Conceptual Physics Fundamentalsalee3/Physics 11/Powerpoint Lectures/Chap9.pdf• Heat Transfer and Change of Phase ... walking barefoot without ... •transfer of heat involving only

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley

Conceptual Physics

Fundamentals

Chapter 9:

HEAT TRANSFER AND

CHANGE OF PHASE


This lecture will help you

understand:

• Conduction

• Convection

• Radiation

• Newton’s Law of Cooling

• Global Warming and the Greenhouse Effect

• Heat Transfer and Change of Phase

• Boiling

• Melting and Freezing

• Energy and Change of Phase


Heat Transfer and Change of

Phase “You won’t fully appreciate the frontiers of

physics until you’re familiar with its foothills.”

—Iain MacInnes



Phase Objects in thermal contact at different

temperatures tend to reach a common

temperature in three ways:

• conduction

• convection

• radiation


Conduction

Conduction

• transfer of internal energy by electron and

molecular collisions within a substance,

especially a solid


Conduction

Conductors

• good conductors conduct heat quickly

– substances with loosely held electrons

transfer energy quickly to other electrons

throughout the solid

example: silver, copper, and other solid metals


Conduction

Conductors (continued)

• poor conductors are insulators

– molecules with tightly held electrons in a substance

vibrate in place and transfer energy slowly—these are

good insulators (and poor conductors)

example: glass, wool, wood, paper, cork, plastic foam,

air

• substances that trap air are good insulators

example: wool, fur, feathers, and snow


If you hold one end of a metal bar against a piece of ice,

the end in your hand will soon become cold. Does cold flow

from the ice to your hand?

A. yes

B. in some cases, yes

C. no

D. in some cases, no

Conduction

CHECK YOUR NEIGHBOR


If you hold one end of a metal bar against a piece of ice,

the end in your hand will soon become cold. Does cold flow

from the ice to your hand?

A. yes

B. in some cases, yes

C. no

D. in some cases, no

Explanation:

Cold does not flow from the ice to your hand. Heat flows from your

hand to the ice. The metal is cold to your touch, because you are

transferring heat to the metal.

Conduction

CHECK YOUR ANSWER


Conduction

Insulation

• doesn’t prevent the flow of internal energy

• slows the rate at which internal energy flows

example: rock wool or fiberglass between walls slows

the transfer of internal energy from a warm

house to a cool exterior in winter, and the

reverse in summer


Conduction

• Insulation (continued)

dramatic example: walking barefoot without

burning feet on red-hot

coals is due to poor

conduction between

coals and feet


Convection

Convection

• transfer of heat involving only

bulk motion of fluids

example:

• visible shimmer of air above a

hot stove or above asphalt on a

hot day

• visible shimmers in water due

to temperature difference


Convection

Reason warm air rises

• warm air expands, becomes less dense, and is

buoyed upward

• it rises until its density equals that of the

surrounding air

example: smoke from a fire rises and blends with the

surrounding cool air


Convection

Cooling by expansion

• opposite to the warming that occurs when air is

compressed

example: the “cloudy” region above

hot steam issuing from the nozzle of a

pressure cooker is cool to the touch (a

combination of air expansion and

mixing with cooler surrounding air)

Careful, the part at the nozzle that you

can’t see is steam—ouch!


Although warm air rises, why are mountaintops cold and

snow covered, while the valleys below are relatively warm

and green?

A. Warm air cools when rising.

B. There is a thick insulating blanket of air above valleys.

C. both A and B

D. none of the above

Convection

CHECK YOUR NEIGHBOR


Although warm air rises, why are mountaintops cold and

snow covered, while the valleys below are relatively warm

and green?

A. Warm air cools when rising.

B. There is a thick insulating blanket of air above valleys.

C. both A and B


Explanation:

Earth’s atmosphere acts as a blanket, which keeps the valleys

from freezing at nighttime.

Convection

CHECK YOUR ANSWER


Convection

Winds

• result of uneven heating of the air near the ground – absorption of Sun’s energy occurs

more readily on different parts of Earth’s surface

• sea breeze – The ground warms more than water

in the daytime.

– Warm air close to the ground rises and is replaced by cooler air from above the water.


Radiation

Radiation

• transfer of energy from the Sun through empty

space


The surface of Earth loses energy to outer space due

mostly to

A. conduction.

B. convection.

C. radiation.

D. radioactivity.

Radiation

CHECK YOUR NEIGHBOR


The surface of Earth loses energy to outer space due

mostly to

A. conduction.

B. convection.

C. radiation.

D. radioactivity.

Explanation:

Radiation is the only choice, given the vacuum of outer space.

Radiation

CHECK YOUR ANSWER


Which body glows with electromagnetic waves?

A. Sun

B. Earth

C. both A and B

D. neither A nor B

Radiation

CHECK YOUR NEIGHBOR


Which body glows with electromagnetic waves?

A. Sun

B. Earth

C. both A and B

D. neither A nor B

Explanation:

Earth glows in long-wavelength radiation, while the Sun glows in

shorter waves.

Radiation

CHECK YOUR ANSWER


Radiation

Radiant energy

• transferred energy

• exists as electromagnetic waves ranging from

long (radio waves) to short wavelengths (X-rays)

• in visible region, ranges from long waves (red) to

short waves (violet)


Radiation

Wavelength of radiation

• related to frequency of vibration (rate of vibration of a wave source)

– low frequency vibration produces long-wavelength waves

– high frequency vibration produces short-wavelength waves


Radiation

Emission of radiant energy

• every object above absolute zero radiates

• from the Sun’s surface comes light, called

electromagnetic radiation, or solar radiation

• from the Earth’s surface is terrestrial radiation in

the form of infrared waves below our threshold

of sight


Radiation

Emission of radiant energy (continued)

• frequency of radiation is proportional to the absolute temperature of the source ( )

f ~ T


Radiation

Range of temperatures of radiating objects • room temperature emission is in the infrared

• temperature above 500 C,

red light emitted, longest

waves visible

• about 600 C, yellow light

emitted

• at 1500 C, object emits

white light (whole range

of visible light)


Radiation

Absorption of radiant energy

• occurs along with emission of radiant energy

• effects of surface of material on radiant energy

– any material that absorbs more than it emits is

a net absorber

– any material that emits more than it absorbs is

a net emitter

– net absorption or emission is relative to

temperature of surroundings


Radiation

Absorption of radiant energy (continued)

• occurs along with emission of radiant energy

– good absorbers are good emitters

– poor absorbers are poor emitters

example: radio dish antenna that is a good emitter is

also a good receiver (by design, a poor

transmitter is a poor absorber)


If a good absorber of radiant energy were a poor emitter, its

temperature compared with its surroundings would be

A. lower.

B. higher.

C. unaffected.


Radiation

CHECK YOUR NEIGHBOR


If a good absorber of radiant energy were a poor emitter, its

temperature compared with its surroundings would be

A. lower.

B. higher.

C. unaffected.


Explanation:

If a good absorber were not also a good emitter, there would be a

net absorption of radiant energy, and the temperature of a good

absorber would remain higher than the temperature of the

surroundings. Nature is not so!

Radiation

CHECK YOUR ANSWER


A hot pizza placed in the snow is a net

A. absorber.

B. emitter.

C. both A and B


Radiation

CHECK YOUR NEIGHBOR


A hot pizza placed in the snow is a net

A. absorber.

B. emitter.

C. both A and B


Explanation:

The relation tells us that high temperature sources emit

high frequency waves. High frequency waves have short

wavelength.

Radiation

CHECK YOUR ANSWER

f ~ T


Which melts faster in sunshine—dirty snow or clean snow?

A. dirty snow

B. clean snow

C. both A and B


Radiation

CHECK YOUR NEIGHBOR


Which melts faster in sunshine—dirty snow or clean snow?

A. dirty snow

B. clean snow

C. both A and B


Explanation:

Dirty snow absorbs more sunlight, whereas clean snow reflects

more.

Radiation

CHECK YOUR ANSWER


Radiation

Reflection of radiant energy

• opposite to absorption of radiant energy

• any surface that reflects very little or no radiant

energy looks dark

examples of dark objects:

• eye pupils

• open ends of pipes in a stack

• open doorways or windows of distant houses in the

daytime


Radiation

Reflection of radiant energy (continued)

• darkness often due to reflection of light back and forth many times partially absorbing with each reflection

• good reflectors are poor absorbers


Which is the better statement?

A. A black object absorbs energy well.

B. An object that absorbs energy well is black.

C. Both say the same thing, so both are equivalent.

D. Both are untrue.

Radiation

CHECK YOUR NEIGHBOR


Which is the better statement?

A. A black object absorbs energy well.

B. An object that absorbs energy well is black.

C. Both say the same thing, so both are equivalent.

D. Both are untrue.

Explanation:

This is a cause-and-effect question. The color black doesn’t draw

in and absorb energy. It’s the other way around—any object that

does draw in and absorb energy, will, by consequence, be black

in color.

Radiation

CHECK YOUR ANSWER


Eureka video of Conduction, Convection & Radiation

Conduction, Convection and Radiation song

http://www.youtube.com/watch?v=wz6wzOtv6rs

http://www.youtube.com/watch?v=7Y3mfAGVn1c

http://www.youtube.com/watch?v=7Y3mfAGVn1c


Newton’s Law of Cooling

Newton’s Law of Cooling (continued)

• applies to rate of warming

– object cooler than its surroundings warms up at a rate

proportional to T

example: frozen food will warm faster in a warm room

than in a cold room


It is commonly thought that a can of beverage will cool

faster in the coldest part of a refrigerator. Knowledge of

Newton’s law of cooling

A. supports this knowledge.

B. shows this knowledge is false.

C. may or may not support this knowledge.

D. may or may not contradict this knowledge.


CHECK YOUR NEIGHBOR


It is commonly thought that a can of beverage will cool

faster in the coldest part of a refrigerator. Knowledge of

Newton’s law of cooling

A. supports this knowledge.

B. shows this knowledge is false.

C. may or may not support this knowledge.

D. may or may not contradict this knowledge.


CHECK YOUR ANSWER


Global Warming and the

Greenhouse Effect

Greenhouse effect

• named for a similar temperature-raising

effect in florists’ greenhouses



Greenhouse Effect understanding greenhouse effect requires

two concepts: – all things radiate at a frequency (and therefore

wavelength) that depends on the temperature of the

emitting object

– transparency of things depends on the wavelength of

radiation



Greenhouse Effect understanding greenhouse effect requires two concepts (continued)

• example: Excessive warming of a car’s interior when windows are closed on a hot sunny day. Sun’s rays are very short and pass through the car’s windows. Absorption of Sun’s energy warms the car interior. Car interior radiates its own waves, which are longer and don’t transmit through the windows. Car’s radiated energy remains inside, making the car’s interior very warm.



Greenhouse Effect Global warming

• energy absorbed from the Sun

• part reradiated by Earth as longer-wavelength terrestrial radiation



Greenhouse Effect Global warming (continued)

• terrestrial radiation absorbed by atmospheric

gases and re-emitted as long-wavelength

terrestrial radiation back to Earth

• reradiated energy unable to escape, so warming

of Earth occurs

• long-term effects on climate are of present

concern


The “greenhouse gases” that contribute to global warming

absorb

A. more visible radiation than infrared.

B. more infrared radiation than visible.

C. visible and infrared radiation about equally.

D. very little radiation of any kind.

Global Warming and the Greenhouse Effect

CHECK YOUR NEIGHBOR


The “greenhouse gases” that contribute to global warming

absorb

A. more visible radiation than infrared.

B. more infrared radiation than visible.

C. visible and infrared radiation about equally.

D. very little radiation of any kind.

Explanation:

Choice A has the facts backward. Choices C and D are without

merit.

Global Warming and the Greenhouse Effect

CHECK YOUR ANSWER



Phase

Matter exists in four common phases

that involve transfer of internal energy: • solid phase (ice)

• liquid phase (ice melts to water)

• gaseous phase (water burns to vapor) addition

of more energy vaporizes water to vapor

• plasma phase (vapor disintegrates to ions and

electrons)



Phase Evaporation

• change of phase from liquid to gas



Phase Evaporation process

• molecules in liquid move randomly at various

speeds, continually colliding into one another

• some molecules gain kinetic energy while others

lose kinetic energy during collision

• some energetic molecules escape from the

liquid and become gas

• average kinetic energy of the remaining

molecules in the liquid decreases, resulting in

cooler water



Phase Important in cooling our bodies when we overheat • sweat glands produce perspiration

• water on our skin absorbs body heat as evaporation cools the body

• helps to maintain a stable body temperature



Phase

Sublimation

• form of phase change directly from solid to gas

example:

• dry ice (solid carbon dioxide molecules)

• mothballs

• frozen water



Phase Condensation process • opposite of evaporation

• warming process from a gas to a liquid

• gas molecules near a liquid surface are attracted to the liquid

• they strike the surface with increased kinetic energy, becoming part of the liquid



Phase Condensation process (continued)

• Kinetic energy is absorbed by the liquid,

resulting in increased temperature

example:

• steam releases much energy when it condenses to a

liquid and moistens the skin—hence, it produces a

more damaging burn than from same-temperature

100 C boiling water

• you feel warmer in a moist shower stall because the

rate of condensation exceeds the rate of evaporation



Phase Condensation process (continued)

example:

• in dry cities, the rate of evaporation from your skin is greater than the rate of condensation, so you feel colder

• in humid cities, the rate of evaporation from your skin is less than the rate of condensation, so you feel warmer

• a cold soda pop can is wet in warm air because slow-moving molecules make contact with the cold surface and condense


If bits of coals do not stick to your feet when firewalking, it’s

best if your feet are

A. wet.

B. dry.

C. sort of wet and sort of dry.

D. none of these

Heat Transfer and Change of Phase

CHECK YOUR NEIGHBOR


If bits of coals do not stick to your feet when firewalking, it’s

best if your feet are

A. wet.

B. dry.

C. sort of wet and sort of dry.

D. none of these

Explanation:

The energy that vaporizes water is energy that doesn’t burn your

feet.

Heat Transfer and Change of Phase

CHECK YOUR ANSWER


Boiling

Boiling process

• rapid evaporation from beneath the surface of a liquid


Boiling

Boiling process (continued)

• rapid form of evaporation beneath the surface

forms vapor bubbles

• bubbles rise to the surface

• if vapor pressure in the bubble is less than the

surrounding pressure, then the bubbles collapse

• hence, bubbles don’t form at temperatures

below boiling point (vapor pressure is

insufficient)


Boiling

Boiling process (continued)

• boiling water at 100 C is in thermal equilibrium—

boiling water is being cooled as fast as it is being

warmed

• in this sense, boiling is a cooling process


Boiling

• Boiling point depends on pressure

example: buildup of vapor pressure inside a pressure cooker prevents boiling, thus resulting in a higher temperature that cooks the food

• Boiling point is lower with lower atmospheric pressure

example: water boils at 95 C in Denver, CO (high altitude) instead of at 100 C (sea level)


Boiling

demonstration of cooling effect of

evaporation and boiling


The process of boiling

A. cools the water being boiled.

B. depends on atmospheric pressure.

C. is a change of phase below the water surface

D. all of the above

Boiling

CHECK YOUR NEIGHBOR


The process of boiling

A. cools the water being boiled.

B. depends on atmospheric pressure.

C. is a change of phase below the water surface.

D. all of the above

Boiling

CHECK YOUR ANSWER


Boiling and Freezing

Freezing by evaporation

• dish of room temperature water is placed in a

vacuum jar

• as pressure in the jar is slowly reduced by a

vacuum pump, water begins to boil

• molecules with highest kinetic energy escape

and the remaining water is cooled

• pressure is reduced further, more faster-moving

molecules boil away until the remaining water

reaches 0 C


Boiling and Freezing

Freezing by evaporation (continued)

• as cooling continues by boiling, ice forms over

the surface of the bubbling water, resulting in

frozen bubbles of boiling water


Melting and Freezing

Melting

• occurs when a substance changes phase from a

solid to a liquid

• opposite of freezing

• When heat is supplied to a solid, added vibration

breaks molecules loose from the structure and

melting occurs.


Melting and Freezing

Freezing

• occurs when a liquid changes to a solid

• opposite of melting

• When energy is continually removed from a

liquid, molecular motion decreases until the

forces of attraction bind them together and

formation of ice occurs.


Energy and Change of Phase


• From solid to liquid to gas phase

– add energy

• From gas to liquid to solid phase

– remove energy



example of both vaporization and

condensation processes • Cooling cycle of refrigerator pumps a special fluid that

vaporizes and draws heat from stored food. The gas that

forms along with its energy is directed to the

condensation coils outside the fridge where heat is

released and the fluid condenses back to liquid.

• air conditioner pumps heat energy from one part of the

unit to another



Heat of fusion

• amount of energy needed to change any

substance from solid to liquid and vice versa

example:

• heat of fusion for water is 334 joules/g

• farmers in cold climates replaced frozen tubs of water

with unfrozen ones in their cellars to prevent jars of

food from freezing



Heat of vaporization

• amount of energy needed to change any

substance from liquid to gas and vice versa

example:

• heat of vaporization for water is 2256 joules/g

• In briefly touching a hot skillet, energy that normally

would flow into your finger instead vaporizes water.

Hence, you’re not burned.


Change of Phases for Water


Backup


When snow forms in clouds, the surrounding air is

A. cooled.

B. warmed.

C. insulated.

D. thermally conducting.


CHECK YOUR NEIGHBOR


When snow forms in clouds, the surrounding air is

A. cooled.

B. warmed.

C. insulated.

D. thermally conducting.

Explanation:

The change of phase is from gas to solid, which releases energy.


CHECK YOUR ANSWER


Which involves the greatest number of calories?

A. condensing 1 gram of 100 C steam to 100 water

B. cooling 1 gram of 100 C water to 1 gram of 0 C ice

C. cooling 1 gram of 0 C ice to near absolute zero

D. all about the same


CHECK YOUR NEIGHBOR


Which involves the greatest number of calories?

A. condensing 1 gram of 100 C steam to 100 C water

B. cooling 1 gram of 100 C water to 1 gram of 0 C ice

C. cooling 1 gram of 0 C ice to near absolute zero

D. all about the same

Explanation:

540 calories is more than the 100 calories for B, and half of 273

calories to cool ice (the specific heat of ice is about half that for

liquid water).


CHECK YOUR ANSWER


Ice is put in a picnic cooler. To speed up the cooling of

cans of beverage, it is important that the ice

A. melts.

B. is prevented from melting.

C. be in large chunks.

D. none of these


CHECK YOUR NEIGHBOR


Ice is put in a picnic cooler. To speed up the cooling of

cans of beverage, it is important that the ice

A. melts.

B. is prevented from melting.

C. be in large chunks.

D. none of these

Explanation:

For each gram of ice that melts, 540 calories is taken from the

beverage.


CHECK YOUR ANSWER

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Conceptual Physics Fundamentalsalee3/Physics 11/Powerpoint Lectures/Chap9.pdf• Heat Transfer and Change of Phase ... walking barefoot without ... •transfer of heat involving only