GREENWOOD PUBLIC SCHOOL, ADITYAPURAMOUR MOTTO-DEVELOPMENT WITH DELIGHT

CLASS-VIII SUBJECT- PHYSICS

TERM-2 SYLLABUS

CHAPTER - 4SOME NATURALPHENOMENA CLASS

Introduction to Natural Phenomena

Any naturally occurring calamity or physical process is called natural phenomenon.

Two destructive natural phenomena are: lightning and earthquakes.

Static Charges: Staticcharge is the result of an imbalance between negative

and positive charges in an object.

Methods of Charging

A body can be charged by rubbing due to friction or by induction. The electrical charges produced by rubbing are called static charges.

Charging by rubbing

When objects like plastic get rubbed with hair, it acquires a charge. Similarly, a glass rod gets charged when rubbed with a silk cloth.

They get charged as they gain or lose electrons and that is why they can attract or repel the small pieces of paper depending on the type of charge they carry.

Charged objectsObjects that carry a charge by means of rubbing or other processes are called as charged objects. Like charges repel, while unlike charges attract.

Transfer of charges:-Charges can be transferred through good conductors (metals) to another conducting material.

The charge transfer is due to the movement of the electron from one atom to another.

ElectroscopeA device that can detect the presence of charge is called as an electroscope.

Discharged ObjectsWhen objects lose their charge by transfer, they are known as discharged objects.

EarthingThe process of transferring charges from a charged object to the earth is called earthing.

Lightning:-The bright flash (streaks) of light which we can see in the cloud is called lightning.

Lightning: Introduction

During thunderstorms, air currents move upwards and water droplets move downwards. This causes separation of charges between clouds and between clouds and earth.

When the magnitude of charges increases, air (normally a bad conductor) starts conducting and allows the flow of electricity. This is called as lightning, as this flow of charge is accompanied by bright streaks of light and sound.

Electric DischargeThe process of flow of charge from cloud to cloud or from cloud to earth due to the separation of positive and negative charges is called an electric discharge.

Lightning safety: Steps to follow

Outside the house: (a) Find a safe place or shelter under small trees (b) If inside a car or vehicle, stay inside with doors and windows shut (c) Stay away from metal poles (d) Do not lie on the ground. Instead, crouch with your head in between your hands.

Inside the house: (a) Avoid contact with telephone and electrical wires (b) avoid bathing (c) Unplug electrical appliances.

Lightning Conductors

Lightning conductors help to protect buildings during lightning. They consist of a metal rod that is taller than the building which is installed

within the walls during construction. They run all the way to the earth and act as a direct passage for electric discharge during lightning.

Earthquake :- The sudden violent shaking ,rolling or tremor of the earth surface is called Earthquake.

Earthquake

Earthquake is the sudden trembling of the surface of the earth due to disturbance deep inside the earth’s crust.

Causes large-scale damage to life and property. Cannot be predicted.

Causes of earthquakeEarthquakes are caused due to movement or collision of tectonic plates in the uppermost layer of the earth’s crust.

Movement of platesEarth’s crust is fragmented and each such fragment is known as a plate. These plates are constantly in motion and sometimes can collapse under another, causing an earthquake on the surface.

Seismic/fault ZonesBoundaries of the plates on the earth’s crust are weak zones where earthquakes are most likely to occur. These are known as seismic or fault zones.

Power of earthquake

Power of an earthquake is expressed in terms of magnitude on a scale. This scale is called as a Richter Scale.

An earthquake with magnitude > 7 on the Richter scale is considered destructive.

This scale is not linear. i.e. an increase of 2 in magnitude implies 1000 times more destructive energy.

Seismic WavesTremors deep inside the earth produce waves, which are called seismic waves.

Seismograph

An instrument that records seismic waves is called a seismograph. Consists of a vibrating rod or pendulum that starts vibrating when tremors

occur.

Protection from an earthquake: Steps to follow

Outdoors: (a) Find a clear spot, away from overhead lines and buildings (b) if in a car then go to a clear spot and stay inside the car till the tremors stop.

Indoors: (a) stay under a table till tremors stop (b) Avoid staying near tall heavy objects.

Structural measures: (a) in highly seismic areas build mud houses with light roofs in order to minimize damage (b) Fix cupboards and shelves to the walls (c) During an earthquake, some buildings catch fire and therefore must have proper working firefighting equipments.

CHAPTER-5

LIGHT

Light:(i) Light is a form of energy, which includes the sensation of vision in our eyes and make us able to see various things present in our surrounding.(ii) When light falls on an object, some part of light is reflected back to our eyes. Then, we are able to see an object because of light being reflected from the object.

Properties of Light:(i) It is form of energy.(ii) It travels in straight line.(iii) Light can form shadows.

Laws of Reflection: The two laws of reflection are:(i) The angle of incidence is equal to the angle of reflection.

˂ i = ˂ r(ii) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.These laws of reflection are applicable to all types of reflecting surfaces including spherical surfaces.

Formation of Image by Plane Mirror:In figure PQ is a plain mirror and O is an object. Incident rays OA and OC are coming from the object. These incident rays are reflected from the mirror and

reflected rays emerge as AB and CD. If reflected rays AB and CD are extended behind the mirror, they appear to meet at point ‘I’. This is that point where image is formed.

      

Some features of image formed by a plain mirror:(i) Obtained Image is always formed behind the mirror.(ii) Obtained image is virtual, which means it cannot be obtained on screen.(iii) Image is erect and laterally inverted.(iv) Image size is equal to object size.(v) Distance of object from the mirror = Distance of image from the mirror.Distance of the object and image is the same from the mirror.

Regular and Diffused Reflection:

1. Regular Reflection:(i) When the reflected surface is smooth and well – polished, the parallel rays falling on it are parallel to another one, the reflected light goes in one particular direction and also parallel to each other. This is regular reflection. E.g. plane mirror, reflection from still water.

                                   

2. Diffused and Irregular Reflection:When the reflecting surface is rough, the parallel rays falling on it are reflected in different directions. Such reflection is known as diffused or irregualar reflection. For example, reflection of light from the wall of a room or tree etc.

Multiple Images:(i) When two or more mirrors are placed at an angle to one another, we can see multiple images of an object.(ii) For example in the hair dresser shop two mirrors are placed opposite to each other. If a man is sitting between them, His image is formed in both the mirrors. The image in one mirror would act as an object for another mirror and this sequence would continue. This will give result in the formation of multiple images.

(iii) The number of images formed depends on the angle between the two mirrors. This can be calculated by using the following formula:

Number of Images = 

(iv) If the given two mirrors are at a right angle to each other, 3 images will be

obtained. If the given mirrors are at 60° angle, we shall get 5 images.(v) When the two mirrors are kept opposite and parallel to each other, we will get infinite number of images formed.

Kaleidoscope:(i) The idea of number of images formed by mirrors placed at an angle to one another is used in a kaleidoscope to make numerous beautiful patterns.(ii) To make a kaleidoscope, get three rectangular mirror strips about 15 cm long and 4 cm wide each. Join them together to form a prism. Fix them in a circular cardboard tube or tube of a thick chart paper.

Human Eye:We see objects only when light coming from objects enters our eyes. Eye is one of our most important sense organs which give us the sense of vision. 

Structure of Human Eye:

                   

1. Cornea: It is located at the front portion of the eye. It is a transparent window that bulges outwards and responsible for the maximum refraction (bending) of the light that enters the eye. 

2. Iris: It is a thin circular, coloured part structure. It works like the shutter of eye. It controls the amount of light entering the eye.

3. Pupil: It is an adjustable circular opening that is located at the centre of the iris. Light enters into eye through the pupil. When the light amount is high, iris contracts and allows less light into the eye. When the light amount is low, iris dilates and allows more light into the eye.

4. Lens: It is made of a jelly transparent material and is a biconvex structure. It is present behind the pupil. The size and thickness of the lens change as per the distance of an object by the ciliary muscle.

5. Retina: The screen of eye is referred to as retina because the light ray comes through the pupil and passes through the lens and converges on a screen called retina. Retina is the light – sensitive inner lining of the back of the eye.Retina contains several nerve cells. There are two kinds of cells(i) Cone cells, which are sensitive to bright light and sense colour.(ii)Rods cells, which are sensitive to dim light.

6. Optic Nerve: It is located at the back of the eye. Optic nerves are the bundles of over one million nervefibres that carry visual messages from the retina to the brain.

7. Blind Spot: It is the small region where the optic nerve and the retina meet. It has no sensory cells. Hence no image is formed at this spot on the retina.

Working of Eye:(i) Light reflects off from the objects and enters the eyeball through a transparent layer of tissue at the front of the called the cornea.(ii) The cornea bends the light rays through the pupil- the dark opening in the center of the colored portion of the eye.(iii) The adjusted light passes through the eye’s natural crystalline lens.(iv) The eye lens converges all the light rays on the retina and image is formed on retina real, small and inverted.(v) The retina converts these light rays into electrical signals that are relayed to

the brain via the optic nerve. The brain processes the information it receives, so that in turn, we can see.

Persistence of Vision:(i) The impression of an image does not vanish immediately from the retina. An image persists there for about 1/16th of a second. This feature is called persistence of vision.(ii) If still images of a moving object are flashed on the eye at a rate faster than 16 per second, then the eye perceives this object as moving.Examples: Due to this persistence of vision, when we see many still images in a sequence, they give the illusion of moving images. Movies and animation are made by exploiting this property of the human eye.

Care of Eyes:(i) Eyes are very important sense organ and they are sensitive too. So, It is necessary to take proper care of our eyes. Some points for care of eyes are as follows:(ii) If there is any problem one should go to an eye specialist.(iii) Too bright or too dim light is bad for eyes. Insufficient light causes eyestrain and headaches.(iv) Do not look at the sun or a powerful light directly.(v) Never rub eyes. If particles of dust go into theeyes, wash the eyes with clean water. If there is no improvement go to a doctor.(v) Always read the book and news paper at the normal distance for vision. Do not read by bringing the book too close to the eyes or keeping it too far.

Vision Defects:Some of the vision defects are as follows:

1. Myopia: A person with myopia can see near by objects clearly while distant objects appear blurred. This disease is also called short sightedness or near sightedness. Spectacles with concave lenses can improve the vision in a myopic person. 

2. Hypermetropia: A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. This disease is also called long sightedness. Spectacles with convex lenses can improve the vision in a hypermetropic person.

3. Cataract: During old age, the crystalline lens becomes milky and cloudy in a cataract patient and thus the person cannot see clearly. Cataract is treated by surgery. In cataract surgery, the cloudy lens is removed and an artificial lens is transplanted in the eye.

Visually Challenged Person:(i) Some people including children face with disability of vision. This disability of vision can be partial or complete. Such persons are called visually challenged persons.(ii) A visually challenged person usually shows a marked development of other senses; like the sense of hearing and sense of touch. However, additional resources can enable them to develop their capabilities further. Resources can be of two types: Non-optical aids and optical aids.

1. Optical Aids: Optical aids help a person who is partially visually challenged. These aids enlarge an image or a text so that they could be visible. Bifocal lenses, contact lenses, tinted lenses, magnifiers and telescopic aids, magnifying devices and telescopic devices come under this category.

2. Non-optical Aids: Non-optical aids help a person who is completely visually challenged. Non-optical aids include visual aids (magnify words), tactual aids (using the sense of touch), auditory aids (using the sense of hearing) and electronic aids.(i) Visual aids can magnify words; provide suitable intensity of light and material at proper distances.(ii) Tactual aids, including Braille writer slate and stylus, help the visually challenged persons in taking notes, reading and writing.(iii) Auditory aids include cassettes, tape recorders, talking books and other such devices. Electronic aids, such as talking calculators, are also available for performing many computational tasks.(iv) Closed circuit television, also an electronic aid, enlarges printed material with suitable contrast and illumination.(v) Now a day, use of audio CDs and voice boxes with computers are also very helpful for listening to and writing the desired text.

Louis Braille:The most popular resource for visually challenged persons is known as Braille. Braille code is used for common languages, mathematics and scientific notation. Louis Braille was born in 1809 in France. He became blind due to an accident in childhood. He developed the Braille script in 1824.

                                               Braille Script: This system has 63 raised dot patterns or characters. Each character or pattern represents a letter, a combination of letters, a common word or a grammatical sign. Dots are arranged in cells of two vertical rows of three dots each. A reader needs to touch the Braille script to read it. Example of dot patterns used in Braille System:

CHAPTER-6

STAR AND THE SOLAR SYSTEM

Celestial Objects: Things like stars, planets, moons, etc. which are present in the space are known as celestial objects.

The Solar System:1. It consists of the Sun and other celestial objects present in the space revolving around the sun.The gravitational attraction between the Sun and these objects keeps them revolving around it. The various entities like planets, meteors, comets, etc. are part of our solar system.2. The eight planets in their order of distance from the Sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

1. The Sun:(i) The Sun is the nearest star amongst all the stars present in the universe.(ii) It is the main source of heat and light for all the planets specially Earth.

2. The Planets: The planets are like stars, but they do not have light of their own. They merely reflect the sunlight that falls on them.  The planets keep changing their position with respect to the stars.These are the objects revolving around the sun in fixed path. This fixed path is termed as an orbit.1. Period of revolution: It is the time taken by planet to complete full revolution around the sun.2. Period of rotation: It is the time taken by planet to complete one rotation around its own axis.3. Satellite: These are the celestial bodies present around other celestial bodies.

Example Moon is a satellite of the Earth.4. Artificial satellite: These are the man-made satellites present around the Earth.

Planets Revolving Around the Sun:1. Mercury (Budh)(i) It is nearest planet to the Sun and the smallest planet present in our solar system.(ii)  It is very difficult to observe it, as most of the time it is hidden in the glare of the Sun.(iii) Mercury has no satellite of its own.

2. Venus (Shukra):(i) It is the nearest planet to Earth and It is the brightest planet visible in the night sky.(ii) It is also called a morning or an evening star, although it is not a star.(iii) Venus rotates on its axis from east to west.(iv) Venus has no moon or satellite of its own.

3. The Earth:(i) It is the only planet in the solar system on which life exists.(ii) Earth appears blue green due to the reflection of light from water and landmass on its surface.(iii) The axis of rotation of the Earth is not perpendicular to the plane of its orbit. The tilt is responsible for the change of seasons on the Earth.(iv) It has only one moon or satellite of its own.

EARTH

4. Mars (Mangal):(i) It looks somewhat reddish and, so, it is also known as the red planet.(ii) Mars has two small natural satellites.

5. Jupiter (Brihaspati):(i) It is the largest planet of the solar system. It rotates very rapidly on its axis.(ii) Jupiter has a large number of satellites. And also has faint rings around it.(iii) It is about 1300 times larger than that of earth and its mass about 318 times than that of our Earth. It has 28 satellites of its own.

6. Saturn (Shani):(i) It is a yellowish colored planet.(ii) There are beautiful rings around this planet, which are not observed in any other planets.(iii) It is the least dense among all the planets. Its density is less than that of water.(iv)This planet has 30 satellites of its own.

7. Uranus:(i) It is the one of the outermost planet of the solar system.(ii) The most distinctive feature of Uranus is that it has highly tilted rotational axis. (iii) This planet has 21 satellites or moons of its own.

8. Neptune:(i) It is the outermost planet of the solar system. (ii) This planet is not visible with naked eyes. (iii) This planet has 8 satellites or moons.

Some Other Celestial Members of the Solar System:1. Asteroids:

(i) The small rocks like substances that are orbiting around the Sun are called asteroids.(ii) They are found in large gap between the orbits of Mars and Jupiter.(ii) They are visible only through large telescopes.

2. Comets:(i) They revolve around the Sun in highly elliptical or circular orbits.(ii) A Comet looks like as if it has a bright head with a long tail and it is been seen that the tail keeps growing in size as it keeps approaching the sun.

3. Meteors:(i) These are small objects that sometimes enter the Earth’s atmosphere.(ii) They occasionally enter the earth’s atmosphere with very high speed. Due to friction of earth's atmosphere, they glow and then evaporate quickly. That is why their bright steak lasts for a very short time.

4. Meteorite:(i) These are the leftover object that reach the Earth.(ii) Meteorites help scientists in investigating the nature of the material from which the solar system was formed

5. Artificial Satellites:(i) These are the man-made satellites that revolve around the Earth.(ii) Aryabhatta was the first Indian satellite. Some other Indian satellites are INSAT, IRS, Kalpana-1, EDUSAT, etc.(iii) They are used for forecasting weather, transmitting television and radio signals. They are also used for telecommunication and remote sensing.

The Moon:- The moon is the natural satellite of the earth. Sometimes it shines bright to cast shadow and sometimes it becomes invisible.1. Phases of the moon:The different shapes of moon visible during various days in a month are called as phases of the moon..

Different Phase of Moon

2. Full moon day: It is the day on which the full moon is visible.3. New Moon day: After the full moon day, the moon appears to become thinner and thinner. On the fifteenth day the moon is not visible. This day is known as the new moon day.4. Why does Moon have different phases?(i) Moon does not have its own light and actually it reflects the light coming from sun which is responsible for various phases.(ii) After new moon day, the visibility size of illuminated part of moon seen from Earth keeps increasing and after full moon day it keeps decreasing.

Moon is visible due to reflected sunlight

Positions of the moon in its orbit and its corresponding phases

5. The Moon’s Surface:(i) The Moon’s surface is dusty, barren and has many different size craters on it. Moreover, there are many big and small mountains.(ii) The Moon has no atmosphere and water.

The Stars:(i) These are objects which twinkle in the sky at night.(ii) Sun is the nearest star to our planet which is approx 150 million km away from Earth.Light Year:(i) It is defined as the distance travelled by light in one year. And it is the unit to measure large distances.(ii) The speed of light is about 300,000 km per second.(iii) The distance of the Sun from the Earth may be said to be about 8 light minutes

Pole Star: It is the star which is situated in the direction of the earth’s axis.

Constellations:The different known shapes that are formed by a group of stars, is known as a constellation.

Some constallations in the night sky1. Ursa Major:(i)  This constellation consists of seven stars.(ii)  It looks like a big ladle or a question mark. There are three stars in the handle of the ladle and four in its bowl(iii)  It is also known as Big Dipper, the Great Bear or the Saptarshi.

2. Orion:(i) It is another well known constellation visible during late evenings in winter.

(ii) It also consists of seven or eight bright stars.  The three middle stars in Orion represent the belt of the hunter and the remaining four bright stars are arranged in the form of a quadrilateral.(iii) Orion is also called the Hunter.

3. Sirius:(i) It is the brightest star in the sky.(ii) To find the location of Sirius, imagine a straight line passing through the belt of three middle stars of Orion.

4. Cassiopeia:(i) It is another popular constellation in the northern sky. It is visible during winter in the early part of the night. It looks like a distorted letter W or M.