based on smell and type of meltingclassify them as natural or artificial fibres. (AS3)

11. Prepare a table of various synthetic fibers which are used to make household articlesfrom them? (AS4)

12. Explain the differences between the thermoplastics and thermosetting plastics withthe help of a diagram explaining in terms of arrangements of monomers? (AS5)

13. Prepare a chart which can explain recycling codes, full name and acronym of plasticand its usage for various household articles , recycled or not, if recycled what willbe made from that? (AS5)

14. Introduction of synthetic fibres in the textile industry brought revolutionary changeacross the world in the dressing patterns irrespective of culture and customs. Howdo you appreciate this? (AS6)

15. How synthetic fibres changed our everyday life? (AS7)16. Nibha wants to buy clothes to parents for winter wear. What type of clothes would

you suggest? Specify reasons. (AS7 )17. If plastic is not properly disposed, what could be the consequences? (AS7)18. Indiscriminate usage of plastic is a serious threat to bio-diversity. What are the

efforts of Government and Non-Government organizations in this regard? (AS7)

50 Metals and Non-Metals

In the previous classes, you have studiedabout some materials that are usedin our daily life.You studied about naturalfibres and their properties. You also studiedthe soil and its properties. You learnt acids,bases and salts. You also studied changesaround us like rusting etc. In this chapteryou learn about the properties of metals andnon-metals

You are familiar with a number ofmaterials like aluminium, copper, gold,iron, etc.

Can you name some objects made ofmetals?Observe the fig-1. Try to name the

metals from which the objects are made.Add names of more metals that you knowto the list.

Your first answer is gold. Some of youmay also have added aluminium, silver,lead, iron, copper, tin, mercury etc.

Did any of your friends add steel to thelist of metals?Do you think that steel is a metal?Let us learn the properties of metals

so that you are able to answer thesequestions at the end of the chapter. You alsolearn about another type of materials, callednon-metals, which may be new to you.

Now observe carefully all the materialsthat you have listed above as metals.

Do all these look alike?Do all of them shine?Are they hard or soft?Do they break easily?Can you group materials into twocategories by looking at theirproperties? We try to find two groups, then discuss

and compare them in detail in this chapter.

Physical PropertiesBefore we start this section, you will

need to collect pieces of iron (iron nails),copper, zinc, sulphur powder, aluminium,carbon, magnesium and iodine for carryingout the activities.

Chapter

4METALS AND NON -METALS

Fig-1

51Free distribution by T.S. Government

AppearanceIn previous classes, you learnt that

the materials which have a bright surfaceand reflect light are called lustrousmaterials and materials that do not shineare non-lustrous.

Activity-1

Observing appearance andcolour of some materials

Observe the appearance of yoursamples. Look at their colour. Decidewhether they appear shining or dull andrecord your observations in table - 1. If thesurface seems dirty, clean it with sandpaper.

Table–1

Your observations in the table showsthat some materials are shining and someare dull.

Which of the samples did not shine evenafter you cleaned them with sandpaper?Generally metals are lustrous. Do all

lustrous materials are metals?We all know that mirror reflects light.

Can a mirror be called metal?No, so you need to look at several

properties to decide if a given material ismetal or not.Sonority

While Aryan was carrying a geometrybox he slipped and fell down. He noticedthat the box made a ringing sound when ithit the hard floor. It was similar to thesound that of ringing bell.

Have you observed material used tomake school bell or bells in temple?Why are wooden bells not used inschools?Do all materials produce sound whenthey dropped on hard surface?Let us find

Activity-2

Listening the sound produced bysome material

Fig-2

Drop a piece of coal on the floor andlisten to the sound.

Do you think coal is sonorous?Take the pieces of zinc, copper,

aluminium, magnesium and tightly packedpackets of sulphur, carbon and iodine. Dropthem one by one, on a hard surface. Listencarefully to the sound produced and recordyour observation in table 2.

IronZincCopperSulphurAluminiumCarbonMagnesiumIodine

Sample AppearanceShining/not shining Colour

52 Metals and Non-Metals

Table-2

What similarity do you notice amongmaterials which produce sound?You may notice that some of the

materials produce sound and some of themdo not. Materials which produce ringingsound are called sonorous materials.Generally, most of the metals are sonorous.The materials other than metals are notsonorous.

Lustre and sonority are the propertiesassociated with the metals. But there is noneed that all metals should posses thisproperty. For example, though mercury isa metal, it is not sonorous.

Which property of metals firstattracted the attention of human beings?The story of early tools will give you a

hint.

Story of Early ToolsDo you think tools were always made of metals? Early

human beings made their tools from which were easilyavailable - stone and wood. Later, they used the bones ofanimals.

Then they discovered metals like copper and iron.Tools made of copper and iron are much stronger thantools made of stone and wood. Metals had the advantageof not just being harder but they can be heated in a fireand moulded or cast into different shapes. So it becamepossible to make a wider range of tools with such metals.

Fig-3 : hammers made ofdifferent materials being used

to hammer nail

Material Sample thatProduce sound

Material Sample thatdo not Produce sound

MalleabilityHave you ever noticed the thin silver

foil used to decorate sweets or the thinaluminium foil used for packing food?

Try to observe a blacksmith at work. Hebeats a hot iron piece repeatedly till itsshape changes.

Do you bring a similar change in theshape of a clay material by beating it? Not all materials can be converted into

thin sheets to make the desirable objects.

Activity-3Identifying malleability ofmaterial

Take a hammer and beat the materialsamples which are used in Activity-2 andobserve the changes in material samples.Record your observations in the table-3.

Observing thechange Name of sample

Flattens Iron, …..Breaks/ convertsinto powder

No change

Table-3

53Free distribution by T.S. Government

Some of the samples, when beatenhard, were flattened whereas somematerials broke into pieces or became apowder. The materials which can beflattened into thin sheets are calledmalleable materials. Malleability is one ofthe properties associated with metals.

What did you observe in the case ofiron? You may not be able to flatten it butthe blacksmith can do it. He heats it beforebeating. So we can say that materials differin the range of their malleability. Metalslike aluminium, silver and gold are highlymalleable.Ductility

We use wires in different situations inour daily life. Look at the samples given inthe table-4.

Have you ever seen the wires made upof materials mentioned in table-4.Write yes, if you have seen wires madeof them.Take help from your teacher, friends andelders to decide which of the material canbe used to make wires.From the above table you may infer thatsome materials can be drawn into wires andsome materials cannot be drawn into wires.

Fig-5The property of drawing a material to

make fine wires is called ductility. Mostmetals are ductile.

Is ductility the only property ofmetal to use them as connecting wires inelectric circuits?

Let us explore another property ofmetals.

Electrical ConductivityYou might have seen an electrician

using the screwdriver.What materials does it contain?Why does not a screwdriver used byelectricians has metal handle?

Fig-4

IronZincCopperSulphurAluminiumCarbonMagnesiumIodine

Sample Can we convert itinto Wires (Yes/No)

Table-4

yes

54 Metals and Non-Metals

Activity-4

Identifying electricconductivity of a material

Arrange an electric circuit with abattery and bulb (remember the simpleelectric circuits from previous classes).Close the circuit using an iron nail, asshown in figure 6.

Fig-6Observe whether the bulb glows or not.Record your observation in table-5.Repeat the same experiment using theother samples and record your observationsin the same table.

Table-5

IronZincCopperSulphurAluminiumCarbonMagnesiumIodine

Sample Does the bulb glow?(Yes/No)

Think and discuss

Do all the samples allow the bulb toglow?

Materials that allow electricity to passthrough them and make the bulb to gloware called electric conductors. Most metalslike iron, copper and aluminium are goodconductors of electricity.

Talk to an electrician. Look at thehandles of his tools.

Are the handles made of the samematerial? If not, why? Note the precautions to be taken while

working with such tools.The handles of both electrical

appliances and cooking utensils are notmade of metals. Electrical appliancesconduct electricity.

What do cooking appliances conduct?

How will you close the circuit usingsulphur, carbon or iodine? They may bein powder form. Try to tightly pack thepowder in a straw and use it. Think ofother ways!

Activity-5Observing heat conduction bymetals

Take an iron rod. Stick pins on it withthe help of wax (see fig-7). Now fix therod to a stand as shown in the fig-7. Heatone end of the rod with a spirit lamp andsee how the pins fall off?

Why did the pins fall from iron rod?Pin of which end fell first?What could be the reason for this?

55Free distribution by T.S. Government

Fig-7You know that the pin fell because of

the heat supplied to the iron rod makes thewax to melt at one end. The wax closer tothe flame melted first. This activity clearlyshows that heat moves from one end of theiron rod to the other. This property of amaterial is known as conductivity of heat.All metals conduct heat.

All materials do not have equalconductivity. Iron, copper and aluminiumcooking vessels are preferred due to theirhigh heat conductivity.

Do it!On the basis of all the activities carried

out, fill in the table 6.

We find that it is the metal whichpossess all the properties of lustre,malleability, ductility, sonority, conductionof heat and electricity. Non-metalsgenerally do not show these properties.

Chemical propertiesThe properties you have studied so far

are all physical properties. Though theseproperties are quite reliable, chemicalproperties are better indicators ofdetermining as to whether a given materialis metallic or not. Let us try to see whathappens when metals and non-metals reactwith other substances.Reaction with Oxygen

Aim: To know the reaction of oxygen withmetals and non-metalsMaterial required: One metal sample(magnesium) and one non-metal sample(sulphur), spirit lamp or Bunsen burner andlitmus papers, etc.

Lab Activity

Table-6

Lustrous Sonorous Conductsheat

Conductselectricity ductileMalleableMaterial sample

IronZincCopperSulphurAluminiumCarbonMagnesiumIodine

56 Metals and Non-Metals

Procedure: Take a small strip of magnesium and

note its appearance. Burn it. Note theappearance after burning.

Take a small quality of powderedsulphur in a deflagrating spoon and heat iton a spirit lamp. (You can make your ownspoon using a metal bottle cap and wrappinga wire around it).

(Be cautious, do not inhale fumes, theyare harmful).

Collect the ashes of magnesium ina test tube and add some distilled water toit. Test the solution with red and blue litmuspapers. Note the colour change in table-7.

As soon as sulphur starts burning,introduce the spoon into a gas jar/tumbler.Cover the tumbler with a lid to ensure thatthe gas produced does not escape.Remove the spoon after some time buttry to keep the jar covered. Add a smallquantity of water into the tumbler andquickly replace the lid. Shake the tumblerwell. Check the solution with red and bluelitmus papers. Record the changes intable7.

What do you observe?When the samples are burnt, they

react with oxygen in air to give differentproducts. See how.

Fig-8

Table-7

Sample

Magnesium

Sulphur

Physical Appearancebefore Reaction

PhysicalAppearance After

Reaction

Effect on LitmusPaper

57Free distribution by T.S. Government

Think and discuss

Something similar happens with othermetals also. Magnesium ribbon is dullwhen exposed to air and shiny if we cut it.Silver objects and jewellery becomes black.Copper statues and vessels become dullgreen. All these metals react withcomponents in air. But gold jewellery doesnot become dull.

What could be the reason?Thus, different metals react with the

components of air in a different manner atdifferent rates and conditions. There aresome metals which do not react with thecomponents of air. Gold and platinum aresuch metals which do not rust.

Reaction with waterNote:- This is a demonstration to be

carried out by the teacher. Sodium isextremely reactive and dangerous andstudents should see the demonstration froma distance.

Take a 500 ml beaker or a big glasstrough and fill half of it with water. Takethe sodium which is stored in kerosene andkeep it on a sheet of filter paper to blot thekerosene and cut a very small piece ofsodium from it.

Put the remaining sodium back againin kerosene. Standing away from the troughput the sodium piece in water using forceps.

Magnesium+Oxygen → Magnesium oxide (s) (g) (s)

Sulphur + Oxygen → Sulphur dioxide (s) (g) (g)

All the products in the above reactionsare oxides, but are they same in nature? Theoxide of magnesium turns red litmus toblue. The oxide of sulphur turns blue litmusto red. From this knowledge, you can saythat magnesium oxide is basic and Sulphurdi oxide is acidic.

You can also infer that non-metals reactwith oxygen to give oxides which areacidic, while metals react with oxygen togive oxides that are basic in nature.

Is our body a metal or non-metal?Most of the human body is made up

of water (H2O). It isn’t surprising thatmajority of a human body’s mass isoxygen. Carbon, the basic unit of organicmolecules is the second. 99% of themass of the human body is made up ofjust six elements. Oxygen (65%),carbon(18%), hydrogen (10%), Nitrogen(3%), calcium (1.5%), phosphorus(1.0%).

Rusting of MetalsIn class VII we studied rusting of iron

in detail. Recall that iron rusts when it is incontact with air that contains oxygen andmoisture. When it is covered with paint, itcannot come in contact with air, hence itdoes not rust. If paint is gone on iron thenthe rust forms on it.

Fig:9

58 Metals and Non-Metals

The piece of sodium floats on thesurface of the water with a ‘hissing’ sound.This shows that sodium is reactingextremely fast with water. After thereaction is complete, test the solution withlitmus paper.

Repeat the same experiment usingaluminium or iron. You will not see anychange even after five minutes. This isbecause these metals react extremelyslowly with water.

Does non-metals react with water?Generally they do not react with water.

Reaction with Acids

Take the samples given in the table 8in separate test tubes. Add 5ml of dilutehydrochloric acid to each of the test tubeswith the help of a dropper.

Observe the reactions. If you find noreaction, heat the test tube gently. If youstill see no reaction, add 5 drops of conc.Hydrochloric acid. Now bring a burningMatch stick near the mouth of the test tubeand observe what happens. Record yourobservations in the table-8.

Do you find any difference in thesereactions?When do you notice a pop sound with aburning match stick?This sound indicates the presence of

hydrogen.You found that some metals react with

dilute hydrochloric acid liberatinghydrogen but non-metals usually do notreact with acids.

Repeat the same experiments withsulphuric acid and record your observationsin table-8.

Reactivity of metalsYou have already seen that some metals

react with air and others do not. The reactionis fast in some cases like Magnesium andslow in case of silver and copper. Similarly,different metals react with water and acidsunder different conditions. Let us explorethis reactivity further.

Table-8

Iron

Zinc

Copper

Sulphur

Aluminium

Carbon

Magnesium

Iodine

Reaction withdilute

hydrochloricacid

Reactionwith dilutesulphuric

acidSample

Fig:10

59Free distribution by T.S. Government

Take five beakers and label them a, b,c, d and e. Take 50ml of water in each beakerand dissolve a spatulaful of copper sulphatein beakers 'a' and 'b'. Dissolve a spatulafulof zinc sulphate, iron sulphate and zincsulphate in beakers c, d and e . Now put:

Fig:11 a

* Zinc granules in beaker ‘a’* Iron nail in beaker ‘b’* Copper turnings in beaker ’c’* Copper turnings in beaker ’d’* Iron nails in beaker ’e’

Fig:11 bLeave the beakers undisturbed. Record thechanges in the colour of the solutions inthe table-9.

The blue colour of copper sulphatedisappears and a powdery red mass ofcopper is deposited at the bottom of thebeaker ’a’.

Also notice that in beaker ’b’ redcopper is found at the bottom of the beaker

and on the nail leaving light green ironsulphate solution.

What could be the reasons behind thesechanges?In beaker ‘a’ zinc displaces copper from

copper sulphate giving rise to colourlesszinc sulphate. Iron displaces copper fromcopper sulphate in beaker ’b’ leaving lightgreen colour of Iron sulphate.Copper sulphate+Zinc → Zinc sulphate + Copper

Copper sulphate + Iron→Iron sulphate + Copper

Similar to the reactions in beakers ’a’and ‘b’, is there displacement of zinc fromZinc sulphate by Copper in beaker ‘c’ andIron from iron suphate by Copper in beaker‘d’and Zinc from Zinc sulphate by Iron in‘e’?

Do you find any changes in beakers c,dand e?Since we do not see any change in the

other three beakers (c,d and e) we can inferthat.i) Copper is unable to displace zinc from

zinc sulphate.ii) Copper is unable to displace iron from

iron sulphate.iii) Iron is unable to displace zinc from

zinc sulphate.We can understand with the above

reactions that a more reactive metal canreplace a less reactive metal, but a lessreactive one cannot replace a more reactivemetal. That is why there are no displacementreactions in the beakers c, d and e.Some uses of non-metals

You are all aware of the uses of metals.Non-metals are also useful. The three non-metals studied by us are sulphur, carbon andiodine. Let us know about their uses.

a b c d e

a b c d e

Table-9

Beaker 'a'

Beaker 'b'

Beaker 'c'

Beaker 'd'

Beaker 'e'

Solutions Observations

60 Metals and Non-Metals

Sulphur is used in making fireworks,crackers, gun powder, matchsticks andantiseptic ointments. It is found in onions,garlic, eggs, hair and nails.

Activated carbon is used as adecolourising agent and also in waterpurification systems.

Tincture iodine is used in medicalpurposes.

Uses of metalsHave you ever noticed a thin silver foil

decorated on sweets and thin aluminium foilused in inner packing of food materials andtoffees. Aluminium and copper mixture isused in currency coins, medals and statues.Zinc and iron mixture used in making ofiron sheet. Most of the agricultural

instruments are made by iron. Electricalappliances, automobiles, satellites,aeroplanes ,cooking utensils, machinery,decorative materials made by metals dueto their malleability, ductility.

Try thisRecall the names of the some of the

laboratory acids and bases that you know.Write down their names in (table -10) andidentify metal/non-metal present in them,which form oxides when react with oxygen.Take the help of your teacher (table -10).

Have you seen a periodic table?Try to find the metals and non-metals

that you come across in the periodic table.

Key wordsMetals, non-metals, lustrous, sonority, malleability, ductility, good conductors

of heat and electricity, oxides of metals and non-metals, displacement reaction.

The materials which show brightness on surface and reflect the light are called lustrousand which do not shine are non-lustrous material.

The property of materials by which they can be beaten into thin sheets is calledmalleability.

What we have learnt

S.No. Name of the Base Metalpresent in it

Name of the Acid

Non- Metal presentin it

1. Calcium hydroxide Calcium Sulphuric acid Sulphur

Table-10

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The property of drawing material to make fine wires is called ductility.

The ability of materials to produce a particular sound when it is dropped on the hardsurface is termed as sonorous.

Metals often possess all of the following properties. They are lustrous, hard, malleable,ductile, good conductors of heat and electricity and sonorous Ex: copper, magnesium,aluminium, iron, zinc etc.

Most of the metals exist in solid state.

Some metals react with the components of air in different manner with different ratesand in different conditions.

Gold and platinum are the metals which do not react with air.

Metals react with acids and liberate hydrogen gas.

Metals can displace each other according to their reactivity.

Oxides of non-metals are usually acidic in nature.

Oxides of metals are usually basic in nature.

1. Explain the physical properties of metals with suitable examples (AS1)

2. You are given two samples. How do you distinguish which one is metal and which isnon-metal? (AS1)

3. Which metals are used in making jewellery? Why? (AS1)

4. Which substance liberates hydrogen when reacts with metals? (AS1)

5. In a chemical reaction iron is unable to displace zinc from zinc sulphate. Why? (AS1)

6. Why cooking pans don’t have metal handles? (AS1)

7. Sulphur dioxide is------------ (AS1)(a) basic oxide (b) acidic oxide (c) neutral oxide (d) amphoteric oxide

Improve your learning

62 Metals and Non-Metals

8. Match the following: (AS1)

1) Making into thin sheets ( ) (a) ductility

2) Shinning materials ( ) (b) conductivity

3) Making into wires ( ) (c) sonority

4) Transmission of heat ( ) (d) lustrous

5) Making ringing sound ( ) (e) malleability

9. Which gas makes a ‘pop’ sound if exposed to lighted matchstick? (AS1)

10. Why are bells made up of metals instead of wood? (AS1)

11. Imagine the human life without metals, write briefly about the consequences. (AS2)

12. After completion of metals and non metals chapter, Raheem understood that metalsare hard and non metals are soft. During the discussion with his brother he came toknow that Diamond is a hardest material and it is a non metal. Similarly mercury is asoft material and it is a metal. These findings from the discussion raised some questionsin Raheem’s mind. Can you guess those questions? Write them. (AS2)

13. Discuss the acidic and basic nature of the metals and non-metals with suitableexperiments. (AS3)

14. How do you appreciate wide range utility of aluminium right from utensils to spacecraft? (AS6)

15. How is malleability of metals used in our daily life? (AS7)

16. Dumping of waste material made up of metals and non-metals leads to environmentpollution. Do you support the statement? Give your justification with suitable examples.(AS7 )

* * * * * * *

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SOUNDChapter

5

to sounds of objects, animals. Prepare a listof sounds that we hear and the sources fromwhich they might have originated. Writethem in table-1.

Activity-2

Identifying different soundsMake a student to stand at the black

board such that his face is turned towardsthe black board and ask other student in theclass to make different sounds. The studentat the board should tabulate the sounds heheard and sources of those sounds as shownin table-2.

Table 1

Sound heard Source of Sound

Feeble barking Dog from some distance

Bell ringing

Throughout the day we are constantlyexposed to different sounds. For example,peoples talk, birds chirping, cries ofanimals, sounds of autos, motorbikes,buses, lorries, tractors, trains and musicfrom loudspeakers at public places,television. Sound is an unavoidable andintegral part of our lives. We are alwayssurrounded by sound. It is almostomnipresent. Sound plays an importantrole in our lives. It helps us to easilycommunicate with each other.

Make a list of sounds that you get tohear in your surroundings.

How are these sounds produced?How do sounds travel from one placeto another?Are we able to hear all sounds in nature?We will try to seek answers to some

of these and similar questions in thischapter.Production of sound

Activity-1Listening to sound andpredicting its source

Let us sit quietly for a while and listen

64 Sound

Table 2 S.No. Sound heard way of producing Sound

1. Gala Gala A few stones rattling in a metal box2. Eela (whistling sound) A student has produced the sound

from her/his mouth3 Some one striking the table tab with a

scale4 Someone thumping the ground with

shoes56

Is there any change in the soundproduced in the two situations?How do you feel when you touch the

ringing bell? Remove your hand and ringit again. Do you hear a different sound?Why?

Fig-1: Observing vibrations of bell when itis producing sound

How does the student at the black boardguess the source of sound withoutactually seeing the source?You might have observed many other

sources of sound in your daily life. Try tolisten and identify some more sources ofsound and prepare a list.

How do objects produce sound?What happens when objects made ofmetals are hit by a hammer or fall downfrom a height on a concrete floor?How does a flute or a whistle producesound?What would you feel if you touch a bodywhile it is producing sound?

Activity-3Vibrating body produces sound

Take a brass bell (bell used in poojaroom or in your school). Ring the bell andlisten to the sound carefully. Now hold thebell tightly with your hand as shown in fig-1 and ring it again.

Do you hear sound from the bell?

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Let us do following activities1. Fix a rubber band tightly on an empty

matchbox. (See fig-2). Pluck the rubberband and keep it close to your ear.

Fig-2: Listening to sound from match boxtied with rubber band

Do you hear any sound? Do you feelany vibration in your hands?

2. Blow air into papers of yournotebook as shown in fig-3. What happens?Does the action produce any sound? Do youfind any vibrations in the note book?

Fig-3: Blowing air into papers

3. Fill a plate with water and let thewater settle. Strike the rim of the plate witha spoon as shown in fig-4. What do youobserve? What do you hear? Where do youfind vibrations in this case?

Fig-4: Striking the rim of a plate with a spoon

4. Put a hack-saw blade in between atable and a brick as shown in the figure-5and press it and leave it abruptly. Whathappens? Does it produce sound? What isthe state of the hack-saw blade while it isproducing sound?

Fig-5:Sound produced by a vibrating hack-saw blade

What have you observed while doing theabove activities?

What changes took place in thoseobjects?

You must have noticed that objectsvibrate while they are producing sound. Youcan feel these vibrations if you touch theobjects. We are able to observe thevibrations of the plate and papers of thebook. We have also observed vibrations ofthe hack-saw blade. Thus, we can concludethat a vibrating body produces sound.

66 Sound

Fig-6: Sound has energy

But there are certain instruments whichproduce sound though we cannot see anyvibrations in the instruments like in fluteand clarinet etc.

How do they produce sound?Is it possible to produce sound withoutvibrations?Does every vibrating body producesound?Does sound have energy?Let us find out:

Activity-4

Sound has energyTake a plastic bottle and a cell phone.

Cut the top of the bottle so that it lookslike a glass. Play songs on the mobile phonein high volume and place it in the bottle.Close the mouth of bottle with a balloonusing rubber band so that it covers thebottle as shown in the fig-6 and stretch ittightly so as to behave like a diaphragm.Place some sugar crystals or small size ofsand particles on the balloon diaphragmand observe the movement of particles. Dothe same activity after removing the phonefrom the bottle. What do you notice?

Fig-7

When there is no cell phone inside thebottle, sand particles on the diaphragmremain stationary. While the cell phoneplays songs inside the bottle, the diaphragmvibrates which can be seen through dancingof sand particles. The sound produced bycell phone inside the bottle is responsiblefor these vibrations. Thus, sound has energyto make sand particles vibrate on thediaphragm.

Musical instruments You might have observed many

musical instruments like Tabala, Flute,Harmonium and Gitar. The sounds producedby these instruments are distinct. It is easyfor us to identify which sound is comingfrom which instrument.

How do they produce sound?

Why there is a difference between thesounds produced by various musicalinstruments?

Which part of these instruments isresponsible for production of sound?

Let us doList out the names of musical

instruments and mention the vibrating partof each instrument, write them in table-3.

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Table 3

Name of instrument Vibrating part of it

Tabala Membrane, air insidehollow body

Have you named all the vibrating partsfor each musical instrument? For example,in tabala, not only the stretched membranebut the air inside the hollow body alsovibrates.

Can you name the instrument for whichmore than one part is responsible forthe production of sound?

How do you compare the productionof sound in a flute and sound producedin a water tap when it is turned on, justbefore the water flows out of it.

Activity-5

Producing a sound thatresembles sound of rainfall

Start clapping with fore finger on lefthand palm, add the middle finger and clapagain, then ring finger and lastly smallfinger successively and reverse the processgradually. If all the students in your classdo it simultaneously the sounds producedwould resemble the onset and stopping ofrainfall.

Activity-6

Observing the changes in soundTake 4 or 5 metal glass bowls or

tumblers. Fill them with water in decreasingorder. Strike gently each bowl or tumblerwith a spoon. What do you hear? (This is ajalatarang) (see figure-8)

Fill the bowls or tumblers with equalamounts of water, strike each bowl like inabove case and listen to the sound.

What difference do you notice in thesound produced?

Why is there a variation in the soundproduced due to change in the waterlevel of a bowl?

Thus we conclude that sounds areproduced by vibrating bodies and the airthat passes through orfices of theinstruments.

Fig-8 : Jalatarang

68 Sound

Do you know?Bismillah Khan, the most outstanding and world-famous shehnai player, had attained

astonishing mastery over the instrument. He was born in a small village in Bihar about 80years ago. He spent his childhood in the holy city of Varanasi, on the banks of the Ganga,where his uncle was the official shehnai player in the famous Kasi Viswanath temple.

Bismilla Khan Chitti babu

Chitti Babu (October 13, 1936 - February 9, 1996) was a renowned classical musicianfrom India, and arguably one of the greatest Veena artists, in the field of Karnatic Music aspeciality of South India. He became a legend in his own lifetime. His name wassynonymous with the musical instrument Veena, and he was and still is known in theKarnatic Music world, simply as Veena Chitti Babu.

Sounds produced by humanbeings

We know that all animals producesound to communicate with other animals.Human beings use these sounds moreeffectively. Sounds produced in particularorder and manner constitutes our speech.This order in production of sound isdifferent for different languages orcommunication processes.

Honeybee makes sounds on seeingflowers to communicate to the other beeswho are at a distance. Do they produce thispeculiar sound through their mouth or someother organ used for that purpose?

Majority of communication in humanbeings is through speech. Which organsgive human beings the ability to talk?

Try this :

Imitate different sounds made byanimals. Try to mimic your friends. Whilemaking these sounds, place your fingers onyour throat. What do you feel? Do yourfingers sense any vibrations? Are thevibrations same for all the sounds that youmake?

Structrure of larynx or vioce boxLarynx is the important organ in human

body to produce sound.

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Nasopharynx

OropharynxTongue

Epiglottis

Vocal cord

TracheaEsophagus

Larynx

Laryngopharynx

Fig-9: Anatomy of the larynx

Larynx has two muscular ligamentscalled vocal cords. They are stretchedacross voice box; it leads to a narrow slitbetween them, to allow passage of air, toproduce sounds.

Fig- 10 (a) and 10 (b) : Opened vocal cordsand Closed vocal cords

Obseve Fig- 10(a), Vocal cords openduring breathing to allow air into lungs.

Observe Fig- 10(b), Vocal cords closewhen we speak. The air from the lungspresses between them to cause vibrationand produce sound.

Do you know?Length of vocal cords in men is

nearly 20mm, in women it is about5mm less, whereas in children it isshorter. Can we say that the length ofvocal cords plays any role in decidingthe quality of sound produced by men,women and children?

Activity-7Observing the movements ofvocal cords during speech

Ask a friend to raise his neck up.Stretch a chocolate wrapper across hismouth and ask him to blow air on thewrapper forcibly. Observe the changes inmovement at his throat. Ask him to blowagain slowly and observe the difference inmovements.

What changes do you observe in themovements at the throat on the twooccasions?During the first trial the voice box gets

tensed and produces high sound whileduring the second trial it is close to normalposition of throat and produces lowersound. The sound produced in the aboveactivity is due to a combination ofvibrations produced in the wrapper and thevocal cord.

If the sound is a kind of vibration, howdoes this vibration reach us from thesource? How are we able to hear the soundsproduced at a distance?

Propagation of soundSound needs a medium topropagate

The sound produced by the school bellwill be heard by all of us irrespective ofwhether we are in a room opposite to thebell or in a room at the back of the bell.Obviously, the sound produced by theschool bell travels in all directions andreaches us, propagating through the airpresent between source of the sound andour ears. That is the air surrounding us act

70 Sound

as a medium which allows the sound to passthrough it.

Does sound travel only in air?

Does it travel in any other gaseousmedium?

Does sound also travel in other medialike solids and liquids?

Propagation of sound in different mediaLet us try to know how vibration can

propagate through different media!

Do you know?Can we talk without moving of lips?Ventriloquists make sounds or talk with hardly any movement of lips. Lips are slightlyseparated. The various words are spoken quickly and it is difficult for listeners to noticethe difference. They have a very good control over breathing and movement of lips, throatand the muscles of mouth which aids in pronunciation and delivery of speech withoutmuch movement of lips and throat. They let the breath out of the mouth by vibrating theirlips in accordance to the air supply and relax muscles while doing this. This is one of themost effective vocal activities. In Telangana, Chinchapattana Gomatham Srinivas fromWarangal district is a famous ventriloquist. He has performed more than 6000 showsaround the world. He created a sensational world record by performing a 32 hour non-stop mimicry show in 1990.

MimicryThe mimicry artists maintain a very good control over their voices. The magic they createis solely with their voice. They exercise their vocal parts to keep them fit to be able toenthrall the audience through their voice. Dr.Nerella Venu Madhav is a world famousmimicry artist. He belongs to Warangal District of Telangana State. Govt of India honouredhim with Padma Shree in 2001.Try these yourself and you can acquire a hobby.

Nerella VenumadhavGomatham Srinivas

Activity-8Observing sound propagation insolids

Strike one end of the table with a penand ask your friend to listen to the soundproduced keeping her ear touching the tableat other end and also ask her to listen tothe sound by lifting her head slightly fromthe table (fig-11). Ask your friend whatdifference she noticed while hearing thesounds when her ears were away from thetable and touching the table.

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Fig-11: Propagation of sound in solids

Take a metal or wooden strip. Strike itat one end and ask your friend to hear thesound by keeping his ears at the other endof the strip. Ask your friend what differencehe noticed while hearing the sounds whenhis ears are away from the strip andtouching the strip (fig-12).

Do you know how to make a toytelephone using tea cups?

Take two paper-cups. Make small holesat the bottom of these cups. The holesshould be very small so that only a thinstring can pass through them. Take a longstring. Make sure that the string does nothave any knots in it. Push the string throughthe hole in one of the cups. Fix the stringby putting a knot at the end. Similarly fixthe string to the other cup. Our phone isready.

You and your friend can communicatewith this phone now. Stand away from eachother so that the string is tightly held. Oneof you can speak in the cup while the othercan listen by putting the cup on his ear.

Are you able to hear the sound?What is a medium between you andyour friend which is responsible forpropagation of sound? In the above activities you observed

that sound travels in solid medium likewood, metal, thread, etc.

Does sound travel in liquids?Can we hear the sound produced inwater?Let us find out.

Acivity-9Propagation of sound throughliquids

Take a bucket fill it with water. Take twostones and strike them against each otherkeeping your hands inside the water. Askyour friend to listen to the sound by keepinghis / her ears touching walls of the bucket.

Fig-13

Fig-12

Fig-14

72 Sound

Fig-15: Sound does not travel (propagate)through vaccum

When the tumbler is covered with aplate, the volume reduces but you can hearthe ringtone. As you start sucking more andmore air, you can notice that the volumedecreases gradually. If the air is suckedcompletely, you will not hear the sound atall. But practically it is not so easy. Thisactivity gives an idea about the need of amedium for propagation of sound.

We can demonstrate that sound doesnot propagate through vacuum and itrequires a medium, if we use perfectvacuum pumps to create vacuum which isnot easy with the glass tumbler.

How do we hear sound?We hear the sound produced in our

surroundings with the help of our ears. Thestructure of ears play an important role inhearing the sound. Let us peep into our earand try to understand how we hear sound.

Structure and functioning of theeardrum

Our ear consists of three sections, theouter ear, the middle ear and the inner earas shown in the figure-16. Pinna of externalear collects the sound vibrations. Theyenter into the ear canal. We have learnt thatsound travels in the form of vibrations.These vibrations strike the tympanum (ear-

Ask your friend about the differencebetween sounds produced by striking thestones against each other in water andstriking them in air.

Thus the conclusion is that soundpropagates through matter in all the threestates – solid, liquid and gas.

Think and DiscussWhat is the effect of humidity on

quality of sound propagation? Is thereany difference in propagation of soundin air during the summer and winterseasons? Discuss with your friends.

Activity-10Does sound travel if there is nomedium?

Take a glass or plastic tumbler. Makesure that the tumbler is dry. It should belong enough to accommodate a cellphone vertically. Place a cell phone in theglass and play the ring tone of the mobile.Listen to the ringtone and it’s volume level.Cover the glass with a small plate and againlisten to the ringtone and note thedifference in volume of the sound. Nowsuck the air from the glass keeping it closeto your mouth as shown in the figure15. Ifyou suck air quickly the rim of the glasswill stick around your mouth due to airlock. Listen to the volume of the ringtoneat this stage. And also ask your friend tolisten to the sound for comparing variationin its volume. Is there any change in thevolume of sound observed by you or yourfriend?

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drum) and make it to vibrate.

Fig-16: Structure of the ear drum

The vibrations from the tympanicmembrane reach the middle ear (earossicles), contains three small bonesmalleus (hammer shaped), incus (anvilshaped) and stapes (stirrup shaped). Theymagnify the sound vibrations. The stapestransmits the vibrations to the membraneof oval window. The oval window has thesurface area 1/20th of the ear-drum. Bythis the vibrations increase 30 to 60 times.The vibrations from the oval windowtransmit to the cochlea which is the innerpart of the ear. The cochlea is filled withthick fluids which transmits the vibrations.The motion of the vibrations in the cochleais detected by tiny hairs connected tonerves at this point. The vibrations aretransformed into electrical signals andcarried by the nerves to the brain where thesensation of the sound is realized. Thesound vibrations can also reach the innerear by travelling directly through the bonesof the skull, as you tap your head.

Are there any harmful sounds that wehear? What happens when we hear suchsounds?

Charecteristics of soundWe learnt how sound is produced and

what a vibration is. Now, let us look atdifferent characteristics of sound.

Loudness, feebleness andamplitude

Sometimes people talk loudly andsometimes softly. If we hit a table hard weget a loud sound. If we tap the same tablegently, we hear a feeble sound.

In our daily life we hear many loudsounds and feeble sounds on differentoccasions. For example, sounds emanatingfrom drums during the prayer of assemblyin the school are very loud. But the soundproduced while we are taking pledge in theassembly is a feeble sound. We know thatdiwali is a festival of sounds and light.When we burst crackers we get many loudsounds.

Why are some sounds loud?

Why are some sounds feeble?

Is there any relation between theintensity of sound and vibrations of thebody which produces sound?

Think and Discuss

"Vibrations produce sound and soundproduces vibrations". Which is true inthis? Discuss.

"Our ear has the three media throughwhich sound propagates." Discuss withyour friends as to whether the abovestatement is true.

ear drum

cochlea

outer ear (pinne)auditory canals

middle bone

74 Sound

Aim: To know the relation between theintensity of sound produced by a body andthe vibrations of the body.

Material required: Wooden table, 30cm metal scale or nearly 30 cm hack-sawblade and a brick.

Procedure :

1. Place the blade/scale on the table,with 10cm of the blade on the surface ofthe table and rest of it in air. Keep a heavybrick on one end of the 10 cm blade/scalekept on table (fig-17).

2. Vibrate the blade gently and observethe vibrations and simultaneously listen tothe sounds. Repeat the same 2-3 times andrecord your observations in the table-4.

3. Vibrate the blade using greaterforce. Observe the vibrations and listen tothe sound. Repeat this for 2-3 times andrecord your observations in the table-4.

Fig-17:vibrations of the body andintensity of sound.

Lab Activity -1 Table 4

Force Vibrations of Intensity of the blade / scale sound

Small

Large

When do you hear a loud sound?When do you hear a feeble sound?What difference do you notice invibrations of blade / scale during loudand feeble sounds?The initial position of the scale at rest

along the surface of the table, is calledmean position.

As shown in the figure 18(scale) OA isthe mean position of the vibrating body. OBand OC are the vibrations occurring in thebody.

Fig-18

The to and fro motion of a body fromits mean position is known as onevibration.

The body vibrates from the meanposition OA to OC and comes back to OAand then moves from OA to OB and comesback. The maximum displacement ofvibrating body from its mean position iscalled amplitude. In the picture givenabove the maximum displacement is A toB or A to C.

O

B

A

C

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What difference do you find in theamplitude of vibration for a feeble anda loud sound in the above experiment?

Do you Know?Decibel is the unit for measuring

the intensity of sound. It is denoted asdB.

The unit expressed as decibel in thename of Alexander Graham Bell(1847 – 1942), whose research insound is famous.

The smallest audible sound (nearerto total silence) is 0 dB. A sound 10times more powerful than this is 10 dB.A sound 100 times more powerful thanthat of total silence is 20 dB. A sound1,000 times more powerful than thesound nearer to total silence is 30 dB.Some common sounds and their decibelratings are given below.

Near total silence - 0 dBA whisper - 15 dBNormal conversation - 60 dBA lawnmower - 90 dBA car horn - 110 dBA jet engine - 120 dBA gunshot or firecracker - 140 dB

Problems: From the above data,compute the following.

How many times is a car horn morepowerful than normal conversation?

How many times is a jet enginemore powerful than a whispering sound?

Lab Activity-2

Aim: Identifying pitch or shrillness ofa sound.

Material required: A wooden table,two hack-saw blades or metal scales of30cm length and a brick.Procedure:1. Place the first blade/scale on the table,

with 10cm portion of the blade on thetable and rest of it in air. Keep a brickas weight on the 10 cm portion of theblade/scale kept on the table.

2. Place the second blade/scale on the table(see that the gap between these twoblades is 10cms), with 25cm on thetable and 5cm in air. Keep a brick asweight on the scale/blade (fig-19).

3. Vibrate both blades with same force.Observe the vibrations and listen to thesounds produced.

Fig-19: Vibrations of hack - saw blades

Repeat the same 2-3 times and recordobservations in the table-5.

Table 5

Blade length Vibrations Soundin Air

Blade 1 : 20cmBlade 2 : 5cm

76 Sound

What difference do you notice innumber of vibrations of two blades?

What difference do you notice in thequality of sound produced by them?

You would notice that the number ofvibrations produced are less in the 20cmlong blade when compared to the vibrationsof the 5cm long blade. The sound producedby the 5cm blade is more shrill whencompared to that of the 20cm blade.

The shrillness of a sound is known aspitch.

The number of vibrations per second(vib/sec) is called frequency.

The pitch of the sound depends uponits frequency.

In the above experiment number ofvibrations per second in 20cm blade is less.It denotes that it has low frequency.Whereas the number of vibrations persecond are more in 5cm blade so it has highfrequency.

Thus the sound produced by a shortblade (5cm) has high pitch and soundproduced by long blade (20cm) has lowpitch.

Conduct the above experiment withdifferent lengths of the blades or metalscales and note your observations.

A bird makes high pitch sound and a lionmakes low pitch roar. Which one producesthe sound of more frequency?

Give some more examples of naturalsounds that you come across in your dailylife and differentiate them as low pitchsounds and high pitch sounds.

Do you know?Pitch of the voices of the following

is in ascending order.Lion<adult man<adult women<child

<infant<insectCan you guess the reason why?Do you find any difference in blowingthe whistle and striking the drum?

Normal sound consists of mixedfrequencies

In our daily activities, we hear manydifferent sounds. But we generaly do notconcentrate on every sound that we hear.All these different sounds are producedwith different frequencies and withdifferent amplitudes. For example, thedifferent sounds produced in a classroombefore the commencement of the class. Ifwe keenly observe these sounds, we canfind that no two persons produce the samesound of equal pitch and amplitude.

Why do we produce sounds withdifferent pitches or amplitudes whilespeaking? The variation in pitch and amplitude of

sound during our speech helps us tocommunicate with others in the form oflanguage.

Primitive man had no developedlanguage, but he used to communicate withsigns and some sounds which had nowritten equivalent. Later in the process ofevolution these sounds became a source ofa meaningful communication andeventually were converted into written

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forms. This led to the development of thefull form of language which we all use nowto communicate. Not only human beingsbut animals too have a mechanism ofcommunication by producing sounds withdifferent frequencies and amplitudesaccording to their need.

The parts of the speech organ which areinvolved in producing sounds are

Vocal cordsLipsTeeth & tongueNose & throat Naturally, the words that we utter do

not have a single sound but are acombination of sounds with differentfrequencies and amplitudes. The soundproduced for each letter possess aparticular frequency. Thus, the word is acombination of different letters ofdifferent frequencies i.e., a word is soundof mixed frequencies. Sometimes thesame word is uttered in different ways toexpress different emotions. For example,the word ‘NO’ is uttered in different pitchesto express negativity, anger andfrustration.

Noise and musicHow do you feel when you hear soundsin busy traffic?How do you feel while listening tosongs from a radio?Which of the above sounds are morepleasant to hear?We enjoy the sounds in a music

concert. They are pleasant to hear. But there

are some sounds which are unbearable tohear like the sounds produced when a steelplate or utensil is dropped on the floor.

The sounds which are pleasant to hearare called music. The sounds which are notpleasant to hear are called noise. Noise isan irregular combination of sounds whichare ‘unpleasant’ to hear. Music is acombination of sounds that are producedin an order and pleasant to hear.

Give some more examples for pleasantand unpleasant sounds.

Audible rangeOne of our sense organs, ear, enable us

to hear a number of sounds. Are we able tohear all sounds produced in oursurroundings?

Do we hear the sounds produced bybats?The sounds that a normal human being can

hear are called audible sounds. The soundsthat a normal human being cannot hear arecalled inaudible sounds. Frequency of theaudible sound ranges from 20vibrations/sec-20000vibrations/second. Frequency ofinaudible sounds are less than 20vibrations/sec or greater than 20000vibrations/sec.

Sound pollutionSound pollution is a serious problem

like air, water pollution. It is harmful tohuman beings. We express the loudness ofthe sound in decibels (dB), which we havealready learnt in this chapter. The soundproduced in our normal conversation isabout 60dB. If the loudness exceeds 80dB,

78 Sound

the sound becomes physically painful. If aperson is being exposed to the sound of80dB continuously it may lead to hearingproblems.

Let us observe the sounds that areresponsible for sound pollution.

In our surroundings there are manysounds causing sound pollution, like soundsof traffic, their horns, sounds inconstruction sites, sounds at industries,sounds at mines, sounds during explosionsand bursting of crackers, etc.

The unwanted sound in oursurroundings leads to sound pollution.There are some more sources of soundpollution in our homes like mixer/ grinder,washing machines and motors etc.

Write some more sources of soundpollution in your surrounding.

Effects of sound pollutionWhat are the harmful effects of sound

pollution?The first harmful effect is loss of

hearing. It also leads to several healthrelated problems. eg: sleeplessness, hypertension, increase in blood pressure, etc…,

Discuss and list some more effects ofsound pollution.

Measures to control soundpollution

We cannot stop production of soundbut we can reduce sound pollution by somemeasures.

Let us list the steps which can be takento reduce sound pollution:

Attach silencers to bikes and othermachines to reduce sounds.Manufacture machines that work withless noise

Do you know?M.S. Subbulakshmi was famous for

her melodious music. It would bedifficult to overstate the talent and theimpact made by Smt. M.S.

Subbulakshmi,not just in thefield of Karnaticmusic, but also asa philanthropistand a person who

During the use of TVs and musicplayers tone down volume of sound.Plant trees to reduce sound pollution.Discuss with your friends about some

other measures to limit sound pollution andtabulate them.

placed her life at the service to thecountry and people. She rendered hervoice to devotional songs.

Ghantasala Venkateswar Rao wasfamous playback singer. He wasfamous for his melodious voice. He

sang more than10,000 songs inTelugu, Tamil,Kannada, andMalayalam andworked as a musicdirector for over

100 films. His private songs wereequally popular and his devotionalsongs are popular even today.

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Key wordsVibration, vocal cords, medium, vacuum, ear drum, loudness, feeble,

amplitude, decibel, pitch (shrillness), frequency, noise, music.

Vibrating body produces sound.Human beings are able to produce sound with the help of vocal cords.Sounds travel through solids, liquids, and gases. It cannot travel through vacuum.The vibration of the ear drum caused by the sound produced by a vibrating body gives ussense of hearing.Loudness and feebleness of a sound are determined by amplitude of vibration.The intensity of sound is measured in dB (decibels)Pitch or shrillness is determined by the frequency.The number of vibrations per second is called frequency.Normal sounds consist of mixed frequencies. The hearing limit of sounds by human beings is called audible range. Sounds pleasant to listen are called music and unpleasant to listen are called noise.

1. Fill in the blanks with suitable words. (AS1)a) The to and fro motion of a body from it's mean position is called …………………..b) Number of vibrations per second is called …………………..c) The intensity of sound can be measured in ………………….d) Sound cannot travel in ………………….e) Vibrating bodies produce ……………………f) The maximum displacement of a body from its mean position is called …………..

2. A normal humanbeing can listen to sounds with frequency from ____________to_____________ vibrations per sec. (AS1)

3. How will you differentiate the amplitude and frequency of different sounds? Give twosuitable examples from your daily life. (AS1 )

4. Write any three musical instruments that you know and explain how they produce sound.(AS1)

What we have learnt?

Improve your learning

80 Sound

5. The sounds of crickets (insects) make us close our ears. Why? (AS1)6. Robert observed a musical instrument producing sound. But he didn’t find any vibration

of any part of that instrument. This observation raised many questions in his mind. Canyou guess what are the questions raised in his mind? Write them. (AS2)

7. “Vibrations in a body produce sounds”. How do you prove it? (AS3)8. Can parrots speak? Discuss with your friends and collect information. (AS4)9. Collect the photographs of local musicians and exhibit them in your class. (AS4)10. Collect photographs showing various situations of sound pollution and prepare a scrap

book. (AS4)11. Zakir said “vibrations produce sound. And sound produces vibrations. This is how we

hear every sound”. Establish that the given statement is true with relevant examplesfrom your surroundings. (AS4)

12. Make different musical instruments using local available materials and exhibit them inyour class. (AS5)

13. Explain why we are not able to hear the explosions taking place in the sun. (AS7)14. Write any two slogans to reduce sound pollution.15. Write your suggestions about reducing sound pollution. (AS7)16. How does sound pollution effect Bio diversity? Explain. (AS7)

Do you know?

Golconda Fort - HyderabadIt is a famous fort in India. It is famous for many engineering and architectural

marvels. If you clap your hands at a particular point under the dome it reverberatesand can be heard at the highest point of the fort which is about 1km away.

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Articles/ Activities (A) Articles/ Materials used30-40 years ago (B)

Articles/ Materials usedToday (C)

COAL AND PETROLEUMChapter

6

Deekshita is struggling to take down apickle jar and the jar which at the top mostsection of a shelf.

Grandfather: Be careful, it may falldown and break.

Deekshita: No, I am taking down aplastic jar. It won’t break.

Deekshita realizes that Grandfather wasthinking of the clay jars that were used inolden days. Now a days plastic jars are usedand they are unbreakable.

What else has changed, she wondered?Help Deekshita to find out.

Activity-1

Identifying articles and materials used for various purposesHave a look at table 1. Column (A) gives the names of some activities and items. Ask

your grandparents or other older people about the names of the materials which is used forthe items given in column (A), and write them in column B. Then in column C, write thenames of materials being used at present. Few examples are given to guide you.

Table-1

Containers for storing pickle clay jarsPacking food for a journey plastic tiffin boxesWater pipes in the houseHair combsVessels for cooking food Copper vessels Steel vesselsFuels used to cook foodFuel used in a train engine CoalLuggage for carrying clothes Metal trunksWater buckets, pailsWater storages in houses Plastic bucketsConstruction materialJewelleryFurniture (chairs, cots)

82 Coal and Petroleum

From activity 1, you have a long list ofmaterials. Some of them like wood, metals(iron, silver, gold etc) were used ten years,fifty years and even hundred years ago. Doyour grandparents remember using plasticwhen they were young?

Now think and write the answers:

• How many of these materials wereavailable 10 years ago?

• How many of these materials wereavailable 50 years ago?

• How many of these materials wereavailable 100 years ago?

In case you have any doubt, you cantake the help of your social teacher and yourelders too.

you find that some materials such aswood and gold that are used today, were alsoused even thousands of years ago. Butothers like plastics are of recent origin.

The progress in science and technologygives us new products every day. The branchof science which deals with materials iscalled material science.

Sources of materialsWe know that we get wood from trees.

Do you know that we get metals like iron,copper from their ores? Which material isused to make plastic or glass?

Do you remember reading aboutpetrochemicals in the chapter on syntheticfibres and plastics. How do we obtainvarious materials?

Table 2 gives some answers.

WindMillsWind is an important natural

resource. Sail boats and sailing shipshave been using wind power sincethousands of years. Wind mills wereused to grind corn and to pump seawater to make salt.

Do you know?

Table 2

Material How is it obtained?Glass By melting sand with other materials and then, cooling it rapidly

Clay By mixing the mineral kaolinite with water

Wood From dried up trees

Plastics From petrochemicals

Metals From their respective ores

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After looking at table 2, you find thatthe materials used earlier were obtainedfrom soil (clay, sand), water, ores, etc. Soil,water and air were not only sources ofmaterials used earlier but also sources ofenergy. Now, plastic has replaced manymaterials used earlier. Petrochemicals areused for making plastics. Many materialsthat we use for different purposes todaycome from various sources present innature. Therefore, soil, water, air, petrol etc.are called natural resources.Exhaustible and InexhaustibleResources

Now, we have to think about fewquestions to understand about need andimportance of natural resources.• Will the above resources be available

for ever?• Won't we ever run out of air?• Won't we ever run out of water?• Can these resources be exhausted by

human activities?• Do we have unlimited supply of coal

and petroleum?Some energy sources like air and water

are not exhaustible on usage. Hence theseare called Inexhaustible resources coal andpetroleum etc, are the energy sourceswhich are exhaustible on usage. If thissituation continues, in future we might haveto depend on solar energy.

Is wood an inexhaustible resource?Think about using wood for makingfurniture.• What will happen if all the forests are

cut down and the wood is used forvarious purposes?

• How long do you think it would taketo grow the forests again?

There are many resources like treeswhich will be depleted if we don’t makejudicious use of them.

• How long the fossil fuel like petroleumwill be available? Is it inexhaustible?We need petroleum not just as a fuel

but also for several other uses as we willsee later in this chapter.

Resources therefore can be classifiedas inexhaustible and exhaustible dependingon whether they are expected to last foreveror not.

Activity-2List out the natural resources which are

limited and which are abundant and recordthem in table-3.

Table -3Resources Abundant .............................

Resources Limited ............................

Is petroleum exhaustible?

If we see the history of petroleumproduction, from 1859 to 1969, the totalproduction of oil was 227 billion barrels.(In the oil industry the barrel is the unitfor measuring petroleum volume andone barrel equals 159 litres). 50 percentof this total was produced during the first100 years (1859-1959), while the next50 percent was extracted in just ten years(1959-1969). Today our consumptionrate of oil is far excess than that of therate of its formation. Earth takes morethan one thousand years to form the oilthat we consume in one day. By about2015, we would have consumed half ofthe total reserves of the oil. It wouldbecome more and more difficult toextract oil in future.

84 Coal and Petroleum

Fig-1

• What will happen if fossil fuel like coaland petroleum are completelyexhausted?

• What would be our future energyresources?The entire Research and Development

(R&D) in the field of sources of energyshows that at the present rate of use of theconventional energy sources like fossil

fuels will not last for long. Presently only10% of non-conventional energy resourceslike solar energy, wind energy, tidal energy,etc, are used. These are inexhaustibleresources and abundantly available innature.• Are the fossil fuel resources available

plenty to meet the future energy needs?• What actions required to meet the

future energy needs?

Coal, petroleum and natural gas as fuelsEarlier people used kiln (poyyi) for cooking food but

now people use gas stoves or kerosene stoves. Earlier foodwas cooked using wood as fuel, then coal now keroseneand LPG are being used.

Fuel is needed not only for cooking but also fortransport. Different vehicles (train, bus, cars, two-wheelers)are used for travelling long distances and they use differentfuels. People also travel by ships. What fuel is used there?Fuel is also required for electricity generation.

Bio -diesel an alternative fuel sourceBio-fuels are one of the major

non-conventional energy resources.They are non-toxic and renewable.Bio-diesel is one of the bio-fuelwhich is an alternative or additivefuel source to the standard dieselfuel which is used now. It is madefrom the biological ingredientsinstead of petroleum or crude oil.

Bio-diesel usually made from the plant oils or animal fat through a series of chemicalreactions. It is safe and can be used in diesel engines.(But bio-diesel requires morecultivatable land which may effect shortage of food production in future.)

Do you know?

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Do you know?

Earlier you read that wind power wasused in wind mills. Then the steam enginewhich used coal was invented during theindustrial revolution.

During this period steam engine wasused to power everything from cloth loomsto vehicles on land and water.

Coal was the most important fuel in the19th Century. It is an exhaustible resourcebut we can meet our needs for another 250-300 years at the current rate of use. Till1950, coal accounted for half of theelectricity generation in the world.

Coal was replaced by petroleum withthe invention of more efficient engines indifferent vehicles. Now, coal is mostlyused to produce electricity in thermalpower plants.

Coal is an ancient gift servingmodern society.

Coal has been known and used forthousands of years. It is one of our earliestsource of heat and light. But it becamea fuel of importance only after theindustrial revolution which led to anincrease in demand as fuel.

Do you know that the coal obtainedfrom the fire wood and the coal used invehicles and factories are different? Coalused in factories is mined from the earth’scrust. The coal obtained from the fire woodis usually charcoal.

Petrol and diesel which are used invehicles today are obtained from mineralcalled petroleum. Petroleum has beenknown almost since prehistoric times.More than 4000 years ago, asphalt(Petroleum Product) was used in theconstruction of walls and towers ofBabylon. There are also records fromancient China about shallow wells beingdug to get petroleum. But what did ourancestors do with this petroleum? Theymainly used for making their woodenboats waterproof, as a fuel for lamps and in some traditional remedies. We realized theimportance of petroleum only after science and technology progressed to give uspetrochemicals and petrol to run engines.

Drilling Rig

Earth's crust

Natural gas

Petroleum

Water

Imperviousrock

Natural gas is another importantfuel

Natural gas is found trapped betweenimpervious rocks, sometimes along with

petroleum and sometimes withoutpetroleum. Previously, this gas was allowedto escape or even burnt while pumping outthe petroleum.

86 Coal and Petroleum

(Below 400 c) Fuel gas LPG Fertilizer Syntheticrubber

To makechemicals

Shoepolish

cleaningsolution

petrole dry cleaning fluid Heating torch

Kerosine oil Wick stove Lantern

Diesel Oil Diesel generator Diese ling inewater

Lubricating Oil Lubricating oilgenerator

Fuel Oil IndustrialBoiler

Paints Makingroads

Asphalt

Paraffinwax

Ointment Facecream

Grease MatchBox

candle washpaper

vaseline

Residue

6000c

4000c

1700c

700c

1200c

2700c

and natural gas are going on under the supervision of ONGC (Oil and Natural GasCorporation ) throughout India. In India gasfields have been discovered in Tripura ,Mumbai High, Krishna, Godavari delta andJaisalmer.

Now, the natural gas is treated as equallyprecious because it is safer for theenvironment. Natural gas is stored underhigh pressure as compressed natural gas(CNG).

The exploration for more petroleum

Other uses of coal, petroleum and natural gasMost of us think petroleum is a source of fuel. But advances in our understanding of

various chemical processes has led to the use of both coal and petroleum as the startingmaterials for a wide variety of products.

Petroleum is a complex mixture. It is separated into various components by a separationtechnique known as fractional distillation

Look at the figure-2. We cansee the various productswhich are now being obtainedfrom petroleum. Initially, theseparation techniquesavailable were not advancedand could separate only a fewcomponents from thepetroleum mixture. One ofthe first fractions to beseparated from petroleum

Fig-2 : Uses of coal andpetroleum

was kerosenewhich wasfound to bebetter fuel thanthe petroleum.Now, we canseparate many

more components. Fuel gas, petrol, dieseletc. are all obtained from petroleummixture. These components of petroleumare then used to obtain other products.

Refining of Petroleum

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Activity-3

Identifying various uses of petroleum.Look at the figure-2 and find the other uses of petroleum and its products. Fill in the table.

Table -4Name of thepetroleum product Uses

PetrolFuel Oil

Kerosene

Diesel OilParaffin Wax

Coal:Coal is not as versatile as petroleum but

it is also very useful. Look at the figure-3.

We can see that coal gives us coke, coaltar and coal gas. Each of these componentshas several uses.

When coal is heated in air, it burns andproduces mainly carbon dioxide gas.

Coal is processed in industry to getsome useful products such as coke, coaltar and coal gas.

CokeIt is a tough, porous and black

substance. It is an almost pure form ofcarbon. Coke is used in the manufacture ofsteel and in the extraction of many metals.

Manufactureof steel

Extractionin metals

COKE

COAL

Roofing Materials

PerfumesNepthalene In Roads

Synthetic dyes

Pesticides

Medicines

ExplosivesPaints

Photographic Metals

Syenthetic Fibres

Fuel

Lighting

COALGAS

COALTAR

Fig-3: uses of coal and its products

88 Coal and Petroleum

Aim: To show that when we heat highquality coal (carbon content is more) a gasevolves which can burn.

Material required: Two boilingtubes, Rubber corks, iron stands, deliverytube, jet tube, Bunsen burner.

Procedure: Take a spoon of powderedcoal into a hard boiling tube and fix it to astand as shown in the figure. Close the testtube with a rubber cork and connect it toanother boiling tube which is partially filledwith water and has fixed to other stand, withthe help of ‘U’ shaped delivery tube asshown in the figure. Arrange a jet tube tothe cork of second boiling tube. Heat theboiling tube containing coal strongly withthe help of Bunsen burner.

Coalpowder

Boilingtubes

Delivery tube

water

Bunsen burner

Jettube

Stand

Fig-4: Burning the coal

Lab Activity

Table-5

Coke Coal Tar Coal Gas

Coal tarIt is a black coloured thick liquid with

an unpleasant smell. It is a mixture of about200 substances. Products obtained from thecoal tar are used as starting material formanufacture of various substances likeSynthetic dyes, drugs, explosives,perfumes, plastics, paints, and roofingmaterials etc. Interestingly naphthaleneballs used to repel moths and other insectsare also obtained from coal tar.Coal gas

Coal gas is obtained during the

processing of coal to get coke. It is usedas a fuel in many industries situated nearthe coal processing unit.

Activity-4

Identifying uses of coalproducts.

Observe the figure-3 and list out theuses of coal products in the following table5.You can collect more information bydiscussion with elders and with yourfriends.

89Free distribution by T.S. Government

Do you know?

• What do you observe?You may notice that the brownish black

vapours evolved in the first boiling tube.These vapours are passed through water

in the second boiling tube. Colourless gasbubbles evolve from the water. If you burnthe gas that is evolving through the jet tube,you can witness a bright flame at the nozzle.

Repeat the above experiment by usinglime water, soap water instead of water usedin second boiling tube.• What do you observe? Give reasons for

the changes you observed in twoexperiments.

• What inference can you draw fromthese two experiments?

Natural gas and petrochemicalsNatural gas is not just an important

domestic and industrial fuel but also usedin the manufacture of fertilizers.

Some petrochemical productsPetrochemicals are used in preparation of various articles as substitutes for materials

like wood and metals which are limited. They are also used to manufacture several newproducts that cannot be obtained from wood, soil, metals etc. The table below shows howpetrochemicals are now used in all walks of life.

Look at the table given above. Think ofall materials which you use in a day. Canyou imagine spending a day without usingpetrochemical products? List how oftenyou use products obtained frompetrochemicals. Can you imagine life 100

years ago when these are not available?We use so many products now, which

we did not have 100 or even 50 years ago.Increased consumption has lead toincreased production of waste materialwhich created disposal problems. For

Medical equipments,apparels like clothes,bedding, socks, furniture,paints, washing liquids, fibre,cosmetics, medicines,polishing liquids etc.

Cars, motor boats,communication devices,construction materials,paper industry, belt andstraps, tyres.

Plastic tubes,baskets, storing box,cultivation implements,fertilizers

Agricultural Sector Industrial sector Domestic and other sectors

Table - 6

Petrochemicals: The usefulsubstances which are obtained frompetroleum and natural gas are called petrochemicals. These are used in themanufacture of detergents, syntheticfibers (Polyester, Nylon , Acrylicpolythene etc). (Due to its greatcommercial importance petroleum isalso called as liquid gold).

90 Coal and Petroleum

example, for the last 30 years the use ofmaterials obtained from coal and petroleumto make a wide variety of cheap plastics hasrevolutionised the manner in whichhundreds of materials are manufactured.These materials also created a majorpollution to the environment.

••••• How are coal and petroleum formed?

To understand this, explore theprocesses by which coal and petroleum areformed.

Formation of coalThe plants in large and dense forests in

low lying wetlands got burried under thesoil due to the natural processes like floodsand earthquakes. As more soil depositedover them, the organic matter wascompressed. The temperature also rose. Asthe organic matter sank deeper and deeper,under high pressure and high temperature,these dead plants slowly converted to coal.As coal contains mainly carbon, the slowprocess of conversion of dead vegetationinto coal is called corbonisation. Sincecoal was formed from the remains ofvegetation, it is also called fossil fuel.

Formation of petroleumPetroleum was formed from the

remains of tiny organisms called planktonthat were found in the bottom of seas andoceans. Plankton have tiny droplets of oilinside their bodies.

As these organisms died, their bodiessettled at the bottom of the see or oceanand covered with layers of sand and clay.Over millions of years, due to absence of

air, high temperature and high pressurethese dead organisms transformed intopetroleum and natural gas.

Like coal and petroleum natural gaswere also formed from the dead remains(fossils) of living organisms, they too areknown as fossil fuels.

Why are coal and petroleum soversatile?

Coal consists of mainly carbon whilepetroleum consists of a mixture ofcompounds called hydrocarbons (theycontain mainly hydrogen and carbon).These compounds make good startingmaterials for other compounds based oncarbon. Carbon is very versatile and is thebasis for most of the materials. Therefore,coal and petroleum too are very importantstarting materials for synthesising varietyof useful compounds.

Do you know?

Alternatives to natural gasThe gas resources which are not

conventional like natural gas are knownas Non-conventional gas resources . Ourcountry has enormous non-conventionalgas resources like coal bed methane andgas hydrates. These are not in commercialproduction phase due to the lack inproper technology. In future, when the oilera is expected to end, the only way tomeet the energy demands will be byproducing this non-conventional gas.

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Conserving coal and petroleumWhy does the price of petrol go up all

the time? In simple terms, the price ofanything depends on how much of it isavailable and how essential it is. Both coaland petroleum are exhaustible resources,but we need them, both as fuel and asstarting materials for synthesising newcompounds. Since supplies are limited theyare becoming more expensive as thedemand for them increases. We have toconserve these resources as much aspossible, and also look for alternatives forthese resources.

Let us look at the issue of conservation.We can reduce the consumption of bothcoal and petroleum by either opting for adifferent model of development which doesnot depend on these resources or reducethe wastage of these resources. Since thefirst option is right now impracticable,second option of reducing wastage is to bepracticed. The governments of manycountries are working on this problem offinding alternative technologies to reducethe use of fossil fuels.Misuse of energy resources andconsequences

In our daily life while doing various dayto day activities we use many types ofenergy resources and fuels without givinga thought about the exhaustibility of theseresources and consequences. For examplein urban areas dryers are used in washingmachines which consumes electric energy,eventhough abundant source of heat energyin the form of sunlight is available aroundus. Similarly motor bikes are used even

for shorter distances. Walking shorterdistances or using bicycles saves not onlyfuel but also keeps good health.• Can you give some more examples

where energy or fuel resources aremisused in our daily life?

• Can you suggest alternate ways to savethe fuel resources?

• How is biodiversity effected byexcessive use of fossil fuels?

Group discussion on misuse offuel resources and itsconsequences.

Discuss in small groups how fuelresources are being misused in our dailylife while doing various activities like...

a) transportation,b) cookingc) industrial use

• What are the consequences of misuseof fuels? Prepare a report.

• What methods could you suggest toprevent the misuse of fuels?

Fig-5

Activity-5

92 Coal and Petroleum

Harmful effects caused during use offuels

Most of the harmful effects are due tocarelessness of handling petroleumproducts. For example, crude oil andrefined fuel spills into sea from tanker shipby accident causes damage to natural ecosystem and kill sea birds, mammals, shellfish and other organisms.

Utilisation of these fuels leads to somemore harmful effects. Let us see that• Burning fuels releases carbon dioxide,

a green house gas, which causes climatechanges and leads to global warming.

• Coal fired power plants emits mercury,selenium, arsenic, lead in addition togreen house gases which are harmfulto human health and environment.Many paints made from petroleum and

heavy metals release toxic products intoair. These toxic products cause a variety ofhealth problems including heart, lungsdamage, nausea and dizziness.

Key words

Natural resources, exhaustible resources, inexhaustible resources, petroleum,fractional distillation, natural gas, CNG, coke, coal gas, coal tar, carbonisation,plankton, fossil fuels, petrochemicals.

• Natural resources can be classified into Exhaustible and inexhaustible resources.

• Fossil fuels are formed from the dead remains of living organism under the earth overmillions of years.

• Coal, petroleum and natural gas are fossil fuels.

• Coke, coal tar and coal gas are the products of coal.

• Petroleum is formed at the bottom from the remains of tiny organism called plankton.

• Petroleum gas, Petrol, Diesel, Kerosene, paraffin wax, lubricating oil are obtained byrefining of petroleum.

• The natural gas is found sometimes along with petroleum and sometimes withoutpetroleum.

• The useful substances which are obtained from petroleum and natural gas are calledpetrochemicals.

What we have learnt

93Free distribution by T.S. Government

• Excessive use of fossil fuels causes air pollution, greenhouse effect, global warmingand many health problems.

• Fossil fuel resources are very limited. We should think for the alternatives.

• Advances in science and technology have changed our lives.

1. Complete the given analogy (AS1)

i. coal : exhaustible : : ________ : inexhaustible

ii. coal tar : _______ : : coke : manufacture of steel

iii. petrochemical : plastic : : CNG : __________

iv. carbon dioxide : global warming : : ________ : nausea

2. Match the following (AS1)

i. natural resource [ ] a. carbonisation

ii. coal [ ] b. plastic chair

iii.petrochemical product [ ] c. krishna, godavari delta

iv. natural gas [ ] d. Plankton

v. petroleum [ ] e. water.

3. Tick (√ √ √ √ √ ) the only correct choice among the following. (AS1)

i. Which one of the following is less polluting fuel?

a. Natural gas b. coal c. kerosene d. petrol

ii. The main constituent of coal is

a. Carbon. b. oxygen c. air d. water

iii. Which one of the following material is used for making shoe polish?

a. Paraffin wax b. petrol. c. diesel d. lubricating oil

Improve your learning

94 Coal and Petroleum

4. Fill in the blanks. (AS1)

i. ___________ was used in manufacture of steel.

ii. The biproduct of coal used in synthetic dyes and paints is ____________.

iii. Coal was formed in areas where large amounts of ______________was buried underthe earth.

iv. __________gas which causes climate change and global warming.

5. Name the petroleum products used for roads surfacing. (AS1)

6. Explain the process of formation of petrolleum in Earth? (AS1)

7. Project Work (AS4)Compare a CNG run vehicle with that of a diesel run vehicle. What difference do you

notice in both cases with respect to pollutants released, level of pollution and cost of fuel.Prepare a report on your findings. You can take the help of a driver to make a report on yourobservation.

Name of the fuel Cost of the fuel as on today Pollutants liberated

Diesel/Petrol.

CNG

8. Project Work (AS4)Choose five families of your neighbourhood, collect the information about the measuresthat they adopt to conserve energy resources in transport and cooking.

Name of Number of No of vehicles Amount spent for Amount spent formembers in family using Fuel in one month Cooking purpose

Make a report on your observation

the Head ofthe family

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9. The following Table shows the total power shortage percentage in India from 1991-1997. Show the data in the form of a Bar graph taking shortage percentage for the yearson the Y-axis and the years on the X-axis. (AS4)

S.No. Year Shortage (%)

1 1991 7.9

2 1992 7.8

3 1993 8.3

4 1994 7.4

5 1995 7.1

6 1996 9.2

7 1997 11.5

a) Is the shortage percentage of power increasing or decreasing?b) If shortage percentage of power increases year by year, how will it affect on human

life? Explain.

10. Draw a flow chart showing Exhaustible and Inexhaustible resources and their use. (AS5)

11. Collect information about places were we get coal, petroleum and natural gas in AndhraPradesh and mark the places on outline map of Andhra Pradesh. (AS5)

12. How do you appreciate the efforts of human beings to discover an alternate energysources to Coal and Petroleum for their daily use? (AS6)

13. Harshith said to his father, "Daddy we can save more fuel by using bicycle instead ofbike for going to nearby places?" How do you appreciate Harshith’s comment? (AS6)

14. Why should people look for alternative sources of fossil fuel? (AS7)

15. Assume that you are a driver, what measures do you take to save petrol and diesel? (AS7)

16. “Crude oil and refined fuel when spills into the sea from tanker of ships by accident”.Discuss the consequences of this on environment. (AS7)

17. “The use of CNG and LPG as fuels for automobiles helps us to minimize air pollutionand maintains ecological balance”. Do you agree with this statement? If yes, explain.(AS7)

96 Combustion, Fuels and Flame

COMBUSTION, FUELS ANDFLAME

Chapter

7

We use different kinds ofmaterials as fuels for various purposes athome. You might have observed or heardabout how people used to burn wood, coal,cakes of cow dung, kerosene etc., forcooking food at home. Blacksmiths invillages also use them for heating metals.Both in urban and rural areas, now a day’s,LPG is used as fuel for cooking the food.We use the light from the burning candleor kerosene lamp, when there is no supplyof electricity. You might have also observedburning of a candle or coal. Whatdifference do you notice in the burningprocess?• Why does candle give flame when it is

burnt but why does coal burn withoutemitting a flame ?

• Do all fuels produce same amount ofheat when they are burnt?

• What do we need to burn a material?• Have you ever tried burning a piece of

paper or wood or coal, a small rock ora pebble?

• Do all of them burn?Let us do an activity to know which of

these materials burn and which do not.

Activity-1

Do all materials burn ?You will need a pair of tongs, some

metal or clay dishes and a candle or a spiritlamp.

Using tongs, pick up a small piece of paperand bring it close to the lighted spirit lampand keep it on flame as shown in figure-1.

Record your observation in table 1.

Fig-1

Carry out this experiment with a pieceof charcoal, magnesium ribbon, straw,cotton cloth, nylon cloth, dry wood, pebble,wax, plastic piece etc, and record yourobservations.

You can also try to burn liquids.Take 2mL of water in small plate. Bring

lighted stick near to water in the plate (seefigure 2).

97Free distribution by T.S. Government

• What do you observe in your attemptof burning water?

• Is there any difference in flame oflighted stick?

• What happened to the lighted stick whenit is brought closer to water in the plate?

Carry out this activity using coconutoil, mustard oil, kerosene, spirit, petroletc. (sticks should be long enough toprevent any fire accident if the materialcatches fire)

Record your observation in table 1.• What can we conclude from this

activity?We can conclude that some materials

burn and others don’t. In the aboveactivity you observed that when materialsburn in air, heat and light are produced.

Fig- 2

A chemical process in which a materialreacts with oxygen present in the air togenerate heat is called combustion. Thematerials which burn when brought near aflame are combustible materials. Some ofthem can also be used as fuels. Thematerials which do not burn are called non-combustible materials.• Which of the material in the above

activity are combustible?

Think and discuss

• Why some material burn and whysome do not? Give reasons.

• Why some materials which do not burnat normal temperature burn at highertemperatures?

What is needed for the processof combustion?

We know that we need a match stick ora lighter to burn a material.• How will you prove that air is needed

to burn a material?• Can we burn a material in the absence

of air?

S.No. Name of the material How does it burn burnt Burns immediately Burns slowly Does not burn

1 Magnesium ribbon2 Pebbles

3 Petrol

4

5.

Table 1

98 Combustion, Fuels and Flame

We can carry out an experiment todemonstrate that oxygen helps in burning.

Aim: To prove that oxygen helps in burningMaterial required: Test tube, test tubeholder, spirit lamp, match box, inscencestick (agarbatti), potassium permanganatecrystals.

ProcedureLight a scented / incense stick (agarbatti),and let it burn for 10 s, then put out theflame and keep it aside.

Take potassium permanganate in a testtube. Hold the test tube with a test tubeholder and heat it over the flame of spiritlamp. Oxygen is released on heating ofpotassium permanganate.

Insert the agarbatti with the burningstub, into the test tube as shown infigure 4.

Fig-4Observations• How does scented stick started burning?• Why does not it catch again fire when

it is kept aside in air after putting itsflame off?You observe that stick burns with a

flame. Here the oxygen supportscombustion by helping Agarbathi to burnwith bright flame.

Fig-3

Activity-2

Testing the necessity of air forburning

Take a small burning candle and put iton a table. Invert a glass tumbler over it.The candle continues to burn for some time.Then flickers and finally flame goes off.(See figure 3)

Remove the tumbler and again light thecandle. Put the tumbler back over thecandle. When the candle flame begins toflicker, remove the tumbler. What happensto the candle? Notice carefully.

We find that putting the glass tumblerover the candle cuts off the supply of airand the candle flame goes off.

This experiment proves that air isneeded to burn a material. Some otherexperiments need to be conducted to provethat the oxygen present in air supports thecombustion.

If you lift the glass tumbler (Whichis placed over a burning candle) to 1cmheight what happens? Why?

Think and discuss

Lab Activity

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Think and discuss

• How do you say that the gas releasedin the above experiment is oxygen?

• Can we replace potassium permanganatewith any other substance to releaseoxygen?

• Is there any other procedure to provethat oxygen is needed for burning?

A few more examples of combustionare given below. Can you explain thereasons for the changes taking place ?

• A slow fire bursts into a flame whenair is blown on it, but a candle burningwith flame goes off when air is blownon it. Why?

• If a large quantity of dry grass is seton fire in forests then it is verydifficult to put off the fire. Why?

• When an object catches fire, the fireis put off by covering with sand or ablanket. Why?

The examples and activities discussedabove shows that combustion takes placeonly in the presence of air. We know thatsome materials catch fire as soon as theyare brought near a flame, but some materialstake a long time to start burning though theyare kept near the flame.• What could be the reason for this

variation in burning patterns amongcombustible materials?Let us explore this in the following

activities.

Ignition TemperatureIn activity 1, a candle is used to burn a

piece of paper. Can we burn paper with outthe help of flame ?

Activity-3

Burning a paper with sun raysOn a sunny day, go out and focus the

sun rays on a piece of paper using amagnifying lens (Figure-5). Touch the spotafter some time. How do you feel?

Fig-5You must have heard about people in

ancient times rubbing pieces of stonestogether to produce sparks. Have you triedit? Rub two stones hard together and touchthem. What do you feel?

Now recall some of your experiences:• Does a matchstick burn by itself?• Why do you rub the match stick on the

side of the match box to burn it?• Can you burn a piece of wood by bringing

it close to a lighted matchstick?• Why do we use paper or kerosene oil

to start fire in wood or coal?On the basis of above observations and

previous experiences, we can conclude thata combustible substance has to be initiallyheated to catch fire or burn. The lowest

100 Combustion, Fuels and Flame

temperature at which a substance catchesfire is called its ignition temperature.When a substance starts burning heat isproduced and it helps to burn the substancecontinuously. The ignition temperature isdifferent for different substances. Theignition temperature of a substance decidesquickness of catching fire.

The substances which have very lowignition temperature and easily catch fireare called Inflammable Substances.Examples of inflammable substances arepetrol, alcohol, liquified petroleum gas(LPG) etc.,• Can you make a list of some more

inflammable substances?

Activity - 4Understanding ignition temperature

Take two small paper cups. Pour waterin one of the cups. Put the two cups ondifferent tripod stands and heat both ofthem using a candle as shown in thefigure-6.

Fig-6• Which cup burns first?• Does the water in the cup become hot?

Why?Explain why one cup burns quickly but

the other does not, on the basis of ignitiontemperature. When heat is supplied to

cups, the heat received by second cup istransferred to water in it. The water in thiscup prevents the paper to reach its ignitiontemperature and hence it does not burn.• When does the second cup start burning?

Make a guess and discuss with yourteacher.Types of Combustion

During summer time, dry grass catchesfire in the forests. It may spread to the treesand very soon the whole forest will be onfire. It is very difficult to control such forestfires.

If the head of a match stick is rubbed onthe side of a match box, it starts burning.

• What makes match sticks to catch fire?A mixture of antimony trisulphide,

potassium chlorate and white phosphoruswith some glue and starch was applied onthe head of a match stick made of suitablewood. When it struck against a rough surfacewhite phosphorus got ignited due to the heatof friction. This starts the combustion of thematch stick. However, white phosphorusproved to be dangerous both for the workersinvolved in the manufacturing of matchboxes and for the users.

These days the head of the safety matchstick contains only antimony trisulphide andpotassium chlorate. The rubbing surface haspowdered glass and a little red phosphorus(which is much less dangerous). When thematch stick is struck against the rubbingsurface, some red phosphorus gets convertedinto white phosphorus. This immediatelyreacts with potassium chlorate in thematchstick head to produce enough heat toignite antimony trisulphide and start thecombustion.

empty cup cup with water

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FuelsWe know that combustion gives heat

and light. The sources of heat for domestic,automobile and industrial purposes aremainly wood, charcoal, petrol, kerosene,LPG, CNG etc. These substances are fuels.In the previous chapter, we studied aboutthe fossil fuels and their use in variousways. We not only use fossil fuels but alsoother kinds of fuels for different purposesat home, in automobiles and in industries.List different fuels that are used for !Domestic purpose …….................................Automobiles/ Aircraft/ Trains/ Rockets........................................................................Industry .......................................................Classify the above fuels into solid, liquid,gases and write them in table 2.

Table - 2 Solid Liquid Gas

Look at the fuels in the table 2.• Can you decide the best fuel among

them?• What is the criteria to decide a best

fuel?Discuss with your friends.Deciding the best fuel depends upon

the purpose of its use. A best fuel forcooking may not be a best fuel for runningan automobile.

The type of combustion in whichmaterial suddenly burns into flameswithout the application of any externalagent is called spontaneous combustion.

Turn on the knob of the gas stove in thekitchen and bring a burning match stick ora gas lighter near it. The gas burns rapidlyand produces heat and light.

Such combustion is known as rapidcombustion. Materials such as spirit,petrol and camphor burn even with a sparkfrom a gas lighter.

Do you know?1. You might have noticed words

written as highly inflammable on petroltankers. This is a warning to the publicto keep flame away from the inflammablematerial.

2. We generally enjoy sound andlight from fire works on festival days.When a cracker is ignited a suddenreaction takes place with the evolutionof heat, light and sound. A large amountof gas is also liberated in this reaction.Such a reaction is called explosion.Explosions can also take place ifpressure is applied on the crackers.

Think and discuss

• Why is phosphorus preserved inwater? (Hint: think about the role ofignition temperature in combustion)

• Why Kerosene stoves and Bunsenburners have small holes in them?(Hint; Think about the role of air incombustion)

• It is hard to ignite match stick in rainydays. Why ?

102 Combustion, Fuels and Flame

In general, there are several factorsthat have to be kept in mind while choosinga fuel, like purpose of use, fuel efficiencyand availability, reasonable price, easy tohandle and safe to store, easy to ignite andput off etc. The fuel should also burn at amoderate rate and cause less pollution. Inaddition, it should have a high calorificvalue.

• What is calorific value ?

Suppose 1 kg of coal and 1 kg of cowdung are burnt. Which one produces moreheat? Different substances producedifferent amounts of heat on burning. Heatis measured in kilo joules. Calorific valueof a fuel is the amount of heat energyproduced on complete combustion of 1 kgof that fuel. It is measured in kilo joulesper kg (kj / kg).

Fire controlYou must have seen or heard about fire

breaking out in houses, shops, factories,etc.

• How can we put off the fire if it breaksout ?

We use many methods to extinguish afire but they all follow one principle, whichis the principle of elimination of factorswhich support the combustion.

Let us recall the factors which supportthe combustion:

(a) Presence of a combustible material orthe fuel

(b) Supply of air or oxygen

(c) High temperature (More than theignition temperature)

So, elimination of any one of the threefactors will help in controlling the fire. Letus see some examples.

Example

If a fire breaks out in a house or in anybusiness establishment the fire brigade willimmmediately put off the electric mainsand then start spraying water on the fire.

• Why the fire brigade start the work byputting of the electric mains?

• How water helps in eliminating thefactors, which support the combustion?

Initially, the water spray cools thecombustible material below its ignitiontemperature. This prevents the fire fromspreading.

Fuel Calorific value (kj/kg)

Cow dung 6000-8000Wood 17000-22000Coal 25000-33000Petrol 45000Kerosene 45000Diesel 45000CNG 50000LPG 55000Biogas 35000-40000Hydrogen 150000

Do you know?

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Then the heat turns the water intovapours which surround the burningmaterial and prevent supply of oxygen tothe burning materials. So, the fire isextinguished.

The most common fire extinguisher iswater. But water works only when thingslike wood cloth and paper are on fire. Ifelectrical equipment is on fire water mayconduct electricity and harm those tryingto douse the fire.

Water is also not suitable for firesinvolving oil and petrol, because water isheavier than the oil, it sinks below the oiland oil keeps burning on the top.

Since it is difficult to remove thecombustible material from a fire, cuttingof air supply and lowering the temperatureare better methods.

In cases where water cannot be used,carbon dioxide gas is the best choice usedwhich is heavier than oxygen. It can bestored as a liquid in cylinders under highpressure. When released from the cylinder,it expands and brings down the temperature.

It also covers the fire like a blanket.Since the contact between the fuel andoxygen is cut off, the fire is controlled.

That is why it is an excellent fireextinguisher. The added advantage of carbondioxide is that in most cases it does notharm the electrical equipment. It ismandatory for offices, educationalinstitutions and multistoried buildings toinstall fire extinguishers.

Flame

Activity - 5Observing the behaviour ofdifferent solid fuels

Collect some fuels like candle, coal,domestic gas, charcoal, magnesium ribbon,wood, cakes of cow-dung, camphor, wickof the oil lamp, wick of kerosene stove, etc.Burn each of them one by one with the helpof spirit lamp and note the time they taketo catch fire. Also observe how do theyburn?• Do all of them burn in the same

manner? If not, what difference do younotice?

• Do all of them form a flame while theyare burning?Record your observation in the

following tableTable - 3

Material Forms Does not flame form flame

CandleMagnesiumWick ofKerosene stoveCharcoalDomestic gasCamphor

Cow dung cake You may observe that a candle burnswith flame where as charcoal does not.Some materials burn with flame, some donot. Kerosene oil and molten wax risethrough the wick become gas and formflames. But charcoal cannot be vapourised.

104 Combustion, Fuels and Flame

So it does not produce a flame. A fuelcatches fire immediately if it is in the formof gas. Cooking gas catches fireimmediately. Spirit and petrol turn into gasat room temperature. Hence, they catchfire quickly.

Think and discuss

A wax candle burns with a yellowflame. The domestic gas burns with ablue flame. Why?

Structure of a flame

Activity-6

Observing the structure of theflame

Light a wax candle and watch the flame.Carefully note the different coloured zonesin the flame. How many colours are therein the flame?

Starting from the base of the flame,how many flame zones do you observe?What is the colour of the outer most zoneof the flame?

Fig-7

hottest part

moderatelyhot

least hotdark zone

middle zone

outer most zone

blue zone

Observe the innermost zone which isdark. What do you observe there? Is therecombustion takes place? In this zone wax getsvapourised. This is a dark zone. See figure 7.

Observe near the base of the flame.Vapourised wax gets completely oxidizedand burns with a blue flame. It is blue zone.

Do you know?A candle is mainly a source of light

but heat is also released. A candle is madeof wax in which a thick thread is inserted.Wax in the candle melts when it is lightedby a match stick. A little of the wax formsvapour. This vapour combines withoxygen in the air to form flame. The heatof the flame melts more wax from thetop of the candle. The liquid wax movesupward through the thread. It also changesto vapour when it reaches the top of thewick and burns with the flame.

Activity-7Observing what happens in differentzones of candle flame

Light a candle. Hold a glass tube with apair of tongs and introduce its one end in thedark zone of a non flickering candle flame.Keep lighted match stick near the other endof the glass tube as shown in figure-8. Whatdo you observe? Do you see a flame? If sowhat is it that produces a flame? Notice thatthe wax near the heated wick melts quickly.

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Fig-8

When the candle's flame is steady,introduce a clean glass slide into theluminous zone (yellow zone) of the flameand hold for 10 seconds. See figure 9. Whatdo you observe?

Fig-9

A blackish circular ring is formed onthe glass slide. What is it? It indicates the

deposition of un-burnt carbon particlespresent in the luminous zone of the flame.Incomplete combustion takes place in thiszone.

Hold a thin long copper wire just insidethe flame for about half a minute as shownin figure-10. What do you observe? Thecopper wire just outside the flame gets redhot. It indicates that the non-luminous zoneof the flame has high temperature. It is thehottest part of the flame. It is blue in colourand complete combustion takes place dueto good supply of oxygen.

Fig-10

Key words

Combustion, combustible and non - combustible materials, ignition temperature,inflammable material, spontaneous combustion, rapid combustion, explosion, fuels,calorific value.

• Burning a material in the presence of air (oxygen) is called combustion.

• Oxygen or air is needed for combustion to take place.

• The lowest temperature at which a substance catches fire is called its ignitiontemperature.

What we have learnt

106 Combustion, Fuels and Flame

• The type of combustion in which material suddenly burns into flames without theapplication of any external agent is called spontaneous combustion.

• The type of combustion in which material burns rapidly and produces heat and light iscalled rapid combustion.

• The amount of heat energy produced on complete combustion of 1kg of fuel is calledthe calorific value of that fuel.

• Wax does not burn in the dark zone of the candle flame.

• In the blue zone of the candle flame, vaporized wax burns compleatly due to goodsupply of oxygen.

1. Give four examples of combustible materials. (AS1)

2. Give four examples of non combustible materials. (AS1)

3. Why should not we store spirit or petrol near our living place? (AS1)

4. Give an example of a good fuel. How do you choose that fuel? Explain. (AS1)

5. The oil fires should not be sprayed with water. Why? (AS1)

6. What precautions are to be taken while pouring water on fire? (AS1)

7. Why a wick is not used in gas burners? (AS1)

8. Water is not used to control fires involving electrical equipment. Why? (AS1)

9. It is difficult to burn a heap of green leaves but not a heap of dry leaves. Explain why?(AS1)

10. Give supporting arguments for both the statements (1) fire is useful (2) fire isharmful (AS2)

11. In a few years the fuels on earth will be exhausted. Think, what would happen to humancivilization ? (AS2)

12. What would happen if oxygen stops to support combustion? – Make a guess. And if itis the situation for what other works fuels are useful. (AS2)

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13. Use of more fuels in our daily life causes air pollution and it is harmful to humanbeing and the other life on earth. Suggest some remedies to avoid this. (AS2)

14. Let us assume that you are on the moon. If you try to focus sun light on a paper usingmagnifying glass, does the paper catch fire ? or not? Why ? (AS2)

15. Can you heat water in a paper vessel? How is it possible? (AS3)

16. “Is combustion possible without the supply of oxygen?'' Discuss with your teacher(AS3)

17. Expalin giving reasons: In which of the following situations water will get heated in ashorter time? (AS3)

a) Srikar kept water beaker near the wick in the yellow part of a candle flame.

b) Sonu kept water beaker in the outer most part of the flame.

18. Project work: Collect information about the experiments of Joseph Priestly. Write atwo page report describing Priestly’s experiments proving that oxygen is needed forburning. (AS3)

19. List the ways adopted by fire fighters to combat fires. (AS4)

20. Collect information available on different fuels. Find out the cost per kg. Comparethe cost with calorific value. Prepare report on that. (AS4)

21. Collect the information about annual fuel consumption in different parts of theworld. How many years more the fossil fuels last? Make a poster with this informationand issue an appeal to save fuel. (AS4)

22. Draw the diagram of candle flame and label all the zones. (AS5)

23. Where do you find spontaneous combustion and rapid combustion in your daily life?(AS7)

24. How do you organize your daily works with fuels to conserve bio-diversity? (AS7)

25. How do you feel about “Fuels have become a part of human life”? (AS7)

108 Electrical Conductivity of Liquids

ELECTRICAL CONDUCTIVITYOF LIQUIDS

Chapter

8

pieces of connecting wires. Set up theelectric circuit as shown in the figure-1.

Fig-1 : Testing conductivity of material

Place the key on drawing pin. The bulbbegins to glow as soon as the key touchesthe drawing pin. Now replace the key by anail. Does the bulb glow?

Repeat the activity using different typesof materials instead of the nail, say a stripof paper, a piece of chalk, a drinking straw,a piece of plastic, a paper clip, a rubbereraser, pencil graphite etc.

Note in each case whether the bulbglows or not and enter your observationsin Table-1.

Sometimes we read in newspapersabout farmers getting electricshocks while starting water pumps of thewells, especially because of contacts madewith switches or starters by wet hands. Doyou know the reason behind getting electricshock while working with wet hands?

Our elders caution us about touchingelectric heater immersed in water. Why dothey instruct us to stay away from it? Howdoes the electric current flow throughwater?

In the earlier class you have learnt thatelectric current can pass easily throughmetals like copper, aluminium, etc. Do yourecall some other materials which conductelectric current? Let us revisit the activityand do it now.

Activity-1Testing the material to knowwhich allows electric current topass through it

Take a torch bulb or LED (LightEmitting Diode), a dry cell, Wooden sheet,two drawing pins, a key (safety pin) and

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Take care that whenever the bulb glows,it should not be kept in the 'ON' positionfor a long time to avoid the early dischargeof dry cell.

In the above activity, we observed thatsome materials allow electric current topass through them. We call them as goodconductors of electricity.

In general, all metals are goodconductors of electricity. On the otherhand, the materials that do not allow currentto pass through them are called bad or poorconductors of electricity.

Think and discuss

Why some material allows electriccurrent to pass through them and whysome do not?

Electric conductivity is a property ofany given material. We can say that amaterial has good electrical conductivity

Table 1

S.No. Object Material Bulb glows Good conductor/ Bad orYes/No Poor conductor

1 Nail Iron Yes Good conductor

2 Eraser Rubber No Bad/Poor Conductor

if it allows electric current to pass throughit easily.

Electrical conductivity of liquidsIn the activity-1, we have tested

conductivity of objects like nail, paperstrip, chalk, etc. All of these are solids.What about liquids? Do the liquids allowelectric current to pass through them?

Let us do another activity to find outwhether a given liquid allows electriccurrent to pass through it or not.

Activity-2Testing the electric conductivityof liquids

Take a LED, dry cell, metal pins, rubbercap of injection bottle and wires for makingconnections. Set up an electric circuit asshown in the figure 2.

110 Electrical Conductivity of Liquids

LED

cell

Fill the rubber cap with differentliquids, one after another and in each case,check whether the LED glows or not. Startwith distilled water (you can get distilledwater from battery stores or from medicalshop). Pour distilled water in the rubber captill the two metal pins come in contact withit. Check whether the LED glows or not.

Then take water that you drink in schooland repeat the procedure. Do this activitywith liquids like coconut oil, kerosene,lemon juice, mustard oil, sugar solution,etc. After testing each of the liquids,carefully wipe and dry the cap and the pinsbefore filling in the next liquid. In eachcase, note your observations in Table2.

From your observations, decide whichliquids are good conductors of electricityand which are poor or bad conductors. Noteyour inference also in Table 2.

Fig. 2: Testing conductivity of liquids

See that the two metal pins, passthrough the cap should have a very smallgap (around 2 mm) between them so thatthe pins are fairly closer but not touchingeach other. The LED should not glow whenpins are separated by the small distance.

Now, join the free ends of the pinstogether by pressing them for a momentand make sure that the LED glows. Releasethe pins, they get separated and LED shouldnot glow. This becomes our tester. We willuse this tester to check the conductivityof liquids.

Rubbercap

Liquid

Table 2

S.No. Liquid LED glows Good conductors/ poor orYes/No bad conductors

1 Distilled Water No bad conductor

2 Drinking water Yes good conductor3 Coconut oil4 Lemon juice5 Vinegar6 Kerosene7 Vegetable Oil8 Sugar solution910

Let us think about the above table.

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Why doesn’t the LED glow in all thecases? Or why doesn’t the LED remainoff in all the cases?

In activity 1, we saw that when currentflows through the object inserted in the gap,the bulb glows. Similarly, we can see thatwhen the liquid between the two pins ofthe tester allows electric current to passthrough, the circuit is completed (closed)and the LED glows. Then we say, the liquidis a good conductor.

On the other hand, when the liquid doesnot allow the current to pass through, thecircuit is incomplete (Open) and the LEDdoes not glow. Then we say the liquid is abad conductor.

List out the good conductors from table 2.

In the above activity, you may haveobserved that in all those cases where theLED glows, its brightness (intensity) is notthe same. Sometimes it may be brighter andsometimes it may be relatively dimmer.Why is that so?

The intensity of the glow of the LEDdepends on the flow of electric currentthrough the circuit. Although a liquid maybe a conductor, it may not allow currentthrough it as easily as a metal does.

As result, although the circuit iscompleted and the LED glows, due to weakcurrent in cases of some of the liquids, theintensity of glow would be lower comparedto other liquids.

Do you know?

Why do we use LED in the testerinstead of a bulb?

LED glows even when a very weakcurrent is passing through the circuit.Thus, it helps in testing flow ofelectricity in conductors when meagercurrent is passing through the circuit.

Since LEDs glow even with a verylittle current passing through them, theyare used as ‘indicators’ in electricalappliances like mobile phones,televisions, transformers, etc. to indicatewhether the device is working or not.

There are two wires called leadsattached to an LED. One lead is slightlylonger than the other. See figure 3.

Fig. 3 (LED)

While connecting to the LED to thecircuit, the longer lead is alwaysconnected to positive terminal of thebattery and the short lead is connectedto the negative terminal of the battery.

112 Electrical Conductivity of Liquids

three different containers. Dissolve smallquantity of common salt in the water of firstcontainer. Dissolve the Copper Sulphate(Mylatuttam), lemon juice in 2nd and 3rd

containers respectively.

Use the tester that we used in activity2, and repeat the activity 2. Note yourobservations in Table 3.

(Caution: Wash and wipe the pins oftester dry after testing with each liquid.)

Table 3S.No. Material Does the LED glow? Good conductor/ bad

Yes/No or poor conductor1 Distilled water No Bad conductor

2 Dist. Water + salt

3 Dist. Water +

4 Dist. Water + lemon juice

From the above observations, what canwe infer? Distilled water does not allow theelectric current to pass. Water in its pure(distilled) form is a bad conductor ofelectricity. But when water contains salts oracids, it allows passage of electric currentand turns into a good conductor ofelectricity.

The water that we get from sources suchas taps, hand pumps, wells and ponds is notpure like distilled water. It contains somesalts and minerals dissolved in it. Some ofthese minerals are useful for our health. Thiswater is a good conductor of electricity.

On the other hand distilled water is freeof all salts, minerals, acids, etc. and is apoor conductor of electricity.

Do you now understand why you areadvised not to touch electric applianceswith wet hands?

Water with salts is a good conductorof electricity and the current flowingthrough house hold electric appliances isvery high. Therefore, we should never touchthe electrical appliances with wet hands.

Like water in the above activity, mostliquids that conduct electricity aresolutions of acids, bases or salts.

When do liquids conductelectricity?

In the last activity we observed thatdistilled water does not conduct electricity.Can we make poor conductors like distilledwater to conduct electricity? Let us try itout.

Activity-3

Electric conductivity ofelectrolyte

Take same amount of distilled water in

coppersulphate

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LED

Chemical effect of electriccurrent

If solutions of different salts and acidsconduct electricity, what do vegetables andfruits do? Let us try to find that out.

Activity-4

Testing the effect of electriccurrent on potato

Take a potato. Cut into two halvesand take one half of it. Construct testerwith LED bulb, insert two copper wires ofthe tester into the potato leaving somedistance (around 1 cm) between them. asshown in figure 4.

Fig. 4 Does the LED glow?

Leave the inserted wires for 20-30minutes.

What do you observe on the surface ofthe potato?A greenish blue spot is seen on the

potato around the wire connected to thepositive terminal of the battery. But no suchspot is seen around the other wireconnected to the negative terminal. Thisgreenish spot is due to chemical change inthe potato.

What could be the cause behind thischange?Will other vegetables also show suchan effect?Try it out with vegetables like carrot,

beetroot, cucumber, raddish, brinjal, sweet-potato, etc.

Think and discuss

If a battery is packed in a box and ifonly two wires from two terminals aregiven out, how can we decide thepositive and negative terminal of thebattery?

Electrolytic cellIn all the above activities, the battery

we have used is made up of dry cells. Inlower classes you have studied about drycell. Can you produce electric current in

another way?Do you know, how was the first cellmade?

114 Electrical Conductivity of Liquids

How the first cell was made?People in Europe began

experimenting with electricity around400 years ago. They generated electricityin different ways and conducted variousexperiments. However, they faced onemajor problem which prevented themfrom understanding electricity in depth.They did not have a stable and permanentsource of electricity. This may soundlike a minor problem today, but it tookscientists nearly 200 years to find asolution.

That solution came in the year 1780. Andit came almost by chance. A biologistnamed Luigi Galvani from Bologna, Italy,once saw a frog’s leg hung from a cop-per hook twitching violently when ittouched another metal. It seemed as ifthe frog’s leg had suddenly come to life.

Galvani did many more experimentswith the legs of dead frogs. He finallycame to the conclusion that frog’s legstwitched every time electricity flowedthrough them. Galvani thought he haddiscovered living or biological electricity.He presented his theory to the world,saying that all living beings containedelectricity and it was this electricity thatwas their main source of life.

Galvani’s experiments took thewhole of Europe by storm. Manyscientists began performing similar

experiments with various species ofanimals. Among them was AlessandroVolta of Italy. He, too, performedexperiments with frog’s legs. However,he discovered that if a frog’s leg hungfrom an iron hook is touched withanother iron rod, it does not twitch. Voltawas a bit puzzled..

If the reaction in a frog’s leg is dueto the electricity in its body, why are twodifferent metals required to make ittwitch, he wondered? After a lot ofthinking he arrived at the conclusion thatelectricity does flow through the frog’sleg when two different metals touch it.However, this electricity is not containedin the leg of the frog but is generated bysome other process.

Volta repeated his experiment usingdifferent liquids instead of frog’s legs.He found that it did not require ananimal’s body to generate electricity. Itis possible to generate electricity if twodifferent metals are placed in someliquids.

These experiments showed the wayto a steady source of electricity. Voltamade his first cell in 1800 using zinc andcopper plates dipped in sulphuric acid.His discovery made him famous in therealm of science. The cell he made iscalled a Volta cell in his honour. Theword voltage is also derived from hisname.

Do you know?

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Fig. 5Cu Zn Cu Zn

Take an LED. Attach two wires to itstwo terminals. Touch the wire from oneterminal to the zinc plate and the wire fromthe other terminal to the copper wire. Didthe LED light up? If not, change theconnections vice-versa. Did the LED glow?

Repeat the above activity using lemonjuice, tamarind juice and tomato juice oneby one instead of sulphuric acid to makeyour cells.

What other liquids can be used to makethe cell?Will detergent solution be useful?Find it out yourself.How does the above cell function?After a few seconds of immersion of

zinc and copper wires into dilute sulphuricacid, zinc slowly begins to dissolve in thesulphuric acid. We can see bubbles formingon the copper rod.

The current is passed from copper rodto zinc rod. These rods are known aselectrodes and dilute sulphuric acid isknown as electrolyte.

Here the chemical energy is convertedinto electric energy by “electrolysismethod”.

Can you compare this cell with dryCell?

Which is good one? Why?

Think and discuss

What is electrolysis?Discuss with your teacher or collect

the information about electrolysismethod form your school library books.

Let us make a cell with the same metalsand chemicals used by Volta.

Activity-5

Make your own cellCollect two injection bottles. Cut two

5 cm-long bits of thick copper wire. Usesandpaper to scrape about 1 cm of thecoating from both ends of the wires.

Break open a exhausted dry cell andremove its outer metal covering (made ofzinc). Cut two 2 mm-wide and 5 cm-longstrips from this zinc plate. Insert the copperwires and zinc strips into the rubber capsof the injection bottles as shown in figure5.Ensure that the copper wire and zinc stripsdo not touch each other.

Now take a wire and connect the copperwire of one bottle with the zinc plate ofthe other bottle. Fill both bottles withdilute Sulphuric acid of the bottlescarefully with the caps in which the copperwires and zinc strips are inserted. Your cellis ready.

How do you test it?

dil.sulphuric acid

116 Electrical Conductivity of Liquids

Procedure:

Dissolve crystals of copper sulphate inpure water to prepare concentrated solution(deep blue in colour). Pour the solution ina beaker and add a few drops of dilutesulphuric acid to it. (Acid helps inincreasing the conductivity of electrolyte.)

Tie one end of a connecting copperwire to the iron object (key) to be coatedwith copper. Connect its other end to thenegative terminal of a battery. Suspend thetied iron object into the copper sulphatesolution. Suspend the copper plate intocopper sulphate from positive end of thebattery through a switch as shown in fig-6.

Care should be taken that the key andplate do not touch each other and are a littleaway from one another. Put the switch onfor about 10 minutes. Switch off the circuitand take the iron key out.Observations :

Does the iron key get coated with ashiny, brown colour?What is the colour due to?What will happen if you interchange thebattery terminals?

Fig. 6: Electro plating

ElectroplatingCan you list some objects around you

that keep shining? For example, the clip ofyour writing pad or rim of a newly boughtbicycle.

However, if these objects are scratcheddeliberately or accidently, their shinediminishes. Scratching of such objectsremoves some coating from their surfaceand we can see a relatively dull surfacebelow the coating. Safety pins, when theyare new, shine brightly. However, withrepeated use, they lose the brightness ofshining. Repeated handling makes thecoating of the pins wear off and the non-shiny metal beneath is exposed.

In the above examples, the materialunderneath contains a coating of anothermetal. How is this achieved? How is theshining metal coated on the dull object?

Well, let us try doing it ourselves.

Aim: Coating an iron key with copper byelectroplating method.Required material: Copper plate of size2 cm x 5 cm, crystals of copper sulphate(blue vitriol), a key made by iron, glassbeaker, water, sulphuric acid, Battery andsome connecting copper wires. (You maytake a thick copper wire and hammer it toflatten it instead of the above mentionedcopper plate.)

Lab Activity

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Why does copper get deposited on theiron key?When electric current is passed through

the copper sulphate solution, in which thecopper sulphate is present in the form ofcopper and sulphate ions, the free copperions are drawn to the electrode connectedto the negative terminal of the battery anddeposited on it.

Thus one metal is coated with anothermaterial. This process is known aselectroplating.

If the key is to be coated with zinc oraluminium instead of copper, what changesdo we need to make in the aboveexpeirment?

In electroplating, an inferior metal(metals which are effected by theatmospheric humidity, carbon dioxide, etc.)is coated with a superior metal (metalswhich are not effected by the humidity oroxygen). The following are therequirements for fine coating.

(a) The object to be electroplated shouldbe free from greasy matter.

(b) The surface of the article should berough so that the metal deposited stickspermanently.

(c) The concentration of the electrolyteshould be so adjusted as to get smoothcoating.

(d) Current must be the same throughout.

Electroplating – usesElectroplating is widely used in

industry for coating metal objects with a

thin layer of different metals.For example, metals like iron which are

easily corroded by atmospheric air,moisture and oxygen are coated withdeposits of nickel or chromium which aremost resistant to such corrosion by electroplating method. Machinery parts are oftenchromium plated to protect them fromcorrosion and at the same time to give themgood polish. (see Figure 7)

Sometimes, electroplating is done witha view to repairing worn out parts ofmachinery. In such cases the suitable metalis deposited on the affected parts of themachinery by electro plating method.

Electroplating is also used inornamentation and decoration. For exampleseveral articles made of copper or itsalloys, such as table wares, decorationpieces are coated with silver or gold. (seeFigure 8)

Fig. 7

118 Electrical Conductivity of Liquids

Fig. 8In general the processed food items

are preserved in tin coated iron cans. Tinis less reactive to the food than iron. Sothe cans are made by electroplating tin oniron. (see Figure 9)

Fig. 9

Improve your learning

Key wordsGood conductors, Poor conductors, Electrodes, Electrolyte, Electrolysis,

Electroplating

1. Some liquids also conduct electricity as some solids do.2. Some liquids are good conductors of electricity and some are poor conductors.3. Distilled water does not allow the current to pass through it.4. Most Liquids that conduct electricity are solutions of acids, bases and salts.5. Electrolyte is a solution of a substance through which electric current can pass.6. Electroplating is possible through electrolysis.

1. Give examples for good solid conductors and liquid conductors. (AS1)2. Give examples for poor solid conductors and liquid conductors. (AS1)3. What do you add to distilled water for making it to conduct electricity? (AS1)4. What is an electrolyte? (AS1)5. Which energy is cause for glowing of bulb in electrolytic cell? (AS1)6. Write the uses of electroplating. (AS1)

When iron is coated with zinc metal,iron becomes more resistive to corrosionand formation of rust. So, zinc coated ironis used for bridges and in automobiles.

What we have learnt

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7. In case of a fire, before the fire men use the water, they shut off the main electricalsupply for the area. Explain why they do this? (AS1)

8. We get some items made from iron wire in which iron wire is coated with plastic. Isplastic coated by the process of electroplating? Why plastic cannot be coated on ametal by the process of electroplating? (AS1)

9. Kavya observed that a discharged dry cell which kept in sun light by her father for fewhours got ability to glow LED. She got many doubts and questions to raise. Can youguess those questions or doubts? (AS2)

10. Explain the process of coating copper on iron key. Draw the circuit diagram. (AS3)11. Make a battery from four lemons and test it with a LED in the circuit. (AS3)

12. Refer to the Activity 3 in the chapter. Start with distilled water. The LED would notglow. Add two drops of some acid to distilled water and check the glow of LED. Addtwo more drops and check the intensity of the glow. Repeat the activity 5 to 6 times byadding 2 drops of the same acid each time. Do you see any difference in the intensity ofglow with increasing acid content of water? What can be inferred from the aboveobservations? Repeat the entire activity by taking a solution of baking soda and addingdrops of it to distilled water instead of acid. Write differences and similarities.(AS3 )

13. In many of the activities in this chapter, we have used a tester made up of LED. Can weavoid LED and use something else as a tester? Magnetic compass needle could be analternative tester. We know that when we take a current carrying wire near magneticcompass needle, it shows deflection. Use this property to make a tester of magneticcompass needle. You may refer to the following figure. (AS5)

14. How do you appreciate the efforts of Luigi Galvani and Alessandro Volta in discoveringa cell and making a stored Electric energy available to human beings ? (AS6)

15.Collect the information and make list of good conductors and bad conductors. How doyou use this information in your daily life works? (AS7)

120 Some Natural Phenomena

SOME NATURALPHENOMENA

Chapter

9

In the earlier classes you have learntabout winds, storms and cyclones.You have learnt that cyclones can cause alot of damage to life and property. You alsolearnt that we can protect ourselves fromthese destructive phenomena to someextent.

In the present chapter, we shall discusstwo other natural phenomena that causedestruction. These are lightning andearthquake. We shall also discuss whatsteps we can take to reduce the destructioncaused by these phenomena.

LightningYou might have seen sparks on an

electric pole when wires become loose orwhen wind blows and shakes the wires. Youmight also have seen sparks when a plug isloosely put in its socket and the switch isput on. (Do not try this if you have neverseen such a spark.)

Lightning is also an electric spark, buton a huge scale. In ancient times peopledid not understand the cause of thesesparks.

They were, therefore, afraid oflightning and thought that the wrath of godsor evil spirits was striking them. Now, of

course, to some extent we understandreasons for lightning.

We have to take some precautions toprotect ourselves from the deadly sparksof lightning.

The Sparks that the GreeksKnew About !

The ancient Greeks knew, as early as in600 B.C. that when amber (amber is a kindof resin) was rubbed with fur, it attractedlight objects such as hair.

You might have seen that when you takeoff woollen or polyester clothes, especiallyin dry season the hair on your skin standson end.

If you take off these clothes in the dark,you even see a spark and hear cracklingsound.

Why does hair get attracted towardsclothes?In 1752, Benjamin Franklin, an

American scientist, showed thatlightning and the spark from yourclothes are essentially the samephenomena.

People before Franklin knew about thephenomena of lightning and hair gettingattracted to clothes but did not have an idea

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Fig-1Take an inflated balloon and rub it

against your clothes. Bring the balloonclose to small pieces of paper.

Take a drinking-straw and rub it againsta smooth wall or against your clothes, thenbring it near pieces of paper.

What do you observe?

Are they able to attract bits of paperafter being rubbed?

Repeat the activity by rubbing each oneof the above mentioned objects (refill,comb, drinking straw, balloon) and usesmall pieces of dry leaf, husk, etc as testingmaterials. Record your observations intable-1.

What can we infer from the aboveactivity?Do the objects like refill or combattract pieces of paper only afterrubbing?Do all objects show this property?Can we rub a comb against our palm andmake it attract the paper bits? Try it andsee.

that these are related. However, to realisethat these two are the same phenomena butat different scales, it took about 2000years.

Scientific discoveries are a result ofhard work by many people. It cansometimes take a long time.

We shall now study some properties ofelectric charges. We shall also see howthey are related to the lightning in the sky.

Let us perform some activities tounderstand the nature of electric charges.Let us recall what you might have playedas a game.

When you rub a plastic scale on yourdry hair, the scale can attract very smallpieces of paper.

Why is the plastic scale not able toattract pieces of paper before it getsrubbed by dry hair?

Charging by rubbingActivity-1

Effect of rubbing Take a used ball-pen refill and bring it

near small pieces of paper. The refill shouldbe close enough but not touch the piecesof paper.

Check what happens to the paper-pieces. Now, rub the refill vigorously witha piece of polythene.

Bring it close to small pieces of paper.Note your observation. Take care that therubbed end is not touched by your hand orwith a metallic object.

Now, take a comb and move it throughdry hair a few times. Take the comb nearsmall pieces of paper and check whathappens.

122 Some Natural Phenomena

Let us try to find out whether all objectsshow this property. We will also try to findout whether attracting paper-pieces is onlydue to rubbing or rubbing with specificmaterials.

When a plastic refill is rubbed withpolythene, it acquires a small electriccharge. Similarly, when a plastic comb isrubbed with dry hair, it acquires a smallcharge.

These objects are called chargedobjects. In the presence of a charged refillor plastic comb, bits of paper and hair alsoget charged. Let’s try to charge some otherobjects that are familiar to you.

Aim: To find effects of charged bodieswhich have been rubbed by differentmaterials.

Material required: A ball pen refill, aballoon, a comb, an eraser, a steel spoon,polythene sheet, plain paper, woollen cloth,etc.

Procedure :

Rub the above objects against materialslisted in table-2. In each case, bring therubbed object near small pieces of paperand note whether they attract pieces ofpaper or not. Record your observations intable-2 by writing 'yes' or 'no'.

Object Material used for testing Effect before rubbing Effect after rubbing

Bits of Paper Paper bits Paper bits getremain stationary attracted to the refill

Pieces of dry leavesHuskBits of PaperPieces of dry leavesHuskBits of PaperPieces of dry leavesHuskBits of PaperPieces of dry leavesHusk

Refill

Comb

Balloon

DrinkingStraw

Table-1

Lab Activity

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What do you conclude from the abovetable?

Some objects like refill, comb, etcwhen rubbed with some specific materialsare able to attract light objects like bits ofpaper. But some objects like steel spoondo not attract pieces of paper even afterrubbing.

Why do not some materials attractpieces of paper even after rubbing?

Let us try to explore this phenomenonfurther.

Object Material used forrubbing

Whether paper piecesare attracted?

Refill

Inflated Ballon

Comb

Eraser

Steel spoon

Plain Paper

Polythene sheet yes

Woollen clothDry hairPolythene sheetWoollen clothPlain PaperPolythene sheet

Woollen cloth

Dry hair

Polythene sheet

Woollen clothPlain Paper

Polythene sheet

Woollen cloth

Table-2

Types of charges and theirinteraction

Activity-2

Understanding types of charges

Fig-2(a) Fig-2(b)

Inflate two balloons and hang them insuch a way that they do not touch each other.Rub both the balloons with a woollen clothand release them.

124 Some Natural Phenomena

Let us summarise our observationscarefully.1. A balloon rubbed with woollen cloth

repelled another balloon of the sametype.

2. A refill rubbed with polythene repelledanother refill rubbed with similarmaterial.

3. A ballon rubbed with woollen clothattracted by a refill rubbed withpolythene sheet.What can we conclude from theseobservations?Does the repulsion between chargedballoons indicate that they possessimilar charge?Does the attraction between a chargedballoon and a charged refill indicate thatthey posses different charges?Does this activity remind you some ofexperiments that you have done in ‘Playingwith Magnets’ chapter of class VI?We know that magnets attract objects

made up of magnetic materials like Iron,Nickel, Cobalt, etc.

We also know that unlike poles ofmagnets (North-South or South-North)attract each other and like poles (North-North or South-South) repel each other.

Can we say that something similar ishappening in above activities?

Does it indicate that the charge on theballoon is of a different kind from thecharge on the refill!Can we say that there exist two kindsof charges?

(Make sure that your hand doesn’ttouch the balloon while rubbing withwoollen cloth). What do you observe?

Take a refill and rub it with a polythenesheet. Keep it gently in a plastic tumbler.Take another refill and also rub it with thesame polythene sheet.

Bring the second refill near the firstone in the tumbler. Take care that you donot touch either of the rubbed portions ofthe refills with your hand. Is there any effect on the first refill in

the tumbler?Do they attract each other or repel eachother?

Bring two ballons rubbed with samesilk cloth together what happend ?

Now, take a rubbed balloon near therubbed refill in the tumbler and check theaction (fig-3).

Do they attract each other or repel eachother?In the first two parts of the above

activity, two objects that were made of thesame material have brought near to eachother after being rubbed with someappropriate material.

In the third part, objects made of differentmaterials were brought near to each otherafter being rubbed with different material.

Fig-3

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Fig-4: Testing the presence of charge

Bring a glass rod which is rubbed witha silk cloth near the suspended ball. Whathappens? Does it get attracted towards theglass rod or move away from it.

Now touch the silver foil on thethermocol ball with charged glass rod.Remove the glass rod from the ball andagain rub it with silk cloth and bring it closeto the suspended ball.

What do you observe?Does it get attracted towards the glassrod or move away from it?You may notice in the first instance that

the thermocol ball is attracted towards theglass rod and in second instance it movesaway from the glass rod.

What could be the reason for thischange in movement of the ball?If a charged body is brought near an

uncharged body it induces an oppositecharge on it and it gets attracted by thebody.

In the above experiment when a chargedbody (glass rod rubbed with silk cloth) isbrought near an uncharged body(thermocol) it induces an opposite chargein it and hence it get attracted by the glassrod.

In second case (above experiment) wehave charged the thermocol ball bytouching it with a charged glass rod. Hence

Can we also say that the charges of thesame kind repel each other, whilecharges of different kinds attract eachother?It is a convention to call the charge

acquired by a glass rod when it is rubbedwith silk cloth positive and the chargeaquired by the silk cloth is negative.

It is observed that when a charged glassrod is brought near to a charged plasticstraw which is rubbed with polythene sheet,there is attraction between the two. What do you think about the kind of

charge on the plastic straw?You may guess that the plastic straw

would carry a negative charge.Is your guess correct or wrong?Discuss with your teacher.The electrical charges generated by

rubbing are static. They do not move bythemselves.

When charges move, they constitute anelectric current. You studied about thecurrent in a circuit which makes a bulbglow, or the current that makes a wire getheated in class VI & VII.

The electric current in a circuitrepresents motion of charges.

Activity-3

To find the presence of chargeon a body

Make a small ball of thermocol.Collect thin silver foil used to decoratesweets. Wrap this thin silver foil to coverthe thermocol ball and suspend it from astand with the help of a thread as shown inthe figure 4.

126 Some Natural Phenomena

Do the foil strips behave in the sameway in all cases?Can this apparatus be used to detect thepresence of charge on a body or not?Can you explain why the strips repeleach other?The strips of aluminium foil receive the

same charge from the charged refill throughthe paper clip. The strips carrying similarcharges repel each other and hence theymove apart.

This device can be used to test whetheran object is carrying charge or not. Thisdevice is known as electroscope. (Earlierdays gold foil used in electroscope). In theabove activity you can observe thatelectrical charge can be transferred from acharged object to another through a metalconductor.

Touch the end of the paper clip gentlywith hand and you will find a change in thefoil strips. They move closer and comeback to their original state.

Repeat charging of foil strips andtouching the paper clip. Every time you willfind that the foil strips collapse as soon asyou touch the paperclip with hand.

Why does it happen?The reason is that the foil strips lose

charge to the earth through your body. Wesay that the foil strips are discharged.

The process of transferring of chargefrom a charged object to the earth is calledearthing.

Earthing is provided in buildings toprotect us from electrical shocks due toany leakage of electrical charge.

when we brought the glass rod near the ball,as both of them have similar charge the ballgets repelled by the glass rod.

From the above experiment we canconclude that attraction is not a sure testto know the presence of a charge on a body.

Activity-4Demonstrating transfer of charge

Take an empty jam bottle. Take a pieceof cardboard slightly bigger in size than themouth of the bottle.

Pierce a hole in it so that a metal paperclip could be inserted. Open out paper clipas shown in the fig-5

Fig-5Cut two strips of aluminium foil about

4 cm×1 cm each and hang them on the paperclip.

Insert the paper clip having the stripsof aluminium foil in to the cardboard lidso that it is perpendicular to it as shown inthe fig-5.

Charge a refill and make it touch theend of the paper clip. Observe what happens.

Is there any effect on the strips ofaluminium foil?Do they repel each other or attract eachother?Now bring other charged bodies andmake them touch the end of the paperclip.

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The Story of Lightning

Fig-6It is now possible to explain lightning

in terms of the charges produced byrubbing. You have learnt in Class VII thatduring the development of a thunderstormthere will be fast movement of air currents.

The clouds moving in air acquire acharge on their surface due to the frictionwith particles of air. As the surface area ofa cloud is very large, the amount of chargeaccumulated on its surface is very high.

When a charged cloud comes close toanother cloud it induces an opposite chargeon the later and the accumulated chargetries to move from one cloud to anothercloud.

But the air present between them beinga poor conductor of electricity resists theflow of charge between them.

When the magnitude of theaccumulated charge becomes very large,the air, which is normally a poor conductorof electricity, is no longer able to resisttheir flow.

Hence discharge takes place betweennegative and positive charges whichproduce streaks of bright light and sound.We see streaks as lightning. The process iscalled an electric discharge.

The process of electric discharge canoccur between two or more clouds, orbetween clouds and the earth. Today weneed not get frightened by lightning like theancient people did. Now we understand thephenomenon.

Scientists are trying hard to improveour understanding. However, lightningstrike could destroy life and property. It is,therefore, necessary to take measures toprotect ourselves.

Lightning Safety1. Which is the safe place during a thunder

storm?A house or a building of low heightIf you are travelling in a bus or in a caryou are safe inside provided that doorsand windows are closed.If you are in a forest, taking shelterunder shorter trees than a taller tree isa good idea during the thunder storm.

2. Which is not a safe place during athunder storm?Travelling in an open place.Standing under tall trees in open fieldsor in a garden / park.Staying in multi storied building whichdo not have lightning conductors.Standing near electric poles ortelephone poles.Speaking on landline telephones.Using electric appliances like T.V andComputers.

Lightning ConductorsLightning Conductor is a device used

to protect buildings from the effect oflightning. A metallic rod, taller than the

128 Some Natural Phenomena

building, is installed in the walls of thebuilding during its construction.

One end of the rod is kept out in the airand the other is buried deep in the ground asshown in fig-7. The rod provides an easy routefor the transfer of electric charge to the ground.

The projected end of the metal rod isat a height more than the height of thebuilding. Hence it receives the charge firstduring lightning because it is closer to thecloud than the building.

As it is a good conductor of electricity,it allows all the charge to flow through itthereby causing no damage to the building.

Fig-7The metal columns used during

construction and water pipes in thebuildings also protect us to some extent.But do not touch them during athunderstorm.

Earthquakes Natural phenomena like lightning,

floods, cyclones etc., can cause largescale destruction of human life andproperty. Fortunately, these phenomena canbe predicted to some extent.

The metrological department can warnabout a thunderstorm developing in somearea. If a thunderstorm occurs there is

always a possibility of lightning andcyclones accompanying it.

So, we get time to take measures toprotect ourselves from the damage causedby these phenomena.

There is, however, one naturalphenomenon which we are not yet able topredict. It is an earthquake. It can causedamage to human life and property on a hugescale.

A major earthquake occurred in Indiaon 8th October 2005 in Uri and Tangdhartowns of North Kashmir. Before that amajor earthquake occurred on 26th January2001 in Bhuj District of Gujarat.

Activity-5

Collecting information about thedamages caused by earth quakes

Ask your parents about the hugedamages to life and property caused bythese earthquakes. Collect a few picturesshowing the damage caused by theseearthquakes from newspapers andmagazines of those days.

Prepare a short report on the sufferingof the people during the earthquakes.

What is an earthquake?What happens when it occurs?What can we do to minimise its effects?These are some of the questions which

we shall discuss here.

What is an Earthquake?An earthquake is a sudden shaking or

trembling of the earth lasting for a veryshort period of time. It is caused by adisturbance deep inside the earth’s crust.

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Collect accounts of the devastationcaused by the tsunami in India from yourparents, or other elders in the family or inthe neighbourhood.

What Causes an Earthquake? What could cause a disturbance inside

the earth?

In ancient times, people did not knowthe true cause of earthquakes. Their ideaswere, therefore, expressed in mythical/folkstories. Similar myths were prevalent inother parts of the world.

Now we know that the tremors arecaused due to the disturbance at deep downinside portion of uppermost layer of theearth. This uppermost layer of the earth iscalled crust.

The outer most layer of the earth is notin one piece. It is fragmented. Eachfragment is called a plate. These plates arein continuous motion. [See fig-9(a), 9(b)]

When they brush past on one another,or a plate goes under another plate due tocollision, they cause disturbance in theearth’s crust.

Crust

Mantle

Inner Core

Outer CoreFig-8

Some mythical/folk stories told that theearth is balanced on the horn of a bull andwhen the bull shifts it to the other horn, anearthquake takes place.

How could it be true?Earthquakes occur all the time, all over

the earth. They are not even noticed. Majorearthquakes are much less frequent. Theycan cause immense damage to buildings,bridges, dams and people.

There can be a great loss to life andproperty. The earthquakes can cause floods,landslides and tsunamis.

A major tsunami occurred in the IndianOcean on 26th December 2004. All thecoastal areas around the ocean sufferedhuge losses.

Activity-6Locating the tsunami affectedareas in the map

Take an outline map of the world.Locate the eastern coast and Andaman andNicobar Islands in India. Mark othercountries around the Indian Ocean whichcould have suffered damage.

Fig-9(a)

Fig-9(b)

130 Some Natural Phenomena

It is this disturbance that shows up asan earthquake on the surface of the earth.However, most earthquakes are caused bythe movement of earth’s plates.

Since earthquakes are caused by themovement of plates, the places atboundaries of the plates are considered asweak zones where earthquakes are morelikely to occur.

The weak zones are also known asseismic or fault zones. In India, the areasmost threatened are Kashmir, Western andCentral Himalayas, the whole of North-East, Rann of Kutch, Rajasthan and the Indo– Gangetic Plane. Some areas of SouthIndia also fall in the danger zone as shownin figure 10.

Can we predict when and where the nextearthquake takes place?How do we know the occurrence of anearthquake?Seismologists use two main devices to

measure an earthquake, a seismograph anda seismoscope. The seismograph is aninstrument that measures seismic wavescaused by an earthquake.

The seismoscope is an instrument thatmeasures the occurrence or the time ofoccurrence of an earthquake. Unlike othermeasuring devices, the seismoscope is asimple device which can be used withoutany technological background.

How do we measure the intensity of theearthquake?The power of an earthquake is

expressed in terms of a magnitude onRichter scale. The destructive earthquakeshave magnitudes higher than 7 on theRichter scale.

Both Bhuj and Kashmir earthquakes hadmagnitudes greater than 7.5.

Although, we know for sure whatcauses an earthquake, it is not yet possibleto predict when and where the nextearthquake might occur. Tremors on theearth can also be caused when a volcanoerupts, or a meteor hits the earth, or anunderground nuclear explosion takes place.

Fig-11

The tremors produce waves on thesurface of the earth (see figure 11). Theseare called seismic waves. The waves arerecorded by an instrument called theseismograph.

Fig-10

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Richter magnitude Earthquake effects

less than 3.5 Generally not felt, but recorded by the sesmograph.

3.5-5.4 Often felt, but rarely causes damage.

5.5-6.0 At most slight damage to well-designed buildings. Can causemajor damage to poorly constructed buildings over smallregions.

6.1-6.9 Can be destructive in areas up to about 100 kilometres acrosswhere people live.

7.0-7.9 Major earthquake. Can cause serious damage over larger areas.

8 or greater Great earthquake. Can cause serious damage in areas severalhundred kilometres across.

The instrument is simply a vibratingrod, or a pendulum, which starts vibratingwhen tremors occur. A pen (stylus) isattached to the vibrating system.

The pen (stylus) records the seismicwaves on a paper which moves under it. Bystudying these waves, scientists canconstruct a complete map of the earthquake.

They can also estimate its power tocause destruction.

Like many other scales, Richter scaleis not linear. This means that an earthquakeof magnitude 6 does not have one and halftimes the destructive energy of anearthquake of magnitude 4.

In fact, an increase of 2 in magnitudemeans 1000 times more destructive energy.For example, an earthquake of magnitude6 has thousand times more destructiveenergy than an earthquake of magnitude 4.

There is another method of measuringthe intensity of an earthquake using themoment magnitude scale which is based onthe amount of displacement that occurredalong a fault zone rather than themeasurement of ground motion at a givenpoint.

The moment magnitude measuresenergy released by the earthquake moreaccurately than the Richter scale. It is theonly magnitude scale that adequatelymeasures the size of large earth quakes.

Fig-12: sesmograph

Table - 3 : Richter scale reading and affects of earthquake

132 Some Natural Phenomena

Protection against earthquakesWe know from the earlier pages that

earthquakes cannot be predicted. We havealso seen that they can be highly destructive.It is, therefore, important that we takenecessary precautions to protect ourselvesall the time.

People living in seismic zones, wherethe earthquakes are more likely to occur,have to be specially prepared. First of all,the buildings in these zones should bedesigned so that they can with stand majortremors.

Modern building technology can makeit possible. It is advisable to make thestructure simple so that it is ‘Quake Safe’.

Consult qualified architects andstructural engineers.In highly seismic areas, the use of mudor timber is better than heavyconstruction material. Keep roofs aslight as possible. In case the structurefalls, the damage will not be heavy.It is better if the cupboards and shelvesare fixed to the walls, so that they donot fall easily.

Be careful where you hang wall clocks,photo-frames, water heaters etc., sothat in the event of an earthquake, theydo not fall on peopleSince some buildings may catch firedue to an earthquake, it is necessarythat all buildings, especially tall

buildings, have fire fightingequipment in working order.

The Central BuildingResearch Institute, Roorkee, hasdeveloped to make quake proofhouses.

In the event that an earthquakedoes strike, take the followingsteps to protect yourself:

Inside the homeTake shelter under a table

and stay there till shaking stops.Stay away from tall and heavy objectsthat may fall on you.

Outside the homeFind a clear spot, away from buildings,trees and over head power lines. Dropto the ground.

Earth quakes in Andhra PradeshDo you know the places in AndhraPradesh where earthquakes haveoccurred and its intensity?According to seismic hazard map Andhra

Pradesh lies in zones II and III. The southeastern districts of Chittoor, YSR Kadapa,Nellore and Krishna and Godavari deltaregion have been placed in zone III and thecity of the Hyderabad lies in zone II. Discussabout zones with your teacher.

Fig-13

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Key words

Crust, discharge, earth’s plates, earthquake, electroscope, lightning, lightningconductor, negative charge, positive charge, richter scale, seismograph,Seismoscope, thunder, thunderstorm, transfer of charge, tsunami, tremor

Some objects can be charged by rubbing with other objects.

There are two kinds of charges — positive charge and negative charge

Like charges repel and unlike charges attract each other.

The electrical charges produced by rubbing are called static charges.

When charges move, they constitute an electric current.

An electroscope may be used to detect whether a body is charged or not.

Attraction is not a sure test to know the presence of charge on a body.

The process of transfer of charge from a charged object to the earth is calledearthing.

The process of electric discharge between clouds and the earth or betweendifferent clouds causes lightning.

Lightning strike could destroy life and property.

Lightning conductors can protect buildings from the effects of lightning.

An earthquake is a sudden shaking or trembling of the earth.

Earthquake is caused due to the disturbance deep inside the earth’s crust.

It is not possible to predict the occurrence of an earthquake.

Earthquakes tend to occur at the boundaries of earth’s plates. These boundariesare known as fault zones.

Destructive energy of an earthquake is measured on the Richter scale. Theearthquake measuring 7 or more on Richter scale can cause severe damage tolife and property.

We should take necessary precautions to protect ourselves from earthquakes.

What we have learnt?

134 Some Natural Phenomena

1. Which of the following cannot be charged easily by friction? (AS1)

(a) A plastic scale (b) A copper rod

(c) An inflated balloon (d) A woollen cloth.

(e) Piece of wood

2. When a glass rod is rubbed with a piece of silk cloth the rod (AS1)

(a) and the cloth both acquire positive charge.

(b) becomes positively charged while the cloth has a negative charge.

(c) and the cloth both acquire negative charge.

(d) becomes negatively charged while the cloth has a positive charge.

3. Identify True or False sentences among the following: (AS1)

(a) Like charges attract each other (T/F)

(b) A charged glass rod attract a charged plastic straw (T/F)

(c) Lightning conductor cannot protect a building from lightning (T/F)

(d) Earthquakes can be predicted in advance (T/F)

4. Sometimes, a crackling sound is heard while taking off sweater during winter.Explain. (AS1)

5. Explain why a charged body loses its charge if we touch it with our hand. (AS1)

6. Name the scale on which the destructive energy of an earthquake is measured. Anearthquake measures 3 on this scale. Would it be recorded by a seismograph? Is itlikely to cause much damage? (AS1 )

7. Suggest three measures to protect ourselves from lightning. (AS1)

8. Explain why a charged balloon is repelled by another charged balloon whereas anuncharged balloon is attracted by a charged balloon? (AS1)

9. List three states in India where earthquakes are more likely to occur. (AS1)

10. Does your habitation lie in a earthquake prone area? Explain. (AS1 )

11. Which place in Andhra Pradesh experiences earthquakes most of the time? (AS1)

Improve your learning

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12. When does a piece of matter have a “charge?” (AS1)

13. What happens if two objects having the same charge are brought close to eachother? What happens if two objects having different charges are brought close?Can you give an example for this? (AS1)

14. Give two examples of effects in your daily life which are caused by transfer ofcharges. (AS1)

15. Inflate two balloons and rub both of them with a cloth first and then with differentmaterial. Will they attract each other in both cases? (AS3)

16. Which country in the world is frequently effected by earth quakes? Collect theinformation and photographs on the recent earthquake in Japan. (AS4)

17. Find out if there is an organisation in your area which provides relief to thosesuffering from natural disaster. Enquire about the type of help they render to thevictims of earthquakes. Prepare a brief report on the problems of the earthquakevictims. (AS4)

18. How do you relate the energy release during the collision of fault lines duringearthquake to the atmospheric variation on the surface of the earth? (AS4)

19. Describe with the help of a diagram an instrument which can be used to detect acharged body. (AS5)

20. Colour seismic zones in India out line map. (AS5)

21. Prepare a model of seismograph. (AS5)

22. How do you appreciate the efforts of scientists to develop an instrument to measurethe intensity and detect the source of earthquake? (AS6)

23. Suppose you are outside your home and an earthquake occurs. What precautionwould you take to protect yourself? (AS7)

24. The weather department has predicted that a thunderstorm is likely to occur on acertain day. Suppose you have to go out on that day. Would you carry an umbrella?Explain. (AS7 )

25. If earthquake occurs in your area what will you do? (AS7)

26. What are the measures you would take in your house when an earthquake occurs?(AS7)

136 Stars and Solar System

Chapter

10STARS AND

THE SOLAR SYSTEM

Observing the night sky is afascinating experience for everyone. Youmight have watched the clear blue sky sometimes and also have observed sunrise andsunset several times. What have youobserved in the sky? What do you knowabout celestial objects and their movement?

Elderly people seem to know a lot ofthings. Some of them can tell the time ofthe day simply by looking at the shadowsof some objects. How do they make suchguesses?

Read the following questions and checkwhat you know about the sky and our earth.

What are the celestial objects that wecan see in the sky?Are the stars moving?Do you see the same stars at night andearly in the morning?Do you see the same stars duringsummer and winter nights?What is the shape of the moon? Whydoes it change? Why doesn’t the sunchange its shape daily like the moon?Where exactly is the sun situated in thesky at noon?Why does the shadow of a tree changefrom morning to evening?

It is definitely interesting to knowanswers to the above questions, but it willbe more interesting to understand how ourancestors came to an understanding aboutall these in the olden days and what theyobserved and how they observed the sky andcelestial bodies without using instrumentslike telescopes.

One thing we have to remember, thatpeople made these observations from theearth and not from any other point in thesky. We shall now perform some activitieswhich will help us understand the abovequestions with some insights.

Activity-1Observing the changes in thelength of shadow

This experiment should be performedon a day when the sky is clear, preferablybetween nine in the morning and four in theevening.

Pick a spot in the open ground whereyou can be sure to have sunlight throughoutthe day. Also there should not be any treesor buildings nearby which can cast a shadowon this spot during the period of theexperiment.

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• Look at your table and figure out thetime of the day of shortest shadow.

• When did you observe the longestshadow in your activity?

• How does the length of the shadowchange with time? Illustrate youranswer with the help of some diagrams.Draw the diagrams of the stick and itsshadow for 5 different times, that is, at9am, 11am, 12noon, 2pm, and 4pm.

• If you continue your activity fromsunrise to sunset, at what times do youthink the shadow would be the longest?

• Where is the sun situated in the sky atnoon? Where does the shadow of stickfall at that time? Think about how yourown shadow will be at that time.

• Do you think that your shadow will bethe same on all the days at noon?

• In which direction does the shortestshadow of the stick fall in your activity?The shortest shadow cast by a vertical

object on the ground always falls in thenorth–south direction. You can use this factto locate directions. The time when theshortest shadow occurs is called the localnoon time at that place.

Think and Discuss

Look at the nails or pegs you havefixed on the ground to keep track of theshadow of the stick throughout the day.From their positions, can you tell howthe position of the sun changes in the skyfrom sunrise to sunset?

The spot should be as flat as possible.You may find such a spot on your schoolplayground.

Take a stick which is a little over ameter long and fix it vertically in theground. Ensure that exactly one meter ofthe stick remains above the surface ofground. You could even build a fencearound your stick as shown in figure-1 tokeep people away from it.

Fig-1: Changes in lengths of shadowMake your first observation at nine in

the morning. Make a mark with a nail orpeg at the point where the tip of the shadowfalls on ground. Measure the length of theshadow.

Then, make similar observations forevery half an hour throughout the day tillfour in the evening.

Use a clock to fix the time for makingyour observations. Enter the measurementsof the length of the shadow and the time ofmeasurement in a table making twocolumns, one for time and another forlength of shadow.

(Since you will be making observationsover the next two weeks at least, you shouldensure that the pegs and stick are notdisturbed.)

138 Stars and Solar System

Continue your observationsObserve on the next day whether the

shadow of the stick falls at the same spotsat the same times throughout the day.

Can you use your stick as a clock (sundial) to tell the time? If your answer is ‘yes’,explain how this is possible.

Two weeks later, once again check tosee whether the stick’s shadow falls at thesame spots at the same times during the day.

If the shadow does not fall on the samespot, what could be the possible reason?

You observed in activity 1 that theposition of the sun in the sky changes duringthe day. If you continue your experimentfor a full year, you will find that the positionof the sun changes from day to day as well.

That is, the position of the sun at 10.amtoday will be different from its position twoweeks later at the same time. If you choosea particular time every week and mark theposition of the sun with a peg at that time,you can build a calendar for the full year.You could use this calendar for thefollowing year to figure out dates.

In the olden days, people used tocalculate the time by observing the shadowsof different objects.

During a period of two weeks you hadmade an observation that the length of theshadow at a particular time is changing dayby day. Did it become longer or shorter?

By observing the direction of shadows,can you guess the arrival of summer orwinter?

Does the sun rise at the same spotthroughout the year? Let us do an activityto understand this.

Activity-2

Understanding the North – Southmovement of the sun.

Fix a spot near your home from whereyou can observe the sunrise. You may haveto go to the terrace of a RCC building orgo to an open field for the purpose. Choosea tree, electric pole or some otherstationary object as a reference point. Overthe next 10 to 15 days, note the spot atwhich the sun rises daily, keeping in mindyour reference point. Make a daily sketchof the rising sun as well as your referencepoint in your note book during this period.(See figure-2).

Fig-2 : Observing the position of sunDoes the spot of sunrise change? If itdoes, in which direction does it seem tomove?

When the sun looks like travelingtowards south of the sky, it is called thedakshinayanam. When it looks liketraveling towards north of the sky it is calledthe uttarayanam. (Ask your parents aboutUttarayanam and Dakshnayanam)• Was the sun appear traveling towards

south or north during the time you madeyour observations?

• Do you think that is the reason for the

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Latitude in DegreeSl.No. Districts North (rounded to

whole number)1. Chittoor 132. Nellore, YSR Kadapa, Anantapur 143. Prakasam, Kurnool, Guntur 154. West Godavari, Krishna, Mahabubnagar 165. East Godavari, Visakha, Ranga Reddy,

Hyderabad, Khammam, Nalgonda 176. Srikakulam, Vizianagaram, Medak,

Nizamabad, Karimnagar, Warangal 187. Adilabad 19

A list of latitudes of districts of Andhra Pradesh is given below in table 1.

change in the length of the shadow ofthe stick day by day in activity 1?

• Assuming that you did not have anycalendar and knowledge of months andseasons, can you use movement of thesun to predict the arrival of winter orsummer?

Think and Discuss

Why does the sun appears to traveltowards north or south? Try to find theanswer by reading your social studieschapter “Earth movements andseasons” along with this lesson.

Collect the information : Are thetimings of rising and setting of the sunsame every day? Collect the informationfrom news papers for at least a period ofa fortnight. Think why the lengths of dayand night are different every day. Try toget answers through internet or fromother books or from teachers.

Activity 1 can be used to make asundial (a clock based on shadows of anobject due to sunlight). But the length ofthe shadow of our stick is changing day today because of the north – south movementof the sun which is a problem in making asundial.

People in olden days overcame thisproblem and made sundials also.

There is only one Sundial in our statewhich is situated in the premises ofSatyanarayana Swamy Temple inAnnavarayam, East Godavari Dist.

How can we make our own sundial?

Activity -3

Make your own sun-dialFirst of all, you will need to cut a right-

angled triangle ABC from a sheet ofcardboard. Angle C of the triangle shouldequal to the latitude of your city or townand angle A should be 90 degrees, as shownin figure 3.

Do you know?

Table -1

140 Stars and Solar System

Fix your cardboard triangle verticallyin the middle of a rectangular woodenboard. Glue strips of paper along both edgesof BC and the wooden board to make thetriangle stand erect.

Place your board with the triangle onlevel ground in an open space which getssunlight throughout the day. Base BC of thetriangle should be placed in the north-southdirection, with B pointing to the north.

At nine in the morning, draw a linealong the shadow of side AC on the woodenboard. Write the time alongside the line.Draw lines of the shadow of side AC at one-hour intervals (use a clock to check thetime) through the day till sunset and markthe time for each line. Your sun-dial isready.

You can tell the time by looking at theshadow on the sun-dial. But remember thatbase BC of the triangle in the sun-dial mustalways be in the north-south direction if youwish to read the time correctly.

(You can find out the north-southdirection at a place by using a stick like inActivity 1, but don’t use magneticcompass.)

Now we will try to know a few thingsabout the moon.

Fig-3 Sun dial

A

B C SN

Fig-4:Drawing the phases of the Moon

• Have you ever observed the movementof moon in the sky?

• Does the moon appear at same point ata particular time every day?

• Is the shape of the moon same on everyday?To understand these issues, let us do

some activities. (You can do these activitieson your own at home.)

Activity-4

Observing phases of the moon1. Note the date of the day after new

moon day (Amavasya), when the moon firstappears in the sky.

Also note the time at night when themoon sets (goes down in the western sky).In the same way every day locate the moonin the sky at the time of sunset orimmediately after sunset.

Record the date and time of the moonset and draw a picture of the moon as yousee it on that day in your note book as shownin figure 4.

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Continue making observations for asmany nights as possible.

2. Observe the moon a few daysbefore full moon day (Pournami) to a fewdays after it. Locate the position of moonin the sky at the time of sunset beforePournami and note the time and positionof moon in the sky at that time.

After Pournami, note the time at whichthe moon rises (comes in the eastern sky)and also note the date. Draw pictures of theshape of the moon on each of these days.

What do you understand from theseobservations?• Can you calculate the number of hours

between one moon rise and the nextmoon rise or the number of hoursbetween one moon setting and the nextmoon setting, with help of theseobservations?

• How many hours lapse between onesunrise to the next, or one sunset to thenext?

• Is the time period same for sun andmoon to appear at selected positionafter completing a cycle in the skyevery day?

• Does the moon appear at the same pointevery day during the time of the sunset?

• What is the shape of the moon? Is itsame every day?You might have observed that the shape

of the moon changes night after night.These changes in its appearance are calledthe phases of the moon. Can you guess whythe shape of the moon changes?

You may have noticed that the timeperiod taken by sun to complete a cycle inthe sky and come to selected position isalmost same every day and it is about 24hours i.e. 1 day. Whereas moon takes about50 minutes more than a day to completethe cycle and which results in the phasesof the moon.

Let us perform two activities tounderstand why the phases of moon occur.

Activity-5A Moon-shaped lemon

Choose a day one week after the newmoon day when the moon is visible in thesky before the sunset.

Take a yellow lemon or a whitewashedclay ball and pivot it on a long needle or ona spoke of bicycle. Hold it up towards themoon as shown in figure 5.

Fig-5 : Observing the shape formed bysun light on a lemon

Ensure that you are standing in thesunshine when you do this activity.

142 Stars and Solar System

Fig-7 : Phases of moon

sun rays

Fig-6

Observe the shape formed by thesunlight on the surface of the lemon.

Is there some similarity between theshape formed and the shape of the moon?

Activity-6

Why does the shape of the moonchange?

(Do this activity around 4p.m.)Wrap a ball tightly with a white

handkerchief or with a piece of white cloth.Assume this is the moon. Hold this ball infront of your eyes in bright sunshine asshown in figure 6 and turn around yourselfslowly. Observe how the shape of theilluminated part of the ball changes.• Does sunlight fall on half the ball at all

times while you turn around?

• Is the shape of the illuminated part onthe ball viewed by you same in allpositions during your rotation?

• Why does this happen?

To understand the reason better, lookat figure7 carefully.

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The large circle in the middle of figure7is the earth and the smaller circles aroundit represent the moon in differentpositions. You can also see the phases ofthe moon on different days in the figure.The sunrays falling on the moon illuminatehalf its surface in all the positions.However, we cannot see the entireilluminated surface from the earth in all thepositions. In some cases we see the entireilluminated surface while in others we seeonly part of it. In one particular position,we cannot see the illuminated surface at all.

The shape of the moon we see is theshape of the illuminated portion visible tous.

In figure 7, the day of the new moon iscalled day 0 or day 28 (position 1). In thisposition, the illuminated surface is notvisible from earth, so the moon cannot beseen from earth.

Four days later, when the moon is inposition 2, a small part of its illuminatedsurface is visible from earth. On day-7, themoon is in position 3, so more of itsilluminated part is visible from earth.

After fourteen days (at position 5) theentire illuminated surface of the moon isvisible from earth. This is the day of thefull moon.

Subsequently, the moon appearssmaller with each day as it passes throughpositions 6 (day-18), 7(day-21) and 8(day-25). After 28 days, the moon is once againin position 1.

Try to duplicate position 1 with the ball.For this, you will have to hold the ball

towards the sun (between your eyesight andthe sun).

In this position, which half of the ballis illuminated?Although half the surface of the moon

is illuminated everyday, we cannot see themoon on new moon day since theilluminated surface is on the side oppositeto the point of observation on the earth. Ona full moon day, the situation is reversed.The illuminated half of the moon faces thepoint of observation, so we see a full moon.

From the above explanation, you mayhave understood that the sun and moonmust be on the same side of the earth on anew moon day and they are on oppositesides of the earth on a full moon day.

During the phases in between, we seedifferent shapes of the moon.

Hold the ball in different positions anddraw pictures of the shapes of the visibleilluminated portion in each case.

The different shapes of the moon in itsdifferent phases, as seen from the earth, areshown in figure 7. Compare the drawingsyou have made with those in figure 7.

Can you now state as to in whichdirection the moon will rise on a fullmoon day?While we observe moon in clear sky

on a full moon day, we think about the spotsthose are visible on the moon. In olden daysalso people were curious about those spotson the moon. They did not know the natureof the surface of moon as we know it today.This led to creation of a lot of stories andmyths about the spots on the moon.

144 Stars and Solar System

Do you know any such stories?Today we have many satellites that

observe the surface of the moon. Humanbeings landed on the moon way back in1969. We have better information aboutmoon’s surface than our ancestors.

The surface of the moonWhen astronauts landed on the moon,

they found that the moon’s surface is dustyand barren. There are many craters ofdifferent sizes. It also has a large numberof steep and high mountains. Some of theseare as high as the highest mountains on theearth. But the moon has no atmosphere likethat on the earth.

Will we be able to hear any sound if wewere on the moon? Why?Can any life exist on the moon? Why?

Do you know?Our country launched Chandrayan-1

(Satellite to Moon) on 22nd October2008 to know about the moon. The objectives of Chandrayan-1 are:

1. To check the possibility of finding water on the moon

2. Finding out the elements of matter onmoon

3. To search for Helium-34. To make 3-dimensional atlas of the moon

5. To study about the evolution of thesolar systemNow India is one of the six countries

which have sent satellites to the moon.Search the findings of Chandrayan-1 oninternet or in news papers.

Think and Discuss

Scientists are planning to buildsettlements on moon and are trying tomake arrangements to live there. Youknow that there is no air on moon. Howwill it be possible to live on the moonthen?

We feel pleasant during the night offull moon. But sometimes on full moonday the moon loses its brightness forsome time, it appears covered – partiallyor fully. This is called Lunar eclipse.Why does the moon get covered? Likethe moon, the sun also gets coveredpartially or fully on some of the newmoon days. This is called Solar eclipse.Let us try to understand thesephenomena.

Solar EclipseA solar eclipse occurs when the

shadow of the moon falls on the earth. Itoccurs only on new moon day.

Types of solar eclipse1. Total Solar eclipse: It occurs when the

moon completely covers the Sun, asseen from earth.

2. Partial Solar eclipse: It can beobserved when only the partially shadedouter region of the shadow cast bymoon (Lunar penumbra) touches theearth.

3. Annular eclipse: It occurs when themoon appears smaller than the sun as itpasses centrally across the solar diskand a bright ring, or annulus, of sunlightremains visible during eclipse.

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4. Hybrid eclipses: These are a rare formof Solar eclipse, which changes froman annular to a total solar eclipse alongits path.

Lunar eclipseA lunar eclipse occurs when the

shadow of the earth falls on the moon. Itoccurs only on full moon day.

Types of lunar eclipse1. Total Lunar eclipse: It occurs when

the earth’s shadow(Umbra) obscures allof the moon’s visible surface.

2. Partial Lunar eclipse: It can beobserved only when part of the moon’svisible surface is obscured by theearth’s shadow.

3. Penumbral Lunar eclipse: It happenswhen the moon travels through thepartially shaded outer region of theshadow cast by the earth(earth’spenumbra).Why does a lunar eclipse occur onlyon a full moon day?

According to figure-7, in whichposition can the shadow of the earth fallon the moon?Can this position occur only on oneparticular day?Can you now explain why a solar eclipseoccurs only on a new moon day?However, why is it that a solar eclipse

does not occur on every new moon day anda lunar eclipse does not occur on every fullmoon day? Let us try and understand thereason.

A total solar eclipse occurred on theafternoon of February 16, 1980 (it wasseen in Mahaboobnagar, Nalgonda andKhammam districts and also in some areasof Krishna district in Andhra Pradesh.)Because the sun was covered during theeclipse, it looked like night during the daytime.

Figure 8 contains a sketch of the timeexposure photograph of this eclipse. Thatmeans the exposures of the different stagesof the eclipse were made at 10-minuteintervals on the same frame.

Fig-8

146 Stars and Solar System

The sketch from left to right shows themoon slowly covering the sun and thenmoving away. The uncovered portion of thesun appears white and the black circlesrepresent the moon in the sketch.

You can easily guess the position of themoon at each stage of the eclipse. Can youdraw lines tracing the paths of the sun andmoon in the sketch?

The white portions of the discs infigure8 represents the sun and the blackportions represents the moon. Each ofthese discs depict the position of the sunand moon at various stages of the eclipse.

Make 2 discs, one white and the otherblack, as the size of sun and moon as infigure 8.

We shall now find the centers of thesun and moon at each stage. To do this, takethe white disc you have made and place itexactly on the white portion of any of thestages in the diagram.

Pierce a hole through the center ofyour disc with a pin to mark the spot at thecenter of the sun’s position at that stage inthe diagram. Remove the white disc andmark the spot with a pencil.

In this way, mark the sun’s center atevery stage of the eclipse in your diagram.Join these spots with a line. This linedepicts the path of the sun.

To find the moon’s path, repeat theexercise, but this time use the black discand mark the centers of the black portionsat each stage of the eclipse. Join thesespots with a line and you will get the pathof the moon during the eclipse.

Do the sun and moon follow parallelpaths or do their paths cross each otherduring the course of the eclipse?

There is another aspect to note infigure8. The sun and moon reached thepoint of intersection of their paths atexactly the same time during the eclipseon February 16, 1980.

If this had not happened, would a totalsolar eclipse still have occurred?Can you now tell why a solar eclipsedoes not occur on every new moon day?What would be the difference in the

position of the sun and moon on new moondays when no eclipse takes place and whenthere is an eclipse? Use the sketch to tryand figure out your answer.

Some other fascinating celestialobjects in the sky are stars. They usuallyappear in groups and members of thesegroups when viewed together formdifferent shapes. People used to assignsome shapes of animals and human beingto those small groups. Those groups arecalled constellations. A group of starswhich contains millions of stars are calledgalaxy. Millions of galaxies together makeour universe.

Let us know something about starsWhen you look at the night sky, do the

stars appear to be moving? If you wish tostudy the movement of stars across the skyand to trace their paths you must observethe pole star, the seven stars of theSaptarishi (Great Bear) constellation andthe six stars of the Sharmistha (Cassiopeia)constellation.

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You can easily recognize the great bearwith its rectangular head in the northern sky(figure 9a).

Fig–9(a): Great bear constellation(saptarishi) - position of pole star

In winter, this constellation rise a fewhours before sunrise (wecan see this fromanywhere in AndhraPradesh). In this season,you can also seeCassiopeia in the samepart of the sky, its six starsforming the letter ‘M’(figure 9b)

Fig-9(b): Cassiopeia constellation(Sharmistha) - position of pole star

You can locate the pole star ( Polaris )with the help of these two constellations.If you are able to spot only the great bear,look at the two stars that form the outerside of its rectangular head. Extend animaginary line from these two stars (asshown in figure 9a). The pole star will belocated on that extended line with adistance of about 5 times the distancebetween these two stars.

If only Cassiopeia is visible, the polestar will be located on a line extended fromthe middle star of the ‘M’ (see figure 9b).

Once you have located the Great Bear,Cassiopeia and pole star in the night sky,do the following activity.

Activity -7

Observing the movement ofconstellations (stars)

Take a 20cm x 20cm square sheet ofpaper and make a 1cm diameter hole in itscenter. Mark a cross ( X ) on one side ofthe sheet of paper as shown in figure 10.

Hold the sheet in front of your eyeswith the ‘x’ mark at the bottom and lookfor the pole star through the hole. Once youhave located the pole star, check in whichdirection the Great Bear or Cassiopeia lie.

Write ‘G’ for Great Bear and ‘C’ forCassiopeia on the paper in the directionsin which you see each of the constellations.Mark the timing at which you made yourobservation in both cases.

Pole star

Pole star

Fig-10

148 Stars and Solar System

Choose a nearby tree or house as areference point. Draw a picture of yourreference point on the paper sheet taken,clearly indicating its location.

Repeat your observations at one-hourintervals. Ensure that you are standing onthe same spot each time you look at thestars. Write G, C in the direction of theposition of the great bear and Cassiopeiaduring each observation and note the timeof the observation next to the letters G andC.

Using the tree or house you havechosen as your reference point, checkwhether the position of the pole star haschanged or not. If it has changed, note thechanged position.

Repeat this activity as many times aspossible, the minimum being four times.But ensure that the ‘x’ mark on your sheetof paper remains at the bottom during allyour observations.

You could also use other known starsor constellations close to the pole star toperform this activity.

Study the picture you have drawn andanswer the following questions.

Do the positions of the stars changewith time?Does the position of the pole star alsochange with time?Does the shape of the great bear andCassiopeia change with time or doesthe position of the entire constellationsin the sky change?What kind of path do theseconstellations trace in the sky?From your observations, you would

have realized that the stars do not remainin the same spot in the sky but revolvearound the pole star. The pole star, however,remains fixed at one place. It takes the stars24 hours to complete a revolution aroundthe pole star. We can observe only half thisrevolution during the course of a night.

If all stars move, why doesn’t the polestar move? Let us try to understand it bydoing the following activity.

Activity -8

Why does the pole star appearsfixed at one point?

Take an umbrella and open it. Makeabout 10 – 15 stars out of white paper having5 cm length, 2.5 cm breadth. Paste one starat the position of the central rod of theumbrella and others at different places onthe cloth near the end of each spoke (figure11).

Fig-11

Now rotate the umbrella by holding itscentral rod in your hand. Observe the starson the umbrella. Is there any star whichdoes not appear moving? Where is this starlocated? Is it located where the rod of theumbrella holds the cloth of the umbrella?

On similar lines, if there were a starlocated where the axis of rotation of the

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earth meets the sky, couldthis star also be stationary?

The pole star is situatedin the direction of the earth’saxis and that is why it does notappear to move even thoughall stars appears that they aremoving because of therotation of the earth.(figure 12).

Fig-13: Saptha rushi Sharmista Orion Leo ( Simha Rashi )

Some of the Galaxies in our Universe

Some of the constellations visible from Andhra pradesh

Fig-14

polaris

Towards

the sun

Earth’sOrbital Plane

Fig-12:Direction of Pole Star

150 Stars and Solar System

Among millions of galaxies, our sun isa star in Milky Way galaxy. Our earth isrevolving around the sun. And moon isrevolving around the earth. Do you knowthat not only the earth but also some othercelestial bodies are revolving around thesun? Let us know something about thosecelestial bodies.

The solar systemThe sun and the celestial bodies which

revolve around it form the solar system. Itconsists of large number of bodies such as

planets, comets, asteroids and meteors. Thegravitational attraction between the sun andthese objects keeps them revolving aroundit.

The earth revolves around the sun. It isa member of the solar system. It is a planet.There are seven other planets that revolvearound the sun. The eight planets in theirorder of distance from the sun are:Mercury, Venus, Earth, Mars, Jupiter,Saturn, Uranus and Neptune.

Figure -15 shows a schematic view ofthe solar system.( Not to the Scale ).

Let us learn about some members ofthe solar system.

The SunThe Sun is the

nearest star to us.

Fig-15It is continuously emitting huge

amounts of heat and light. The sun is thesource of almost all energy on the earth. Infact, the sun is the main source of heat andlight for all planets of our solar system.

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The planetsThe planets look like stars, but they do

not have light of their own. They merelyreflect the sunlight that falls on them.

A planet has a definite path in which itrevolves around the sun. This path is calledan orbit. The time taken by a planet tocomplete one revolution is called itsperiod of revolution. The period ofrevolution increases as the distance of theplanet from the sun increases.

Besides revolving around the sun, aplanet also rotates on its own axis like atop. The time taken by a planet to completeone rotation is called its period ofrotation.

Some planets are known to havemoons/satellites revolving round them. Anycelestial body revolving around anothercelestial body is called its satellite.

The earth revolves around the sun. Doesit make earth a satellite of the sun?

The earth and other seven celestialbodies can be said to be as satellite of thesun, though generally we call them as planetsof the sun. We use the term satellite for thebodies revolving around planets. Moon is asatellite of the earth. There are many man-made satellites revolving round the earth.These are called artificial satellites.

Mercury (Budhudu)The planet mercury

is nearest to the sun. Itis the smallest planetof our solar system.

Because mercury is very close to thesun, it is very difficult to observe it, as mostof the time it is hidden in the glare of thesun. However, it can be observed just beforesunrise or just after sunset, near thehorizon.

So it is visible only at places wheretrees or buildings do not obstruct the viewof the horizon. Mercury has no satellite ofits own.

Venus (Sukrudu)Venus is earth’s

nearest planetaryneighbour. It is thebrightest planetin the night sky.Sometimes, Venusappears in theeastern sky before sunrise.

Sometimes, it appears in the westernsky just after sunset. Therefore, it is oftencalled a morning or an evening star,although it is not a star. Try to locate Venusin the night sky during early winter.

Venus has no satellite of its own.Rotation of Venus on its axis is somewhatunusual. It rotates from east to west whilethe earth rotates from west to east.

Does the sun rise in the east on Venus?

If you get a chance, try to observe Venusthrough a telescope. You will observe thatVenus shows phases just like the moon.

152 Stars and Solar System

The Earth (Bhoomi)The earth is the

only planet in the solarsystem on which life isknown to exist.

Some speciale n v i r o n m e n t a lconditions are responsible for theexistence and continuation of life on theearth. These include just the right distancefrom the sun so that it has the righttemperature range, the presence of waterand suitable atmosphere and a blanket ofozone. We must take special care to protectour environment so that life on earth is notdisturbed.

From space, the earth appears blue-green due to the reflection of light fromwater and landmass on its surface. The earthhas only one moon revolving around it.

Mars (Kujudu / Angarakudu)The next

planet, the firstoutside the orbitof the earth ismars. It appearsslightly reddishand therefore, it is also called the redplanet. Mars has two small naturalsatellites.

Mars science laboratory:National Aeronautic andSpace Administration (NASA) in America started amission called ‘Mars Science Laboratory’ on 26th

November 2011, to know more about Mars. A rovernamed ‘Curiosity’ landed on Mars on 6th August 2012. Itis analyzing the elements in the rocks of the surface ofthe Mars. It found few indications of water on Mars. Itis searching whether the favorable conditions for lifeexist on Mars.

Jupiter (Brihaspati)Jupiter is the

largest planet ofthe solar system. Itis so large thatabout 1300 earthscan be placed insidethis giant planet.However, the massof Jupiter is about 318 times that of ourearth. It rotates very rapidly on its axis.Jupiter has a large number of satellites.

It also has faint rings around, appearsquite bright in the sky. If you observe it withthe help of a telescope, you can also seefour of its large moons.

Saturn (Shani)B e y o n d

Jupiter is Saturnwhich appearsyellowish incolour. Whatmakes it unique in

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Mercury 0.38 5.79 88 days 0Venus 0.95 10.8 225 days 0Earth 1 15 365 days 1Mars 0.53 22.8 687 days 2Jupiter 11.19 77.8 12 years 50Saturn 9.40 142.7 29.5 years 53Uranus 4.04 286.9 84 years 27Neptune 3.88 449.7 165 years 13

Name of thePlanet

ComparativeDiameter with

Diameter ofEarth

Distance fromthe Sun in Crore

km.

Period ofrevolution

No. of satelites(detected so far )

the solar system is that it has rings. Theserings are not visible to the naked eye. Youcan observe them with a small telescope.Saturn also has a large number of satellites.

Uranus and Neptune

These are the outermost planets of thesolar system. They can be seen only withthe help of large telescopes. Like Venus,Uranus also rotates from east to west. Themost remarkable feature of Uranus is thatit has highly tilted rotational axis (figure16). As a result, in its orbital motion itappears to roll on its side.

Fig. 16The first four planets, mercury, venus,

earth and mars are much nearer the sun thanthe other four planets. They are called theinner planets. The inner planets have veryfew moons.

The planets outside the orbit of Mars,namely Jupiter, Saturn, Uranus and Neptuneare much farther away from the sun thanthe inner planets. They have a ring systemaround them. The outer planets have a largenumber of moons.

Table-2 : Comparison between planets

We took Earth’s diameter (12756 Km) as 1 unit. With this information find the diametersof other planets using the comparison given in table-2.

Uranus Neptune

154 Stars and Solar System

AsteroidsThere is a large gap in between the

orbits of mars and Jupiter (figure-17). Thisgap is occupied by a large number of smallobjects that revolve around the sun. Theseare called asteroids. Asteroids exists in thesolar system not only in the Mars-JupiterAsteroid Belt. Asteroids can only be seenthrough large telescopes.

Fig. 17

CometsComets are also members of our solar

system. They revolve around the sun inhighly elliptical orbits. However, theirperiod of revolution round the sun isusually very long. A comet appearsgenerally as a bright head with a long tail.The length of the tail grows in size as itapproaches the sun. The tail of a comet isalways directed away from the sun (figure18).

Fig. 18

Think and Discuss

The diameter of the sun is 13,92,000 Km.The diameter of the earth is 12,756Km.The diameter of the moon is 3,474 Km.The distance from the sun to earth is15,00,00,000 Km.The distance from the earth to moon is3,84,399 Km.

Take the scale as 1 lakh km = 1 cm,and imagine how the arrangement of sun,earth and moon is in our universe. Canyou make this arrangement on yourschool-ground?

Do you know? Till 25th August 2006 we used to say

there are nine planets in our solar system.The ninth planet at that time was ‘Pluto’.In the 26th general assembly ofInternational Astronomical Union it wasdecided that ‘Pluto’ is no more a planet.The decision was taken since it wasobserved that Pluto does not follow therule of “cleared the neighborhood.” Thatmeans, sometimes it is entering into theorbit of Neptune.

Some other members of thesolar system

There are some other bodies whichrevolve around the sun. They are alsomembers of the solar system. Let us knowabout some of them.

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Many comets are known to appearperiodically. One such comet is Halley’scomet, which appears after every 76 years.It was last seen in 1986. Can you tell whenHalley’s comet will be visible again?

Meteors and MeteoritesAt night, when the sky is clear and the

moon is not visible, you may sometimessee bright streaks of light in the sky (fig.19).

Fig. 19These are commonly known as

shooting stars, although they are not stars.They are called meteors. A meteor is usuallya small object that occasionally enters theearth’s atmosphere. It has a very highspeed. The friction due to the atmosphereheats it up. It glows and evaporates quickly.That is why the bright steak lasts for a veryshort time. Some meteors are large and sothey can reach the earth before theyevaporate completely. The body thatreaches the earth is called a meteorite(figure-20).

Fig. 20Meteorites help scientists in

investigating the nature of the materialfrom which solar system was formed.Artificial satellites

You must have heard that there are anumber of artificial satellites which areorbiting the earth. Artificial satellites areman-made. They are launched from theearth. They revolve around the earth muchcloser than earth’s natural satellite, themoon.

India has built and launched severalartificial satellites. Aryabhatta was the firstIndian artificial satellite (figure-21).

S o m eother Indiansatellites areINSAT, IRS,Kalpana-1,E D U S AT,etc.

Artificial satellites have many practicalapplications. They are used for forecastingweather, transmitting television and radio

Fig. 21

156 Stars and Solar System

signals. They are also used for telecommunication, remote sensing(collecting information from a distance) inaviation and military use.

This information about solar system isknown to us for the past 2 to 3 hundredyears, after the telescopes were made. Buthow did the people of olden days know somuch about the celestial bodies and alsoabout earth?

Let us know about some of theinteresting constructions of theirknowledge.How did people come to anunderstanding that earth isspherical?

In olden days people felt that earth isflat because it looks flat. However they hada doubt, if it is flat how does the water inoceans remain there, why does it not spellout of the earth? To get clarity theyassumed a fencing around the flat earth.After that1) They assumed the shape of earth asround by observing the shadow of earth inlunar eclipse. In every eclipse they foundthe shape of earth is in round even thoughthere is a chance of getting linear, ellipticalshadows by a circular object.2) Some sailors, who started their journeyin ocean, reached the same place aftertraveling large distance in one directiononly.3) Observing ships approaching the portalso helped to change their opinion aboutthe shape of the earth, that is, usually theysee smoke of the ship first and then the topof the ship after that the whole ship.4) Observations about the movement ofstars and different stars visible fromdifferent places on the earth also helped tothink about the shape of the earth.

Through all such observations made byso many people at so many places on theearth they came to an understanding thatearth is spherical. And then it is get clarifiedin 1969 when man landed on the moon andobserved the earth’s shape from the moon.

How did people come to anunderstanding that earth rotateson its own axis?

People from olden days thought thatearth is located in the center of the universewith sun, moon and stars moving aroundit. They also thought that sun, moon andstars are located on transparent concentricspheres sorrounding the earth, becausethey are not falling down.

The three spheres are rotating on theiraxis from east to west that is why sun,moon and stars appear to revolve from eastto west around the earth. They also assumedthat the sphere on which sun is locatedrotates east to west and oscillates fromsouth to north that is why uttarayanam anddakshinayanam are happening.

Because of the uneven movement ofsome stars (actually they are planets) whichthey observed, it became very difficult toexplain the model of universe whichrequired so many transparent spheresaround the earth. Nicholas Copernicussuggested that the sun is at the center ofthe universe and all other celestial objectsare revolving around the sun. Then how doday and nights occur. It was assumed thatearth rotates on its axis. This model couldexplain the occurence of day and night.

In this way people came to anunderstanding that earth rotates on its axis.

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Improve your learning

Key wordsCelestial bodies, Local noon, Sundial, Dakshinayanam, Uttarayansam, Phases

of the moon, Constellation, Galaxy, Pole star, Solar system, Planets, Satellites,Artificial satellites, Asteroids, Comets, Meteors, Meteorite.

The shortest shadow cast by a vertical object on the ground always falls in North,South direction.The shortest shadow of on object occurs at local noon.The time duration for appearance of sun and moon after completion of a cycle isdifferent.Changes in appearance of moon are called phases of the moon.On the new moon day, sun and moon are on the same side of the earth.On the full moon day, sun and moon are on either sides of the earth.Moon has no atmosphere like we have on the earth.The polestar is situated in the direction of the earth’s axis and hence it appears asnot moving.There are eight planets in our solar system.Among eight planets of solar system earth is the only planet which supports life.Large number of objects that revolve around the sun between the orbits of Mars andJupiter are asteroids.The length of the tail of the comet grows in size as it approaches the sun.A meteor is a small object that occasionally enters the earth’s atmosphere.A body that reaches the earth is called a meteorite.Aryabatta was the first Indian artificial satellite.Fore casting weather, transmitting Television and Radio signals, Telecommunication,remote sensing are the practical applications of artificial satellites.

1. What is your local noon time? (AS1)2. Where do you find moon at night; a) 2days before pournami b) 2days after amavasya

(AS1)3. Why doesn’t eclipse occur on every full moon day or on every new moon day? (AS1)4. Where do you find the pole star? (AS1)5. What is the difference that you find between polestar and other stars? (AS1)6. Why does polestar seem to be stationary? (AS1)7. Name some constellations. (AS1)8. How many planets are there in our solar system? What are they? (AS1)

What we have learnt

158 Stars and Solar System

9. Look at the table-2 and name the smallest and the biggest planets in our solar system.(AS1)

10. Among all 8 planets what is the special thing about earth? (AS1)11. How do day and night occur? (AS1)12. Do the stars appear moving? How can you say? (AS1)13. Is it possible to see the polestar for the people who live in the southern hemisphere of

the earth? Why? (AS1)14. What is the use of artificial satellites in our daily life? (AS1)15. Why is Venus the brightest planet? (AS1)16. Are you curious about going to the moon? Why? (AS2)17. While observing the shadow of a stick from morning to evening, some questions

arose in Ramya’s mind. What may be those questions? (AS2)18. What are the questions that engage your mind when you look at night sky? (AS2)19. Even though we do not have clock, we can know the time by observing some

shadows in day time. Think and discuss with your friends how we can know the time atnight. (AS2)

20. How can you find north – south direction at your place? (AS3)21. In which direction (towards north or south) is the sun moving day by day when you read

this lesson? (AS3)22. What are the planets you have seen in the sky? When do you observe those planets?

(AS3)23. What is the duration of a day and night today? Collect the information about duration of

day night for the past 7 days from the news papers, analyze it and say whether summeror winter is going to come. (AS4)

24. What are the other districts on the same latitude as your district? (AS4)25. Collect information about cosmic dust (wastage) from news papers, internet and make

a poster on your school panel board about the consequences of cosmic dust. (AS4)26. Make a sundial. Explain how you made it. (AS5)27. Draw the different phases of moon. Arrange them in a order from pournami to amavasya.

(AS5)28. Draw the location of polestar showing the direction from Great Bear. (AS5)29. Draw the diagram of the solar system. (AS5)30. How do you appreciate the construction of knowledge about the Universe by our

ancestors? (AS6)31. We launched many artificial satellites around our earth for different purposes. What do you

think about the impact of artificial satellites and their radiation on bio diversity? (AS7)32. Among eight planets of our solar system, earth is the only planet supporting life. Explain

how we should protect our earth and its environment. (AS7)