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PHY205H1F Summer

Physics of Everyday Life

Class 4: Liquids & Gases

• Pressure

• Buoyancy in a Liquid

• Archimedes’ Principle

• What Makes an Object

Sink or Float

• Pascal’s Principle

• Surface Tension

• Capillarity

• The Atmosphere

• Atmospheric

Pressure

• The Barometer

• Boyle’s Law

• Buoyancy of Air

• Bernoulli’s Principle

• Plasma

Atoms • Atoms are the building blocks of all matter

• They are too small to be seen with visible light

• One gram of water has a volume of 1 cm3 and

contains more than 1023 atoms!

• 1023 =

[image from http://www.physics.utoronto.ca/~ifridman/ ]

• This is a scanning tunneling

microscope image of graphite

taken by Igor Fridman, a

graduate student in U of T

Physics

• The dots are individual

carbon

• Atomic structure is composed of:

• An atomic nucleus, which contains

• Orbiting

• The nucleus is composed of and

, which are in turn made of smaller

quarks

[Image retrieved Jan.10, 2013 from http://www.safetyoffice.uwaterloo.ca/hse/radiation/rad_sealed/matter/atom_structure.htm ]

• Protons have electric

charge:

• Electrons have electric

charge:

• All atoms have

the same number of

protons as electrons

The nucleus of an electrically neutral iron atom

contains 26 protons. How many electrons

are in this iron atom?

A. 52

B. 26

C. 24

D. 28

E. zero

Atoms

Check your neighbour

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The Elements Atoms

• Refer to particles that make up a substance

Elemental substance

• Composed of only

– Lightest and most abundant is hydrogen.

• To date, about 115 elements are known.

– 90 occur in nature.

– Others produced in laboratory are unstable.

Words and can be used

interchangeably. [Image retrieved Feb.4 2013 from http://parisbreakfasts.blogspot.ca/2011/09/financiers.html ]

The atomic number of an element matches the

number of

A. protons in the nucleus of an atom.

B. electrons in a neutral atom.

C. Both of the above.

D. None of the above.

Atoms

Challenge Question: Do you know it?

Periodic Table of the Elements

© 2010 Pearson Education, Inc.

What is the approximate density of water?

A. 10−5 kg/m3

B. 0.01 kg/m3

C. 0.1 kg/m3

D. 1 kg/m3

E. 1000 kg/m3

Discussion question: What do you think?

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Density

• Amount of mass per unit volume of a

material.

• Unit of density is kg/m3 or g/cm3.

• Example:

Density of water is kg/m3, or g/cm3.

volume

mass Density =

[Image retrieved Jan. 11, 2013 from http://www.amazon.com/Evian-Water-Liter-Pack/dp/B0041HVMU0 ]

If the volume of an object were to double, with no

change in mass, what would happen to its

density?

A. It would remain unchanged.

B. It would double.

C. It would decrease by a factor of two.

D. None of these.

Atoms

Check your neighbour

• Pressure is the per

unit that one object

exerts on another

Pressure = force area

Pressure

• In equation form:

• Depends on area over which

force is distributed

• Units: N/m2, or ( )

When you stand on one foot instead of two, the

force you exert on the floor is

A. less.

B. the same.

C. more.

Pressure

CHECK YOUR NEIGHBOR

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When you stand on one foot instead of two, the

pressure you exert on the floor is

A. less.

B. the same.

C. more.

Pressure

CHECK YOUR NEIGHBOR Pressure in a Liquid • Force per unit area that a liquid exerts on an

object

• dependent and not dependent

Example: Swim twice as deep, then twice as much

weight of water above you produces twice as much

pressure on you.

Pressure in a Liquid

Effects of water pressure

• Acts to surfaces of a container

• Liquid spurts at right angles from a hole in the surface. – The greater the , the

greater the exiting speed.

Buoyancy in a Liquid

Buoyancy

• Apparent loss of weight of a submerged object

• Amount equals the weight of

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• Displacement rule: A completely submerged object always displaces

a volume of liquid equal to .

Example: Place a stone in a container that is

brimful of water, and the amount of water

overflow equals the .

http://www.dairygoodness.ca/butter/butter-tips-tricks/how-to-measure-butter

• When butter is in hard, irregular shaped

pieces, fill a 2-cup liquid measuring cup with

water to the 1 cup mark.

• Add enough butter pieces until the level

reaches 1-1/4 cups.

• Drain off the water and you’re left with

of butter!

Archimedes’

Principle

• Discovered by Greek

scientist Archimedes in 250

BC.

• Relates buoyancy to

displaced liquid.

• States that an immersed body (completely or

partially) is buoyed up by a equal to the

weight of the fluid it displaces.

• Applies to gases and liquids.

[ image retrieved Jan.17 2013 from http://personal.maths.surrey.ac.uk/st/H.Bruin/MMath/archimedes.html ]

Demonstration Prediction

• A steel mass of 0.75 kg hangs from a spring

scale.

• When it is not accelerating, the spring scale

reads 7.5 N.

• If Harlow dips the mass into an open container

of water, then stops the motion and lets the

scale settle, what will be the reading on the

scale?

A. Less than 7.5 N, but not zero

B. More than 7.5 N

C. About 7.5 N

D. zero

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Flotation

• Principle of flotation: – A floating object displaces a weight of fluid equal to its

own weight.

Example: A solid iron 1-ton block may displace 1/8 ton of water and sink. The same 1 ton of iron in a bowl shape displaces a greater volume of water—the greater buoyant force allows it to .

Ch.13 Problem 8

• Your friend of mass 100 kg can just barely float in fresh-

water. Calculate her approximate volume.

You place an object in a container

that is full to the brim with water on

a scale. The object floats, but

some water spills out. How does

the weight of the object compare

with the weight of the water

displaced?

A. Weight of object is greater than weight of water

displaced.

B. Weight of object is less than weight of water displaced.

C. Weight of object is equal to weight of water displaced.

D. There is not enough information to decide.

Flotation

CHECK YOUR NEIGHBOR

The Falkirk Wheel’s two caisson are brimful of water and

the , regardless of whether there are boats in

them. This makes rotation and lifting almost effortless.

[image from http://en.wikipedia.org/wiki/File:FalkirkWheelSide_2004_SeanMcClean.jpg ]

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Archimedes’ Principle Denser fluids will exert a greater buoyant force on a

body than less dense fluids of the same volume.

Example: Objects will float in saltwater

(density = 1.03 g/cm3) than in freshwater

(density = 1.00 g/cm3).

[ image retrieved Jan.17, 2013 from http://famoustourisms.com/2011/05/dead-sea-enjoy-the-charm-and-sensation-swim-here/ ]

What Makes an Object Float or Sink?

Whether an object floats or sinks depends upon

the

• Weight of the .

• Weight of the .

The weight of the fluid displaced depends on

.

So what really counts is the weight of the object per

.

This is related to the average of the object.

[ image retrieved Jan.17 2013 from http://en.wikipedia.org/wiki/File:Kylpyankka.jpg ]

What sinks? What floats? 1. An object more dense

than the fluid in which it is

immersed will .

2. An object less dense

than the fluid in which it is

immersed will .

3. An object having a

density equal to the

density of the fluid in

which it is immersed will

.

[ from http://www.flickriver.com/photos/rhosoi/popular-interesting/ ]

[ from http://weeboopiper.wordpress.com/tag/pier-7/ ]

[ from http://www.123rf.com/photo_10758041_bluefin-tuna-thunnus-thynnus-saltwater-fish-underwater-blue-sea.html ]

Two solid blocks of identical size are submerged in

water. One block is lead and the other is aluminum.

Upon which is the buoyant force greater?

A. On the lead block

B. On the aluminum block

C. Same on both blocks

D. There is not enough information to decide.

What Makes an Object Float or Sink?

CHECK YOUR NEIGHBOR

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Pascal’s principle:

• Discovered by Blaise Pascal, a scientist and theologian in the 17th century

• States that a change in pressure at any point in an enclosed fluid at rest is transmitted to all points in the fluid

• Applies to all —gases and liquids

Pascal’s Principle • Application in hydraulic press

Example:

– applied to the left piston is transmitted

to the right piston.

– A load on small piston (left) lifts a load of

on large piston (right).

Surface Tension

The tendency of the surface of a liquid to

and thus to behave like a stretched elastic

membrane.

Examples:

• Insects can stand on the

surface of water, even if they

are more dense than water.

• In a small droplet of water, the

surface will contract until it

forms the shape with the

smallest surface area for its

volume: a sphere

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[image from http://www.dailymail.co.uk/news/article-1371416/Photographer-Adam-Gormley-captures-ant-trapped-raindrop.html#axzz2K9hLsZGF ]

Surface Tension

• Surface tension is caused by molecular

.

• Beneath the surface, each molecule is

attracted in every direction by neighboring

molecules.

• A molecule on the surface of a liquid is

pulled only by neighbors on each side and

downward from below; there is no pull

.

• These molecular attractions tend to pull the

molecule from the surface the liquid,

causing surface tension.

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Test on Wednesday during class time

• Location: EX310 (last name A-M), EX320 (last name N-Z)

• Test will begin promptly at 10 minutes after the hour and

will be 1 hour and 50 minutes long.

• Please bring a calculator, and, if you wish, a 8.5x11” aid

sheet upon which you may write anything you wish on

both sides

• Test will cover Hewitt chapters 2-5, 7, 8, 13 and 14, and

will include:

• 6 short-answer problems for which you must show your

reasoning

• 16 multiple choice questions – you fill in a bubble sheet

• Questions will be similar in style and level to the Exercises

and Problems at the end of the chapters in Hewitt

Tips for the 2 hour Test

• No phones / ipods etc allowed. You

will need a regular calculator, and a

watch could be handy as well!

• Time Management:

– Skim over the entire test from front

to back before you begin. Look for

problems that you have confidence

to solve first.

– If you start a problem but can’t

finish it, leave it, make a mark on

the edge of the paper beside it,

and come back to it after you have

solved all the easy problems.

• Bring your T-card or other photo ID,

as we will be collecting signatures

PHY205 Physics of Everyday Life

Chapter 14: Gases

• The Atmosphere

• Atmospheric Pressure

• The Barometer

• Boyle’s Law

• Buoyancy of Air

• Bernoulli’s Principle

• Plasma

[image from http://blog.testfreaks.com/information/thq-and-disney-pixars-up-the-videogame-preview/ ] © 2010 Pearson Education, Inc.

Atmospheric Pressure

The global average

sea-level pressure is

Pa, or 1 atm.

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Atmospheric Pressure in this room What is the force of air pressure on the top of your

outstretched hand?

20 square inches = pounds!

Why don’t you feel that force pushing your hand

down?

What if all the air below your hand was removed (a

vacuum)?

p.s.i.N

pounds

inch

m

m

N000,100

2

2=

Atmospheric Pressure:

Discussion Question

• A suction cup sticks to a wall. It is

A. pulled to the wall by the vacuum.

B. pushed to the wall by the atmosphere.

C. both of these

D. neither of these

[image from http://www.flippersandfins.net/bettabreedingarticle.htm ]

Pressure and “Suction”

A fluid can only walls or objects; a

fluid cannot pull on a wall.

What we call “suction” is when the fluid on

one side has a higher pressure than the

fluid on the other side.

It is the pressure which creates a pushing

force into the lower pressure area (into the vacuum).

This is how we breath:

1. We expand our lung cavity, lowering the

inside.

2. The higher air pressure outside air into

our lungs.

In drinking soda or water through a straw, we

make use of

A. capillary action.

B. surface tension.

C. atmospheric pressure.

D. Bernoulli's principle.

E. none of these

Atmospheric Pressure

CHECK YOUR NEIGHBOR

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• Pressure at the of a column of air reaching to the

top of the atmosphere is the same as the pressure at the

of a column of water m high.

• Consequence: The highest the atmosphere can push water

up into a pump is 10.3 m.

• Mechanical pumps that don’t depend on atmospheric

pressure don’t have the 10.3-m limit.

Atmospheric Pressure The Barometer • The barometer is a device to

measure .

• It consists of a tube

upside down in a dish filled with

mercury.

• The height of the mercury

column tells us the atmospheric

pressure.

• Atmospheric pressure

decreases with increasing

altitude, so it also measures

—an altimeter.

Why don’t barometers use water instead of

mercury?

A. Water cannot be used because it does not exert

pressure.

B. Water cannot be used because it sticks to the

glass.

C. Water can be used but the barometer will be too

tall.

D. None of the above.

Barometer

CHECK YOUR NEIGHBOR Boyle’s Law

• The product of pressure and volume of a given

mass of gas will always remain the same.

=

LARGE

Pressure Small

Volume

LARGE

Volume Small

Pressure

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Discussion Question

• When a party balloon is compressed to

one-third its volume with no change in

temperature, what happens to the gas

pressure in the balloon?

A. It drops by a factor of 3

B. It drops by a factor of 2

C. It remains the same

D. It doubles

E. It triples

Buoyancy in Air

Archimedes’ principle applies to as well

as liquids.

• An object surrounded by is buoyed up by a

force equal to the weight of the displaced

[image downloaded Jan.24 2013 from http://saigon-hobby.com/products/2739-air-swimmer-nemo.aspx ]

Buoyancy Example: If you wish to support a mass of 80 kg

(one person) with a helium-filled balloon, how big should the

balloon be?

• What must change about the water so that

the amount coming in the hose equals the

amount leaving the hose?

A. The velocity must increase.

B. The velocity must decrease.

C. The pressure must increase.

D. The pressure must decrease.

In-class Discussion Question

• You put your thumb over the end

of the a hose, thereby reducing

the area through which water

can exit the hose.

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• We study the steady flow of water from a

water tap, e.g., in your kitchen sink. The

jet of water

A. broadens as it falls.

B. narrows as it falls.

C. does not change its

cross-sectional shape.

D. slows before hitting the

bottom of the sink.

In-class Discussion Question

Bernoulli’s Law

• Consider an ideal fluid, flowing through a tube which narrows.

• It increases its . This means the kinetic energy per volume of the fluid will .

• How can this be? There must be a which does work on the fluid to speed it up.

• The force must come from a difference.

• Pressure must be in the region of increased fluid velocity.

Bernoulli’s Principle

• Discovered by Daniel Bernoulli, a 15th century

Swiss scientist

• States that where the speed of a fluid increases,

internal pressure in the fluid (and

vice versa)

• Applies to a smooth, steady flow

Bernoulli’s Principle

Streamlines

• Thin lines representing fluid motion

• Closer together, flow speed is and

pressure within the fluid is

• Wider, flow speed is and pressure within

the fluid is

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Bernoulli’s demo, lift.

• If the air were flowing equally

above and below the beach-ball,

there would be no force on the ball

due to the Bernoulli effect.

• The ball would then fall due to its

own weight until the point where

most of the flow is over the top of

the ball, reducing the pressure

there.

• The ball sits at a point of stable

equilibrium, where these forces

balance.

Applications of Bernoulli’s principle

• Blow on the top surface of a paper and the paper

rises.

Reason: Pressure of the moving air is than the

atmospheric pressure beneath it.

• Wind blowing across a peaked roof can lift the roof off the house.

Reason: Pressure is as wind gains speed as it flows over the roof. The greater pressure inside the house lifts the roof up.

• When wind speeds up as it blows over the

top of a hill, what happens to the

atmospheric pressure over the hill?

A. It decreases

B. It increases

C. It stays the same

In-class Discussion Question

Plasma

• Plasma is the

state of matter

(after solids, liquids

and gases).

• A plasma is an . The atoms

that make it up are ionized, stripped of one

or more electrons, with a corresponding

number of free electrons.

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• Which of the following is in a plasma

state?

A. Dry ice

B. A torch flame

C. Molten lava

D. Liquid hydrogen

E. Helium gas

In-class Discussion Question

Before Class 5 next Monday

• Please read Chapters 15 and 16, or at least watch

the 20-minute pre-class video for class 5

• Pre-class reading quiz on chapters 15 and 16 is due

Monday June 3 by 10:00am

• NOTE

• Midterm Test: Wednesday 1-3 in TBA

• Test will begin promptly at 1:10 and will be 1 hour and 50

minutes long.

• Please bring a calculator, and, if you wish, a 3x5 notecard

upon which you may write anything you wish on both sides

• Test will cover Hewitt chapters 2-5, 7, 8, 13 and 14, and will

include some multiple choice and some short-answer