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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Conceptual Physics
Fundamentals
Chapter 9:
HEAT TRANSFER AND
CHANGE OF PHASE
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Heat Transfer and Change of
Phase “You won’t fully appreciate the frontiers of
physics until you’re familiar with its foothills.”
—Iain MacInnes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Heat Transfer and Change of
Phase Objects in thermal contact at different
temperatures tend to reach a common
temperature in three ways:
• conduction
• convection
• radiation
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Conduction
Conduction
• transfer of internal energy by electron and
molecular collisions within a substance,
especially a solid
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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
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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
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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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
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
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Conduction
• Insulation (continued)
dramatic example: walking barefoot without
burning feet on red-hot
coals is due to poor
conduction between
coals and feet
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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
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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
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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!
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
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
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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
Explanation:
Earth’s atmosphere acts as a blanket, which keeps the valleys
from freezing at nighttime.
Convection
CHECK YOUR ANSWER
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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.
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Radiation
Radiation
• transfer of energy from the Sun through empty
space
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The surface of Earth loses energy to outer space due
mostly to
A. conduction.
B. convection.
C. radiation.
D. radioactivity.
Radiation
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
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
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Which body glows with electromagnetic waves?
A. Sun
B. Earth
C. both A and B
D. neither A nor B
Radiation
CHECK YOUR NEIGHBOR
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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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
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)
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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
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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
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Radiation
Emission of radiant energy (continued)
• frequency of radiation is proportional to the absolute temperature of the source ( )
f ~ T
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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)
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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
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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)
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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.
D. none of the above
Radiation
CHECK YOUR NEIGHBOR
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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.
D. none of the above
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
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A hot pizza placed in the snow is a net
A. absorber.
B. emitter.
C. both A and B
D. none of the above
Radiation
CHECK YOUR NEIGHBOR
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A hot pizza placed in the snow is a net
A. absorber.
B. emitter.
C. both A and B
D. none of the above
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
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Which melts faster in sunshine—dirty snow or clean snow?
A. dirty snow
B. clean snow
C. both A and B
D. none of the above
Radiation
CHECK YOUR NEIGHBOR
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Which melts faster in sunshine—dirty snow or clean snow?
A. dirty snow
B. clean snow
C. both A and B
D. none of the above
Explanation:
Dirty snow absorbs more sunlight, whereas clean snow reflects
more.
Radiation
CHECK YOUR ANSWER
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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
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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
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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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
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
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Eureka video of Conduction, Convection & Radiation
Conduction, Convection and Radiation song
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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
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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.
Newton’s Law of Cooling
CHECK YOUR NEIGHBOR
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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.
Newton’s Law of Cooling
CHECK YOUR ANSWER
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Global Warming and the
Greenhouse Effect
Greenhouse effect
• named for a similar temperature-raising
effect in florists’ greenhouses
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Global Warming and the
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
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Global Warming and the
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.
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Global Warming and the
Greenhouse Effect Global warming
• energy absorbed from the Sun
• part reradiated by Earth as longer-wavelength terrestrial radiation
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Global Warming and the
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
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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
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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
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Heat Transfer and Change of
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)
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Heat Transfer and Change of
Phase Evaporation
• change of phase from liquid to gas
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Heat Transfer and Change of
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
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Heat Transfer and Change of
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
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Heat Transfer and Change of
Phase
Sublimation
• form of phase change directly from solid to gas
example:
• dry ice (solid carbon dioxide molecules)
• mothballs
• frozen water
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Heat Transfer and Change of
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
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Heat Transfer and Change of
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
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Heat Transfer and Change of
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
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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
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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
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Boiling
Boiling process
• rapid evaporation from beneath the surface of a liquid
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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)
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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
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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)
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Boiling
demonstration of cooling effect of
evaporation and boiling
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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
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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
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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
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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
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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.
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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.
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Energy and Change of Phase
Energy and Change of Phase
• From solid to liquid to gas phase
– add energy
• From gas to liquid to solid phase
– remove energy
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Energy and Change of Phase
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
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Energy and Change of Phase
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
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Energy and Change of Phase
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.
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Change of Phases for Water
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Backup
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When snow forms in clouds, the surrounding air is
A. cooled.
B. warmed.
C. insulated.
D. thermally conducting.
Energy and Change of Phase
CHECK YOUR NEIGHBOR
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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.
Energy and Change of Phase
CHECK YOUR ANSWER
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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
Energy and Change of Phase
CHECK YOUR NEIGHBOR
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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).
Energy and Change of Phase
CHECK YOUR ANSWER
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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
Energy and Change of Phase
CHECK YOUR NEIGHBOR
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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.
Energy and Change of Phase
CHECK YOUR ANSWER