# HEAT TRANSFER (PDF) by wuxiangyu

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```									           HEAT
TRANSFER
Objectives
• Explain how conduction works.
(22.1)
HEAT TRANSFER
• Explain how convection works.
(22.2)
Heat can be transferred by

..........
• Explain how heat can be                             THE BIG                    conduction, by convection,
transmitted through empty
space. (22.3)                                       IDEA                       and by radiation.
• Identify which substances emit
• Compare the ability of an
with its ability to absorb

T
• Relate the temperature                    he spontaneous transfer of heat
difference between an object              is always from warmer objects
and its surroundings to the rate          to cooler objects. If several
at which it cools. (22.6)
objects near one another have dif-
• Identify the main driver of
ferent temperatures, then those that
global warming and climate
change. (22.7)                      are warm become cooler and those
that are cool become warmer, until
discover!                            all have a common temperature.
This equalization of temperatures
MATERIALS  black construction
paper, hole punch or pencil,
is brought about in three ways:
white polystyrene cup                by conduction, by convection, and
EXPECTED OUTCOME Even
though the inside of the cup is
white, the hole looks black.
ANALYZE AND CONCLUDE
1. Both are the same.
2. The hole would no longer
appear dark.
discover!
3. Light entering a small            Does White Ever Appear Black?                           Analyze and Conclude
opening is reflected from the
inside surfaces many times.          1. Using a paper punch or sharp pencil, make            1. Observing Which is darker, the construction
Some of the light is partially          a small hole in the center of a black sheet of          paper or the hole?
absorbed at each reflection             construction paper.                                  2. Predicting What do you think will happen if
until none remains.                  2. Place the paper on top of a polystyrene                 you enlarge the hole?
TEACHING TIP If the aperture            coffee cup or any cup that is all white inside.      3. Making Generalizations Why do openings
is made too large, some light                                                                   such as the pupil of the eye and doorways
entering the hole will find its                                                                 of distant houses appear black even in the
way out of the cavity. Students                                                                 daytime?
may also find that the hole
will not appear black if viewed
under a bright light.

430                                  430
22.1 Conduction
22.1 Conduction                                                                                            Key Terms
conduction, conductors, insulator
If you hold one end of an iron rod in a flame, as shown in Figure
22.1, before long the rod will become too hot to hold. Heat has trans-                                         Common Misconception
ferred through the metal by conduction. Conduction of heat is the                                          Surfaces that feel cooler than others
must have a lower temperature.
transfer of energy within materials and between different materials
FACT Surfaces that have been
that are in direct contact. Materials that conduct heat well are known
FIGURE 22.1                  in the same vicinity for some
as heat conductors. Metals are the best conductors. Among the                                              time should all have the same
Heat from the flame
common metals, silver is the most conductive, followed by copper,             causes atoms and free        temperature—that of the
aluminum, and iron.                                                           electrons in the end of      vicinity! One surface may feel
Conduction is explained by collisions between atoms or mol-               the metal to move faster     colder than another simply
and jostle against others.   because it is a better conductor.
ecules, and the actions of loosely bound electrons. In conduction,
Those particles do the           Teaching Tip Explain that
collisions between particles transfer thermal energy, without any
same and increase the        the physics of the phenomenon
overall transfer of matter. When the end of an iron rod is held in a          energy of vibrating atoms    of walking harmlessly on red-
flame, the atoms at the heated end vibrate more rapidly. These atoms          along the length of the      hot wooden coals with bare feet
vibrate against neighboring atoms, which in turn do the same. More            rod.                         is the same as the physics that
important, free electrons that can drift through the metal are made to                                     allows one to momentarily place
jostle and transfer energy by colliding with atoms and other free elec-                                    one’s hand in a very hot oven
without harm—not because
trons within the rod.                                                                                      the temperature is low but
because air is a poor conductor
Conductors Materials composed of atoms with “loose” outer elec-                                            of heat. Conductivity, not only
trons are good conductors of heat (and electricity also). Because met-                                     temperature, must be considered.
als have the “loosest” outer electrons, they are the best conductors of                                    Explain that since wood has low
heat and electricity.                                                                                      heat conductivity, it is used for
handles on cooking utensils.
Wood is a poor conductor,
FIGURE 22.2                                                           even when it’s red hot. After
The tile floor feels cold to the                                      the surface of a red-hot coal of
bare feet, while the carpet at the                                    low-conductivity wood gives up
same temperature feels warm.                                          its heat, perhaps to a bare foot
This is because tile is a better                                      that has just stepped on it, more
conductor than carpet.                                                than 1 second passes before
appreciable internal energy from
the inside reheats the surface.
So although the coal has a very
high temperature, it gives up
very little heat in a brief contact
Touch a piece of metal and a piece of wood in your immediate                                           with a cooler surface. The physics
vicinity. Which one feels colder? Which is really colder? Your answers                                     of hot-coal walking! The result
should be different. If the materials are in the same vicinity, they                                       would be very different indeed
should have the same temperature, room temperature. Thus nei-                 think!                       should a person try to walk over
red-hot pieces of iron. Caution:
ther is really colder. Yet, the metal feels colder because it is a better     If you hold one end of a
Warn your students not to try
conductor, like the tile in Figure 22.2; heat easily moves out of your        metal bar against a piece
either of these themselves!
warmer hand into the cooler metal. Wood, on the other hand, is a              of ice, the end in your
hand will soon become
poor conductor. Little heat moves out of your hand into the wood, so
cold. Does cold flow from
your hand does not sense that it is touching something cooler. Wood,          the ice to your hand?
wool, straw, paper, cork, and polystyrene are all poor heat conduc-           Answer: 22.1.1
tors. Instead, they are called good insulators.

CHAPTER 22       HEAT TRANSFER     431                                    431
Demonstration
Insulators Liquids and gases generally make poor conductors—
Place blobs of wax or butter
on rods of various metals.
they are good insulators. An insulator is any material that is a poor
Place each rod a similar                                             conductor of heat and that delays the transfer of heat. Air is a very
distance from a hot flame with                                       good insulator. Porous materials having many small air spaces are
the blob of wax or butter at                                         good insulators. The good insulating properties of materials such
the end of each rod farther
as wool, fur, and feathers are largely due to the air spaces they con-
from the flame. Notice how
the heat is conducted along                                          tain. Birds vary their insulation by fluffing their feathers to create air
FIGURE 22.3
the rods at different rates. This   A “warm” blanket                 spaces. Be glad that air is a poor conductor, for if it were not, you’d
demonstration illustrates the       does not provide you with        feel quite chilly on a 25°C (77°F) day!
relative conductivities of the      heat; it simply slows the            Snowflakes imprison a lot of air in their crystals and are good in-
different metals.                   transfer of your body heat       sulators. Snow slows the escape of heat from Earth’s surface, shields
to the surroundings.
Eskimo dwellings from the cold, and provides protection from the
Teaching Tip Discuss the                                          cold to animals on cold winter nights. Snow, like the blanket in
poor conductivity of air, and                                         Figure 22.3, is not a source of heat; it simply prevents any heat from
its role in insulating materials,                                     escaping too rapidly.
e.g., down-filled sleeping bags
and sportswear, spun glass and
Styrofoam insulation, fluffy
blankets, and even snow.

FIGURE 22.4
Snow lasts longest on the
roof of a well-insulated
house. Thus, the snow pat-
terns reveal the conduction,
or lack of conduction, of
heat through the roof. The
houses with more snow on
the roof are better insulated.

Heat is energy and is tangible. Cold is not; cold is simply the
absence of heat. Strictly speaking, there is no “cold” that passes
In conduction,                                           through a conductor or an insulator. Only heat is transferred. We
......

CONCEPT
CHECK        collisions between                                       don’t insulate a home, such as some of those in Figure 22.4, to keep
particles transfer thermal energy,     think!                         the cold out; we insulate to keep the heat in. If the home becomes
without any overall transfer of
matter.                                You can place your hand        colder, it is because heat flows out.
into a hot pizza oven for          It is important to note that no insulator can totally prevent heat
several seconds without        from getting through it. An insulator just reduces the rate at which
Teaching Resources                  harm, whereas you’d            heat penetrates. Even the best-insulated warm homes in winter will
• Reading and Study                   never touch the metal          gradually cool. Insulation slows down heat transfer.
Workbook                           inside surfaces for even a
......

• PresentationEXPRESS                 second. Why?                   CONCEPT
How does conduction transfer heat?
• Interactive Textbook
• Next-Time Question 22-1
• Conceptual Physics Alive!
DVDs Heat Transfer

432                                  432
22.2 Convection
22.2 Convection                                                                                           Key Term
convection
Conduction involves the transfer of energy from molecule to mol-              think!
ecule. Energy moves from one place to another, but the molecules
themselves do not. Another means of heat transfer is by movement              You can hold your fingers         Demonstration
beside the candle flame
of the hotter substance. Air in contact with a hot stove rises and                                         Do the activity in Figure 22.5,
without
warms the region above. Water heated in a boiler in the basement              harm, but                    with ice wedged at the
rises to warm the radiators in the upper floors. This is convection,          not above                    bottom of a test tube. Some
a means of heat transfer by movement of the heated substance itself,          the flame.                   steel wool will hold the ice
Why?                         at the bottom of the tube. It
such as by currents in a fluid.                                                                            is impressive to see that the
water at the top is brought
FIGURE 22.5                                                            to a boil while the ice below
When the test tube is heated at                                        barely melts! (Convection, or
the top, convection is prevented                                       better, the lack of convection,
and heat can reach the ice by con-                                     is illustrated here. If heating
duction only. Since water is a poor                                    were at the bottom and the
conductor, the top water will boil                                     ice cube at the top, the ice
without melting the ice.                                               would quickly melt.)

In convection, heat is transferred by movement of the hot-                                         discover!
Convection ovens are
ter substance from one place to another. A simple demonstration                simply ovens with a fan     MATERIALS  beaker, water, heat
illustrates the difference between conduction and convection. With a           inside, which speeds up     source, dark dye
bit of steel wool, trap a piece of ice at the bottom of a test tube nearly     cooking by circulating
EXPECTED OUTCOME Though the
filled with water. Hold the tube by the bottom with your bare hand             the warmed air.
dye disperses quite rapidly, if
and place the top in the flame of a Bunsen burner, as shown in                                             they watch carefully, students
Figure 22.5. The water at the top will come to a vigorous boil while                                       will see that it follows the
the ice below remains unmelted. The hot water at the top is less dense                                     convection flow pattern.
and remains at the top. Any heat that reaches the ice must be trans-                                       THINK  In smoke, steam, and in
ferred by conduction, and we see that water is a poor conductor of                                         the air over a hot stove
heat. If you repeat the experiment, only this time holding the test
tube at the top by means of tongs and heating the water from below
while the ice floats at the surface, the ice will melt quickly. Heat gets
to the top by convection, for the hot water rises to the surface, carry-
ing its energy with it to the ice.

discover!
Can You See Convection?
1. Bring a beaker full of water to a boil.
2. Drop a small amount of dark dye or food coloring into the water.
What path does it take as it flows through the water?
3. Think Give three other examples of where you can see the paths
of convection.

CHAPTER 22      HEAT TRANSFER      433                                433
Teaching Tip Explain that
the lack of convection in orbiting
vehicles such as the space shuttle
Convection occurs in all fluids, whether liquid or gas. Whether we
has interesting consequences.                                        heat water in a pan or heat air in a room, the process is the same, as
For example, in orbit, one cannot                                    shown in Figure 22.6. When the fluid is heated, it expands, becomes
light a match without it snuffing                                    less dense, and rises. Warm air or warm water rises for the same
out very quickly. This is because
reason that a block of wood floats in water and a helium-filled
of the absence of convection in
orbit. Much of the convection                                        balloon rises in air. In effect, convection is an application of
in fluids depends on buoyancy,                                       Archimedes’ principle, for the warmer fluid is buoyed upward by
which in turn depends on gravity.                                    denser surrounding fluid. Cooler fluid then moves to the bottom, and
In orbit the local effects of                                        the process continues. In this way, convection currents keep a fluid
gravity are not there (because
the shuttle and everything in the     b                              stirred up as it heats. Convection currents also have a large influence
shuttle are freely falling around                                    on the air in the atmosphere.
Earth). With no convection, hot
gases are not buoyed upward                                          Moving Air Convection currents stirring the atmosphere produce
away from a flame but remain                                         winds. Some parts of Earth’s surface absorb heat from the sun more
around the flame, preventing         FIGURE 22.6                     readily than others. The uneven absorption causes uneven heating of
the entry of needed oxygen. The      Convection occurs in all
flame burns out.                                                     the air near the surface and creates convection currents. This phenom-
fluids. a. Convection
currents transfer heat in       enon is often evident at the seashore. In the daytime the shore warms
Teaching Tip Discuss the role
of convection in climates. Call      air. b. Convection currents     more easily than the water. Air over the shore rises, and cooler air from
attention to the shift in winds as   transfer heat in liquid.        above the water takes its place. The result is a sea breeze, as shown in
shown in Figure 22.7.                                                Figure 22.7.
Ask Why does the direction                                            At night the process reverses as the shore cools off more quickly
of coastal winds change from                                         than the water—the warmer air is now over the sea. If you build a
day to night? Land warms faster                                      fire on the beach you’ll notice that the smoke sweeps inward in the
than water, and in the day the                                       day and seaward at night.
land and the air above it are
warmer than the water and the
air above it. The air rises and
FIGURE 22.7
results in a sea breeze from water
to land. At night, the reverse       Convection currents are pro-
happens.                             duced by uneven heating.

cloud, or is a cloud high-altitude
fog? They are the same. Both are
water-saturated air at different
altitudes.

a. During the day, the land is warmer than the      b. At night, the land is cooler than the water,
air, and a sea breeze results.                       so the air flows in the other direction.

434                                  434
discover!
discover!                                                                            On a much larger
EXPECTED OUTCOME When the
scale, convection due
student blows on his or her
Is Your Breath Warm or Cold?                                                         to uneven solar heat-
hand through the smaller gap
1. With your mouth open wide, blow on your hand.                                     ing of Earth’s surface
in the lips, the air feels cooler.
Note the temperature of your breath.                                              combines with the
effects of Earth’s           THINK  The warm breath
2. Now pucker your lips to make a small opening                                      rotation to contribute       expands more when blown
with your mouth and blow on your hand again.                                      to overall global wind       through a narrow gap.
Does the temperature of your breath feel the same?                                patterns.                    Expanding air cools and so
3. Think In which case does your exhaled breath expand more—                                                      feels cooler when on the hand.
when blowing with your mouth open wide or when blowing with
your lips puckered? When did the air on your hand feel cooler?
Explain why.                                                                                                     Teaching Tip Explain that
when a portion of air is heated, it
expands and becomes less dense
than the surrounding air. The
Cooling Air Rising warm air, like a rising balloon, expands. Why?                                                 buoyancy force becomes greater
Because less atmospheric pressure squeezes on it at higher altitudes.                                             than the weight and the warm
As the air expands, it cools—just the opposite of what happens when                                               air rises. When it rises, it expands
and cools.
air is compressed. If you’ve ever compressed air with a tire pump, you
probably noticed that the air and pump became quite hot. The oppo-                                                     Ask Since warm air rises,
why are mountain tops cold
site happens when air expands. Expanding air cools.
and snow covered, and the
valleys below relatively warm
and green? Shouldn’t it be the
other way around? No, nature
is correct—as warm air rises, it
cools. The cool tops of mountains
are a consequence of rising warm
FIGURE 22.8
When a molecule collides with a target molecule that is
receding, its rebound speed after the collision is less than                                                     Demonstration
it was before the collision.
We can understand the cooling of expanding air by thinking of                                                 cannot do the same with your
molecules of air as tiny balls bouncing against one another. Speed                                                 fingers above the flame. (The
air above the flame is hotter
is picked up by a ball when it is hit by another that approaches with                                              than the air beside it because
a greater speed. When a ball collides with one that is receding, its                                               of the convection flow.)
rebound speed is reduced, as shown in Figure 22.8. Likewise for a
table-tennis ball moving toward a paddle; it picks up speed when it
In convection, heat

......
hits an approaching paddle, but loses speed when it hits a receding                                               CONCEPT
paddle. This also applies to a region of air that is expanding; mol-                                              CHECK       is transferred by
movement of the hotter
ecules collide, on the average, with more molecules that are receding                                             substance from one place to
than are approaching, as shown in Figure 22.9. Thus, in expanding                  FIGURE 22.9
another.
Molecules in a region of
air, the average speed of the molecules decreases and the air cools.22.2           expanding air collide more
often with receding mol-          Teaching Resources
......

CONCEPT
How does convection transfer heat?                               ecules than with approaching
CHECK                                                                              ones.
• Transparency 42
• Next-Time Question 22-2

CHAPTER 22        HEAT TRANSFER     435                                     435
FIGURE 22.10
Key Terms                                  Radiant energy is transmitted as electromagnetic waves.
Teaching Tip Discuss the
hot coals in a fireplace and
decreases with distance. Consider
The heat one feels is not so much
because of the sun’s temperature,          a. Radio waves send signals                         b. You feel infrared waves   c. A visible form of radiant
but because the sun is big!                   through the air.                                   as heat.                     energy is light waves.
Teaching Tip Explain that
Earth is warmer at the equator
than at the poles because of
greater solar energy per unit area
(not because it is closer to the
sun). Ask students to compare                                              How does the sun warm Earth’s surface? It can’t be through conduc-
the rays of sunlight striking
Earth with rain that strikes
tion, because there is 150 million kilometers of virtually nothing
two pieces of paper—one held                                               between Earth and the sun. Nor can it be by convection, because there
horizontally and the other held                                            is nothing between the sun and Earth to expand and rise. The sun’s
at an angle in the rain. Dispel                                            heat is transmitted by another process—by radiation.22.3.1 Radiation
the misconception that the paper
is energy transmitted by electromagnetic waves, as shown in Figure
held horizontally must get wetter
than the paper held at an angle            FIGURE 22.11                    22.10. What is being radiated from the sun is primarily light.
because it is closer to the clouds!        Most of the heat from a fire-        Radiant energy is any energy that is transmitted by radiation.
place goes up the chimney           In radiation, heat is transmitted in the form of radiant energy,
by convection. The heat that
warms us comes to us by
radiation.                      microwaves, infrared radiation (such as the heat from the fireplace in
Figure 22.11), visible light, ultraviolet radiation, X-rays, and gamma
rays. These types of radiant energy are listed in order of wavelength,
from longest to shortest.22.3.2
discover!
......

CONCEPT
MATERIALS          heat source, pair of                                   CHECK
glasses
EXPECTED OUTCOME Students
will find that the effects of the                                           discover!
heat are less when they put on
the glasses.                                                                Why Do Glasses Keep You Cool?
THINK The lenses do not                                                     1. Sit close to a fire in a fireplace and
transmit the infrared waves (or                                                feel the heat on your closed eyelids.
heat) from the fire.                                                        2. Now slip a pair of glasses over your
eyes. How do your eyes feel?
3. Think Why did the glasses cause your
......

CONCEPT                                                                         eyes to feel a different temperature?
CHECK       transmitted in the
electromagnetic waves.

436                                        436
22.4 Emission of
All substances continuously emit radiant energy in a mixture                                         Key Terms
of wavelengths. Objects at low temperatures emit long waves, just                Everything around
as long, lazy waves are produced when you shake a rope with little
energy as shown in Figure 22.12. Higher-temperature objects emit                 and absorbs energy
continuously!
waves of shorter wavelengths. Objects of everyday temperatures emit                                       If time is short, Sections
waves mostly in the long-wavelength end of the infrared region, which                                     22.4 and 22.5 may be omitted
without consequence.
is between radio and light waves. Shorter-wavelength infrared waves
absorbed by our skin produce the sensation of heat. Thus, when we

Common Misconception
FIGURE 22.12
Shorter wavelengths                                          FACT All objects continually emit
are produced when                                            radiant energy in a mixture of
the rope is shaken                                           wavelengths.
more rapidly.

The fact that all objects in our environment continuously emit          FIGURE 22.13
infrared radiation underlies infrared thermometers such as the one in       An infrared thermometer
Figure 22.13. How nice it is that you simply point the thermometer at       measures the infrared
something whose temperature you want, press a button, and a digital         radiant energy emitted by
temperature reading appears. The radiation emitted by the object            a body and converts it to
temperature.
whose temperature you wish to know provides the reading. Typical
classroom infrared thermometers operate in the range of about
–30°C to 200°C.
The average frequency f of radiant energy is directly propor-
tional to the Kelvin temperature T of the emitter:
f    T

People, with a surface temperature of 310 K, emit light in the low-
frequency infrared part of the spectrum, which is why we can’t see
each other in the dark. If an object is hot enough, some of the radi-
ant energy it emits is in the range of visible light. At a temperature of
about 500°C an object begins to emit the longest waves we can see,
red light. Higher temperatures produce a yellowish light. At about
1500°C all the different waves to which the eye is sensitive are emitted
and we see an object as “white hot.” You can see this relationship in
the temperatures of the stars. A blue-hot star is hotter than a white-
hot star, and a red-hot star is less hot. Since the color blue has nearly
twice the frequency of red, a blue-hot star has nearly twice the surface
temperature of a red-hot star. The radiant energy emitted by the stars

CHAPTER 22         HEAT TRANSFER     437                                    437
All substances

......
CONCEPT
CHECK       continuously emit
radiant energy in a mixture of        think!                            The surface of the sun has a high temperature (5500°C) and
wavelengths.                                                       therefore emits radiant energy at a high frequency—much of it in
Why is it that light radi-   the visible portion of the electromagnetic spectrum. The surface of
ated by the sun is yellow-
Teaching Resources                  ish, but light radiated by
Earth, by comparison, is relatively cool, and so the radiant energy it
Earth is infrared?           emits consists of frequencies lower than those of visible light. Radiant
Answer: 22.4                 energy that is emitted by Earth is called terrestrial radiation, which
Workbook
is in the form of infrared waves—below our threshold of sight. The
• PresentationEXPRESS
source of the sun’s radiant energy involves thermonuclear fusion
• Interactive Textbook
in its deep interior. In contrast, much of Earth’s supply of energy is
fueled by radioactive decay in its interior. So we see that both the sun
and Earth glow—the sun at high visible frequencies and Earth at low
22.5 Absorption of                                                 infrared frequencies. And both glows are related to nuclear processes
in their interiors. (We’ll treat radioactive decay in Chapter 39 and
thermonuclear fusion in Chapter 40.)
Teaching Tip Explain that                                            When radiant energy encounters objects, it is partly reflected
some materials absorb better                                       and partly absorbed. The part that is absorbed increases the internal
than others. The good absorbers
energy of the objects.
are easy to spot, because they

......
CONCEPT
appear black.                                                      CHECK

Make the distinction that
objects don’t absorb because
they’re black, but are black
If everything is emitting energy, why doesn’t everything finally run
because they absorb so well.
Cause precedes effect.                                            out of it? The answer is that everything also absorbs energy from its
environment.
Absorption and Emission For example, a book sitting on your
Teaching Tip Explain that
desk is both absorbing and radiating energy at the same rate. It is in
though there are various colors                                    thermal equilibrium with its environment. Imagine that you move the
of eyes, all have one thing in                                     book out into the bright sunshine. If the book’s temperature doesn’t
common: The pupils are black.                                      change, it radiates the same amount of energy as before. But because
This is because the light that                                     the sun shines on it, the book absorbs more energy than it radiates.
enters the eyes through the
pupils is absorbed. (An exception                                  Its temperature increases. As the book gets hotter, it radiates more
to this is that flash photography                                  energy, eventually reaching a new thermal equilibrium. Then it radi-
can sometimes produce photos                                       ates as much energy as it receives. In the sunshine the book remains
that show people with red eyes.       A hot pizza placed out-      at this new higher temperature.
This happens because the bright       side on a winter day is           If you move the book back indoors, the opposite process occurs.
flash can be reflected from the       a net emitter. The same
retina of the eye if the eye does                                  The hot book initially radiates more energy than it receives from
pizza placed in a hotter
not have time to adjust to the        oven is a net absorber.      its surroundings. So it cools. In cooling, it radiates less energy. At a
bright light. Some cameras have                                    sufficiently lowered temperature it radiates no more energy than it
a “red-eye reduction” setting.                                     receives from the room. It stops cooling. It has reached thermal equi-
This setting produces multiple
flashes that give the eyes time to
librium again.
taken.)

438                                  438
Teaching Tip Emphasize that
Good emitters of radiant energy are also good absorbers;                think!                       everything that has any
poor emitters are poor absorbers. For example, a radio antenna con-                                          temperature—but everything
structed to be a good emitter of radio waves is also, by its very design,       If a good absorber of        does not become progressively
a good receiver (absorber) of them. A poorly designed transmitting              radiant energy were a        cooler because everything also
poor emitter, how would      absorbs radiation. We live in
antenna is also a poor receiver.                                                its temperature compare      a sea of radiation, everything
A blacktop pavement and dark automobile body may remain                     with its surroundings?       emitting and everything
hotter than their surroundings on a hot day. But at nightfall these             Answer: 22.5                 absorbing. When emission
dark objects cool faster! Sooner or later, all objects in thermal contact                                    rate equals absorption rate,
come to thermal equilibrium. So a dark object that absorbs radiant                                           temperature remains constant.
Some materials, because of their
energy well emits radiation equally well.22.5                                                                molecular design, emit better
than others.
FIGURE 22.14
Even though the interior of
the box has been painted             Demonstration
white, the hole looks black.
Cut a hole in a sturdy box as
shown in Figure 22.14. Paint
the interior of the box white.
When the box is open, the
interior, as seen through the
hole, appears white. However,
when the box is closed, the
interior appears black because
the light that enters through
the hole is reflected from the
inside walls many times, and
is partly absorbed at each
reflection until very little (or
Absorption and Reflection Absorption and reflection are                                                       none) comes back out.
opposite processes. Therefore, a good absorber of radiant energy
energy we call light. So a good absorber appears dark. A perfect
absorber reflects no radiant energy and appears perfectly black. The
pupil of the eye, for example, allows radiant energy to enter with no
reflection and appears perfectly black. (The red “pupils” that appear
in some flash portraits are from direct light reflected off the retina at
the back of the eyeball.)
Look at the open ends of pipes in a stack. The holes appear black.
Look at open doorways or windows of distant houses in the daytime,
and they too look black. Openings appear black, as in Figure 22.14,                                                      Good emitters of

......
CONCEPT
because the radiant energy that enters is reflected from the inside                                          CHECK       radiant energy are
also good absorbers; poor
walls many times and is partly absorbed at each reflection until very
emitters are poor absorbers.
little or none remains to come back out. You can see this illustrated
in Figure 22.15.
FIGURE 22.15                                                               Teaching Resources
• Concept-Development
opening has little chance of leaving
Practice Book 22-1
before it is completely absorbed.
• Next-Time Question 22-3

CHAPTER 22         HEAT TRANSFER     439                                  439
22.6 Newton’s Law
of Cooling                                                                             FIGURE 22.16
Anything with a mirrorlike surface
Key Term                                                                                         reflects most of the radiant energy
Newton’s law of cooling                                                                          it encounters. That’s why it is a poor
Teaching Tip Relate the rate
of cooling to the black and silver
containers that are cooling and
warming. We see the difference                                                                       Good reflectors, on the other hand, are
between a proportionality                                                                        poor absorbers, like the toaster in Figure
sign and an equals sign for                                                                      22.16. Light-colored objects reflect more
the formula here, for the
rate of cooling or warming is                                                                    light and heat than dark-colored ones. In
proportional not only to the                                                                     summer, light-colored clothing keeps
difference in temperatures but                                                                   people cooler.
also to the differences in the                                           On a sunny day Earth’s surface is a net absorber. At night it is
“emissivities” of the surfaces.
a net emitter. On a cloudless night its “surroundings” are the frigid
Teaching Tip Point out that D                                     depths of space and cooling is faster than on a cloudy night, where
means “the change in.”                                              the surroundings are nearby clouds. Record-breaking cold nights
Teaching Tip Relate Newton’s                                     occur when the skies are clear.
law of cooling to Think and                                              The next time you’re in the direct light of the sun, step in and
Explain 34 (cream in the coffee),
35 (cooling a beverage in the
out of the shade. You’ll note the difference in the radiant energy you
fridge), and 37 (thermostat on a                                    receive. Then think about the enormous amount of energy the sun
cold day). These questions make                                     emits to reach you some 150,000,000 kilometers distant. Is the sun
excellent discussion topics.                                        unusually hot? Not as hot as some welding torches in auto shops.
You feel the sun’s heat not because it is hot (which it is), but primar-
Demonstration                                                 ily because it is big. Really big!
CONCEPT How does an object’s emission rate compare with its
Fill a beaker with warm
water and a similar beaker                                         CHECK    ......   absorption rate?
with boiling water. Record
the temperatures of the
two beakers at regular                                             22.6 Newton’s Law of Cooling
intervals as they cool to
room temperature. Note the                                         An object hotter than its surroundings eventually cools to match the
different rates of cooling.                                        surrounding temperature. When considering how quickly (or slowly)
something cools, we speak of its rate of cooling—how many degrees
change per unit of time.
cooling apply to the warming                                            The rate of cooling of an object depends on how much hotter
of a cold object in a warm            think!                        the object is than the surroundings. The colder an object’s sur-
environment? Yes                                                    roundings, the faster the object will cool. The temperature change
Since a hot cup of tea
loses heat more rapidly       per minute of a hot apple pie will be more if the hot pie is put in a
The colder an object’s
......

CONCEPT                                                             cold freezer than if put on the kitchen table because the temperature
than a lukewarm cup of
CHECK       surroundings, the
tea, would it be correct to   difference is greater. A warm home will lose heat to the cold outside
faster the object will cool.
say that a hot cup of tea     at a greater rate when there is a larger difference between the inside
will cool to room temper-     and outside temperatures. Keeping the inside of your home at a high
Teaching Resources                  ature before a lukewarm
temperature on a cold day is more costly than keeping it at a lower
cup of tea will? Explain.
• Laboratory Manual 59
Answer: 22.6                  temperature. If you keep the temperature difference small, the rate of
• Probeware Lab Manual 10                                          cooling will be correspondingly low.
440                                  440
22.7 Global Warming
This principle is known as Newton’s law of cooling. (Guess who is                                   and the Greenhouse
Newton’s law of cool-
credited with discovering this?) Newton’s law of cooling states that        ing is an empirical rela-   Effect
the rate of cooling of an object—whether by conduction, convection,         tionship and not
or radiation—is approximately proportional to the temperature dif-          a fundamental law like      Key Term
ference DT between the object and its surroundings:                         Newton’s laws               greenhouse effect
of motion.
Common Misconception
rate of cooling    T                                                       The greenhouse effect on Earth is
undesirable.
Newton’s law of cooling also holds for heating. If an object is                                     FACT The greenhouse effect
cooler than its surroundings, its rate of warming up is also propor-                                    provides a temperature that
supports life as we know
tional to DT. Frozen food warms up faster in a warmer room.                                             it. Without it, the average
temperature of Earth would be
......

CONCEPT
What causes an object to cool faster?                                                 about 218ºC. What is undesirable
CHECK                                                                                                   is an increase in this effect.
Teaching Tip Discuss the
22.7 Global Warming and the                                                                             greenhouse effect, first for
florists’ greenhouses, and then
Greenhouse Effect                                                                                  for Earth’s atmosphere. The key
idea is that the medium (glass
An automobile sitting in the bright sun on a hot day with its win-                                      for the greenhouse, atmosphere
dows rolled up can get very hot inside—appreciably hotter than            Physics on the Job            for Earth) is transparent to high-
the outside air. This is an example of the greenhouse effect, so                                        frequency (short wavelength)
named for the same temperature-raising effect in florists’ glass          Ecologist                     electromagnetic waves but
opaque to low-frequency (long
greenhouses. The greenhouse effect is the warming of a planet’s            The greenhouse effect        wavelength) electromagnetic
surface due to the trapping of radiation by the planet’s atmo-             is of particular con-        waves.
sphere. Understanding the greenhouse effect requires knowing               cern to the ecologist.
Ecologists study the            Teaching Tip Point out that
about two concepts.                                                                                     Earth’s atmosphere is primarily
relationship between
the living and nonliving     warmed by terrestrial radiation,
Causes of the Greenhouse Effect The first concept has been                                              not solar radiation. That’s why air
previously stated—that all things radiate, and the frequency and           factors in an ecosys-
near the ground is warmer than
tem. Ecologists need
wavelength of radiation depends on the temperature of the object           to use physics when
air above. The opposite would
emitting the radiation. High-temperature objects radiate short waves;                                   be the case if the sun were the
they analyze changes         primary warmer of air!
low-temperature objects radiate long waves. The second concept we          in atmospheric tem-
need to know is that the transparency of things such as air and glass      peratures over time.            Teaching Tip Explain that
Understanding the            terrestrial radiation also cools
depends on the wavelength of radiation. Air is transparent to both
relationships between        Earth, especially on clear nights.
infrared (long) waves and visible (short) waves, unless the air con-                                    Clouds reradiate terrestrial
energy, temperature,
tains excess carbon dioxide and water vapor, in which case it absorbs      and greenhouse gases         radiation. Farmers sometimes
infrared waves. Glass is transparent to visible light waves but absorbs    enables ecologists to        use smudge pots in orchards
infrared waves. (This is discussed later, in Chapter 27.)                  identify processes that      to create a cloud close to the
ground. This enables terrestrial
Now to why that car gets so hot in bright sunlight: Compared with      interfere with Earth’s
the car, the sun’s temperature is very high. This means the wavelengths    natural processes.
smoke) to be reradiated to the
of waves the sun radiates are very short. These short waves easily pass    Ecologists can find
ground resulting in a longer
opportunities in gov-        cooling time for the ground.
through both Earth’s atmosphere and the glass windows of the car. So       ernment and privately
energy from the sun gets into the car interior, where, except for some                                  This helps crops survive nights
funded projects.             without freezing.
reflection, it is absorbed. The interior of the car warms up.

CHAPTER 22       HEAT TRANSFER      441                                   441
Teaching Tip Briefly discuss
the idea of wave frequency.
Review Figure 22.12, showing the                                             The car interior radiates its own waves, but since it is not as
relationship of wave frequency                                           hot as the sun, the radiated waves are longer. The reradiated long
to wavelength. The origin                                                waves encounter glass windows that aren’t transparent to them. So
of electromagnetic waves is                                              most of the reradiated energy remains in the car, which makes the
vibrating electrons in matter.
Explain that the frequency
car’s interior even warmer. (That is why leaving your pet in a car on a
of electromagnetic radiation                                             hot sunny day is a no-no.) As hot as the interior gets, it won’t be hot
emitted by a source increases                                            enough to radiate waves that can pass through glass (unless it glows
with the temperature of the                                              red or white hot!).
source. Electrons vibrate at                                                 The same effect occurs in Earth’s atmosphere, which is transpar-
greater frequencies in hot matter
than in cold matter. The sun                                             ent to solar radiation, as shown in Figure 22.17. The surface of Earth
is so hot that the frequency of                                          absorbs this energy, and reradiates part of this at longer wavelengths, as
electromagnetic waves it emits                                           shown in Figure 22.18. Energy that Earth radiates is called terrestrial
is high enough to activate our                                           radiation. Atmospheric gases (mainly water vapor, carbon dioxide,
visible receptors. Write f , T
and methane) absorb and re-emit much of this long-wavelength
in big letters to indicate large
values of both frequency and                                             terrestrial radiation back to Earth. So the long-wavelength radia-
temperature. This radiation is                                           tion that cannot escape Earth’s atmosphere warms Earth. This global
visible light. It is absorbed by     FIGURE 22.17                        warming process is very nice, for Earth would be a frigid –18°C oth-
Earth, which in turn emits its own   Earth’s temperature                 erwise. Our present environmental concern is that increased levels of
radiation. Write f , T in small      depends on the energy
letters to indicate low values of    balance between incoming
carbon dioxide and other atmospheric gases in the atmosphere may
both frequency and temperature.      solar radiation and outgoing        further increase the temperature and produce a new thermal balance
terrestrial radiation.              unfavorable to the biosphere.22.7
Consequences of the Greenhouse Effect Averaged over a
few years, the amount of solar radiation that strikes Earth exactly
balances the terrestrial radiation Earth emits into space. This bal-
ance results in the average temperature of Earth—a temperature that
presently supports life as we know it. We now see that over a period
of decades, Earth’s average temperature can be changed—by natural
causes and also by human activity.

FIGURE 22.18
Earth’s atmosphere acts as a
sort of one-way valve. It allows
visible light from the sun in,
but because of its water vapor
and carbon dioxide content,
from leaving.

442                                  442
The carbon that is spewed by
burning is the same carbon that
is absorbed by tree growth. So
a realistic step in the solution
to the increased greenhouse
effect is simply to grow more
FIGURE 22.19                                trees (while decreasing the
Shorter-wavelength radiant energy           rate at which they are cut
from the sun enters through the             down)! This would not be an
glass roof of the greenhouse. The           end-all to the problem, however,
soil emits long-wavelength radi-            because the carbon returns to
ant energy, which is unable to pass         the biosphere when the trees
through the glass. Income exceeds           ultimately decay.
outgo, so the interior is warmed.

Adding materials such as those from the burning of fossil fuels
Volcanoes put more
to the atmosphere changes the absorption and reflection of solar
particulate matter into
radiation. Except where the source of energy is solar, wind, or water,       the atmosphere than
increased energy consumption on Earth adds heat. These activities can        industries and all
change the radiative balance and change Earth’s average temperature.         human activity. But
The near unanimous view of climate scientists is that human          when it comes to
activity is a main driver of global warming and climate change. This         carbon dioxide, the
impact of humans is
view is the outcome of a long, painstaking road of successively more
big enough to affect
sophisticated climate models.                                                climate.
Confidence in the models, run by more and more sophisticated
computers, is bolstered by an intriguing outcome: data gathered ear-
lier about Earth and its atmosphere that were fed into the models
successfully “predicted” the recent climate of the past twenty years.
Although water vapor is the main greenhouse gas, CO2 is the gas
most rapidly increasing in the atmosphere. Concern doesn’t stop
there, for further warming by CO2 can produce more water vapor as
The near unanimous

......
well. The greater concern is the combination of growing amounts                                            CONCEPT
of both these greenhouse gases.                                                                            CHECK          view of climate
scientists is that human activity is
An important credo is “You can never change only one thing.”                                          a main driver of global warming
Change one thing, and you change another. Burn fossil fuels and you                                        and climate change.
warm the planet. Increase global temperature and you increase storm
activity. Changed climate means changed rainfall patterns, changed                                            Teaching Resources
coastal boundaries, and changes in insect breeding patterns. How
these changes upon changes will play out, we don’t know.                                                       Workbook
What we do know is that energy consumption is related to popula-       For: Links on global warming    • Laboratory Manual 60
tion size. We are seriously questioning the idea of continued growth.       Visit: www.SciLinks.org         • Transparency 43
(Please take the time to read Appendix E, “Exponential Growth and           Web Code: csn – 2207            • PresentationEXPRESS
Doubling Time”—very important stuff.)
• Interactive Textbook
......

CONCEPT                                                                                                     • Next-Time Question 22-4
How does human activity affect climate change?
CHECK                                                                                                       • Conceptual Physics Alive!

CHAPTER 22         HEAT TRANSFER        443                                   443
REVIEW
For: Self-Assessment
Teaching Resources                                                                                        Visit: PHSchool.com
• TeacherEXPRESS
• Conceptual Physics Alive!
REVIEW                                                    Web Code: csa – 2200

DVDs Heat Transfer; Heat

Concept Summary                    ••••••
•   In conduction, collisions between parti-
cles transfer thermal energy, without any
22.1.1 Cold does not flow from the ice to your
hand. Heat flows from your hand to
overall transfer of matter.                          the ice. The metal is cold to your touch
•   In convection, heat is transferred by
movement of the hotter substance from
because you are transferring heat to the
metal.
one place to another.                         22.1.2 Air is a poor conductor, so the rate of heat
•   In radiation, heat is transmitted in the
form of radiant energy, or electromag-
flow from the hot air to your relatively cool
hand is low. But touching the metal parts
netic waves.                                         is a different story. Metal conducts heat
very well, and a lot of heat in a short time
•   All substances continuously emit radiant
energy in a mixture of wavelengths.
is conducted into your hand when thermal
•   Good emitters of radiant energy are also
good absorbers; poor emitters are poor        22.2    Heat travels upward by convection. Air is a
absorbers.                                            poor conductor, so very little heat travels
sideways.
•   The colder an object’s surroundings, the
faster the object will cool.                  22.4    The answer is that the sun has a higher
•   The near unanimous view of climate
scientists is that human activity is a main
in the infrared because its temperature is
relatively low compared to the sun.
driver of global warming and climate
change.                                       22.5    If a good absorber were not also a good
emitter, there would be a net absorption
Key Terms           ••••••                                of radiant energy and the temperature
of a good absorber would remain higher
conduction (p. 431)      stellar                          than the temperature of the surroundings.
conductors (p. 431)         radiation (p. 437)            Things around us approach a common
terrestrial                      temperature only because good absorbers
insulator (p. 432)                                        are, by their very nature, also good emit-
convection (p. 433)                                       ters.
Newton’s law of
radiation (p. 436)          cooling (p. 441)      22.6    No! Although the rate of cooling is greater
for the hotter cup, it has farther to cool to
energy (p. 436)                                         reach thermal equilibrium. The extra time
(p. 441)                      is equal to the time the hotter cup takes to
cool to the initial temperature of the luke-
warm cup of tea. Cooling rate and cooling
time are not the same.

444                           444
ASSESS
Check Concepts
ASSESS                                                                               1. They transfer energy through
the conducting material.
2. It is a better conductor and
draws more energy from a
person’s skin.
Check Concepts               ••••••              Section 22.3                                           3. A conductor moves heat
quickly, whereas an insulator
9. Dominoes are placed upright in a row, one             moves heat slowly.
Section 22.1
next to another. When one is tipped over, it
1. What is the role of “loose” electrons in                                                            4. They have many air spaces
knocks against its neighbor, which does the           and air is a good insulator.
heat conductors?                                 same in cascade fashion until the whole row        5. Cold is the absence of heat.
collapses. Which of the three types of heat        6. Warmed air is less dense and
transfer is this most similar to?                     is buoyed upward.
10. What is radiant energy?                            7. The land is warmer than the
water during the day, so
the air rises. The opposite
2. Why does a piece of room-temperature
happens at night.
metal feel cooler to the touch than paper,
8. Increases; decreases, if
3. What is the difference between a conductor                                                          9. Conduction
and an insulator?                                                                                  10. The energy in electromagnetic
waves
4. Why are materials such as wood, fur,         Section 22.4                                          11. Higher temperature sources
feathers, and even snow good insulators?     11. How does the predominant frequency of                 produce waves of higher
frequencies.
radiant energy vary with the absolute tem-
12. Good; otherwise there would
perature of the radiating source?                     be no thermal equilibrium.
12. Is a good absorber of radiation a good emit-      13. Black is a better emitter, and
ter or a poor emitter?                                so will cool faster.
14. It absorbs rather than reflects
13. Which will normally cool faster, a black pot          light.
5. What is meant by saying that cold is not         of hot tea or a silvered pot of hot tea?          15. Light entering is absorbed.
a tangible thing?
Section 22.2
6. How does Archimedes’ principle relate to
convection?
7. Why does the direction of coastal winds
change from day to night?
Section 22.5
8. How does the temperature of a gas
change when it is compressed? When           14. Why does a good absorber of radiant
it expands?                                      energy appear black?
15. Why do eye pupils appear black?

CHAPTER 2222
CHAPTER             HEAT TRANSFER
HEAT TRANSFER   445                                  445
16. Cold room; greater DT
17. Yes
18. Radiant energy emitted by                                                                                     For: Self-Assessment
Earth                                                                                                         Visit: PHSchool.com
19. Earth’s temperature is lower,
so it produces waves of longer
ASSESS
REVIEW (continued)                                        Web Code: csa – 2200

length.
20. a. Only short wavelengths
pass back out.
b. Earth                         Section 22.6                                       24. Visit a snow-covered cemetery and note that
16. Which will undergo the greater rate of cool-       the snow does not slope upward against the
Think and Explain                        ing, a red-hot poker in a warm oven or a           gravestones but, instead, forms depressions
21. Same temperature as your             red-hot poker in a cold room (or do both           around them, as shown. Make a hypothesis
hand                                 cool at the same rate)?                            explaining why this is so.
22. No, energy flows from your
hand via the rod to the snow.    17. Does Newton’s law of cooling apply to
23. Fiberglass is a good insulator       warming as well as to cooling?
because of trapped air.
Section 22.7
24. Heat from warm ground
conducted by stone melts         18. What is terrestrial radiation?
snow in contact.
19. Solar radiant energy is composed of short
25. Iron transfers internal energy
very fast.                           waves, yet terrestrial radiation is composed
26. No convection; the CO2
of relatively longer waves. Why?               25. Wood is a poor conductor, which means
around the candle cuts off the   20. a. What does it mean to say that the green-        that heat is slow to transfer—even when
oxygen supply.
house effect is like a one-way valve?           wood is very hot. Why can firewalkers safely
27. Agree; at thermal equilibrium,
b. Is the greenhouse effect more pronounced        walk barefoot on red-hot wooden coals, but
gases have same temperature,                                                            not safely walk barefoot on red-hot pieces of
which means same average                for florists’ greenhouses or for Earth’s
KE.                                     surface?                                        iron?
28. Disagree; having same KE                                                            26. When a space shuttle is in orbit and there
doesn’t mean same speed,
unless all molecules have
Think and Explain                    ••••••            appears to be no gravity in the cabin, why
equal masses.
can a candle not stay lit?
21. At what common temperature will both
29. H2 molecules are faster.                                                            27. A friend says that, in a mixture of gases in
KE 5 1/2 mv2. For fixed KE,          a block of wood and a piece of metal feel
thermal equilibrium, the molecules have the
less mass means more speed.          neither hot nor cool when you touch them
same average kinetic energy. Do you agree
22. If you stick a metal rod in a snowbank, the
28. A friend says that, in a mixture of gases in
end in your hand will soon become cold.
thermal equilibrium, the molecules have
Does cold flow from the snow to your hand?
the same average speed. Do you agree or
23. Wood is a better insulator than glass. Yet         disagree? Defend your answer.
fiberglass is commonly used as an insulator
29. In a mixture of hydrogen and oxygen gases
in wooden buildings. Explain.
at the same temperature, which molecules
move faster? Why?

446                                  446
30. Less mass means higher
speed, so the U-235 has
a greater average speed.
Lighter and slightly faster

ASSESS                                                                                    U-235 diffuse better.
31. They allow convection.
33. The dust absorbs solar energy
30. Which atoms have the greater average speed        36. Is it important to convert temperatures to          and melts the snow.
in a mixture, U-238 or U-235? How would               the Kelvin scale when we use Newton’s law       34. Right away, because whiter
this affect diffusion through a porous mem-           of cooling? Why or why not?                         coffee won’t radiate and
cool so quickly; also, the
brane of otherwise identical gases made                                                                   higher the temperature of
from these isotopes?                                                                                      the coffee compared with
its surroundings, the greater
31. Notice that a desk                                                                                        will be the rate of cooling.
lamp often has                                                                                            And, increasing the amount
small holes near                                                                                          of liquid for the same surface
the top of the metal                                                                                      area slows the cooling.
lampshade. How                                                                                        35. No, it cools faster in the
do these holes keep                                                                                       freezer because its rate of
cooling is proportional to the
the lamp cool?                                    37. If you wish to save fuel on a cold day, and         difference in temperature.
32. Turn an incandescent lamp on and off                  you’re going to leave your warm house for       36. Not important; either gives
quickly while you are standing near it. You           a half hour or so, should you turn your             same differences.
feel its heat, but you                                thermostat down a few degrees, down all the     37. Off altogether; the amount
find when you touch                                   way, or leave it at room temperature?               of heat energy, and thus
fuel, required to raise the
the bulb that it is not                           38. Why is whitewash sometimes applied to the           temperature inside again
hot. Explain why you                                  glass of florists’ greenhouses? Would you           is small compared with the
felt heat from the lamp.                              expect this practice to be more prevalent in        amount of heat energy that
continually escapes.
33. In Montana, the state                                 winter or summer months?
38. Whitewash reduces incoming
top of snow. When the sun comes out, the                                                                  good in summer.
snow rapidly melts. Why?                                                                              39. Earth’s temperature would
decrease and cooling of
34. Suppose that a person at a restaurant is                                                                  the climate would result.
served coffee before he or she is ready to                                                                Conversely, warming of
Earth’s climate would result.
drink it. In order that the coffee be hot-
test when the person is ready for it, should      39. If the composition of the upper atmosphere
cream be added to it right away or just               were changed so that it permitted a greater
before it is drunk?                                   amount of terrestrial radiation to escape,
what effect would this have on Earth’s cli-
35. Will a can of beverage cool just as fast in the
mate? Conversely, what would be the effect
regular part of the refrigerator as it will in
if the upper atmosphere reduced the escape
the freezer compartment? (What physical

CHAPTER 2222
CHAPTER           HEAT TRANSFER
HEAT TRANSFER   447                                 447
Think and Solve
40. 12 L is 12 kg 5 12,000 g. Q 5
For: Self-Assessment
mcDT 5 (12,000 g)(1.0 cal/g°C)
3 (70°C 2 20°C) 5                                                                                                 Visit: PHSchool.com
600,000 cal.
41. Yes; mcDTball lost by ball 5
ASSESS
REVIEW (continued)                                          Web Code: csa – 2200

mcDTwater gained by water.
(50 g)(0.215 cal/g°C)(T 2 37°C)
5 (75 g)(1.0 cal/g°C) 3
(37°C 2 20°C); T 5 155.6°C.
42. From Q 5 mcDT, Q/m 5
Think and Solve                ••••••                43. In a lab you burn a 0.6-g peanut beneath
cDT 5 (800 J/kg°C)(500°C) 5
50 g of water. Heat from the peanut increases
40. An automobile cooling system holds                   the water temperature from 22°C to 50°C.
400,000 J/kg. Time required
is (400,000 J/kg)/(0.03 J/kg?yr)
12 liters of water. Show that when its               a. Assuming 40% efficiency, show that the
5 13.3 million years.                  temperature rises from 20°C to 70°C, it                 food value of the peanut is 3500 calories
43. a. Q 5 mcDT 5 (50.0 g) 3               absorbs 60 kilocalories.                                (3.5 Calories).
(1.0 cal/g C°)(50°C 2 22°C) 5                                                               b. What is the food value in Calories per
1400 cal. At 40% efficiency
41. Austin places a 50-g aluminum ball into an
0.4 3 energy from peanut               insulated cup containing 75 g of water at               gram?
raises water temperature.              20°C. The ball and water reach an equilib-
Heat content is 1400 cal/0.4 5         rium temperature of 37°C. Austin makes
3500 cal (3.5 Cal).                    some calculations and reports that the
b. Food value is 3.5 Cal/0.6 g
5 5.8 C/g.
initial temperature of the ball must have
44. Work done by hammer is
been slightly more than 155°C. Do your
F 3 d; temp change of nail             calculations agree? (Ignore heat transfer to
from Q 5 mcDT. (5 grams 5              the cup.)
0.005 kg; 6 cm 5 0.06 m.)
Then F 3 d 5 500 N 3 0.06 m
5 30 J, and 30 J 5 (0.005 kg) 3
(450 J/kg°C)(DT). Then DT 5
30 J/(0.005 kg 3 450 J/kg°C) 5
13.3°C.

44. Pounding a nail into wood makes the nail
warmer. Suppose a hammer exerts an aver-
42. Decay of radioactive isotopes of thorium
age force of 500 N on a 6-cm nail whose
and uranium in granite and other rocks in
mass is 5 grams when it drives into a piece
Earth’s interior provides sufficient energy to
of wood. Work is done on the nail and it be-
keep the interior molten, heat lava, and pro-
comes hotter. If all the heat goes to the nail,
vide warmth to natural hot springs. This is
show that its increase in temperature
due to the average release of about 0.03 J per
is slightly more than 13°C. (Use 450 J/kg°C
kilogram each year. Show that 13.3 million
for the specific heat capacity of the nail.)
years are required for a chunk of thermally
insulated granite to increase 500°C in tem-
perature. (Use 800 J/kg°C for the specific
heat capacity of granite.)

448                                    448
45. At 25% efficiency, each
square meter of collector
supplies 50 W on average. So
need (3000 W)/(50 W/m2) 5
60 m2 of collector area.
ASSESS
Activities
46. This is a good demo to show.
Steel wool can be used to
45. At a certain location, the solar power per        47. If you live where there is snow, do as Benja-        wedge the ice at the bottom
of the test tube. Be sure to
unit area reaching Earth’s surface is                 min Franklin did more than two centuries             put the top part of the water-
200 W/m2, averaged over a 24-hour day.                ago and lay samples of light and dark cloth          filled tube in the flame.
Consider a house with an average power                on the snow. (If you don’t live in a snowy       47. The snow under the dark
requirement of 3 kW with solar panels on              area, try this using ice cubes.) Describe dif-       cloth melts faster. The dark
the roof that convert solar power to electric         ferences in the rate of melting beneath the          cloth absorbs more energy
power with 25 percent efficiency. Show that                                                                from the sun.
cloths.
a solar collector area of 60 square meters                                                             48. The metal must reach 230°C
48. Wrap a piece of paper around a thick metal           for the paper to do the same.
will meet the 3 kW requirement.
bar and place it in a flame. Note that the
paper will not catch fire. Can you figure out
why? (Hint: Paper generally will not ignite
until its temperature reaches about 230°C.)

Activities      ••••••

46. Hold the bottom end of a test tube full of
cold water in your hand. Heat the top part
in a flame until the water boils. The fact that
you can still hold the bottom shows that
water is a poor conductor of heat. This
is even more dramatic when you wedge
chunks of ice at the bottom; then the
water above can be brought to a boil
without melting the ice. Try it and see.

More Problem-Solving Practice     Teaching Resources
Appendix F                       • Computer Test Bank
• Chapter and Unit Tests

CHAPTER 2222
CHAPTER           HEAT TRANSFER
HEAT TRANSFER   449                                 449

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