HEAT
TRANSFER
HEAT TRANSFER
Objectives
• Explain how conduction works.
(22.1)
..........
• Explain how convection works.
(22.2)
THE BIG
• Explain how heat can be
transmitted through empty
space. (22.3)
IDEA
Heat can be transferred by
conduction, by convection,
and by radiation.
• Identify which substances emit
radiant energy. (22.4)
• Compare the ability of an
object to emit radiant energy
with its ability to absorb
radiant energy. (22.5)
• Relate the temperature
difference between an object
and its surroundings to the rate
at which it cools. (22.6)
• Identify the main driver of
global warming and climate
change. (22.7)
discover!
black construction
paper, hole punch or pencil,
white polystyrene cup
MATERIALS
EXPECTED OUTCOME Even
though the inside of the cup is
white, the hole looks black.
T
he spontaneous transfer of heat
is always from warmer objects
to cooler objects. If several
objects near one another have different temperatures, then those that
are warm become cooler and those
that are cool become warmer, until
all have a common temperature.
This equalization of temperatures
is brought about in three ways:
by conduction, by convection, and
by radiation.
ANALYZE AND CONCLUDE
1. Both are the same.
2. The hole would no longer
appear dark.
discover!
3. Light entering a small
opening is reflected from the
inside surfaces many times.
Some of the light is partially
absorbed at each reflection
until none remains.
Does White Ever Appear Black?
Analyze and Conclude
1. Using a paper punch or sharp pencil, make
a small hole in the center of a black sheet of
construction paper.
2. Place the paper on top of a polystyrene
coffee cup or any cup that is all white inside.
1. Observing Which is darker, the construction
paper or the hole?
2. Predicting What do you think will happen if
you enlarge the hole?
3. Making Generalizations Why do openings
such as the pupil of the eye and doorways
of distant houses appear black even in the
daytime?
If the aperture
is made too large, some light
entering the hole will find its
way out of the cavity. Students
may also find that the hole
will not appear black if viewed
under a bright light.
TEACHING TIP
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430
22.1 Conduction
22.1 Conduction
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 transferred through the metal by conduction. Conduction of heat is the
transfer of energy within materials and between different materials
that are in direct contact. Materials that conduct heat well are known
as heat conductors. Metals are the best conductors. Among the
common metals, silver is the most conductive, followed by copper,
aluminum, and iron.
Conduction is explained by collisions between atoms or molecules, and the actions of loosely bound electrons. In conduction,
collisions between particles transfer thermal energy, without any
overall transfer of matter. When the end of an iron rod is held in a
flame, the atoms at the heated end vibrate more rapidly. These atoms
vibrate against neighboring atoms, which in turn do the same. More
important, free electrons that can drift through the metal are made to
jostle and transfer energy by colliding with atoms and other free electrons within the rod.
Key Terms
conduction, conductors, insulator
Common Misconception
Surfaces that feel cooler than others
must have a lower temperature.
FIGURE 22.1 Heat from the flame
causes atoms and free
electrons in the end of
the metal to move faster
and jostle against others.
Those particles do the
same and increase the
energy of vibrating atoms
along the length of the
rod.
Conductors Materials composed of atoms with “loose” outer electrons are good conductors of heat (and electricity also). Because metals have the “loosest” outer electrons, they are the best conductors of
heat and electricity.
FIGURE 22.2
The tile floor feels cold to the
bare feet, while the carpet at the
same temperature feels warm.
This is because tile is a better
conductor than carpet.
Touch a piece of metal and a piece of wood in your immediate
vicinity. Which one feels colder? Which is really colder? Your answers
should be different. If the materials are in the same vicinity, they
should have the same temperature, room temperature. Thus neither is really colder. Yet, the metal feels colder because it is a better
conductor, like the tile in Figure 22.2; heat easily moves out of your
warmer hand into the cooler metal. Wood, on the other hand, is a
poor conductor. Little heat moves out of your hand into the wood, so
your hand does not sense that it is touching something cooler. Wood,
wool, straw, paper, cork, and polystyrene are all poor heat conductors. Instead, they are called good insulators.
CHAPTER 22
think!
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?
Answer: 22.1.1
HEAT TRANSFER
431
FACT Surfaces that have been
in the same vicinity for some
time should all have the same
temperature—that of the
vicinity! One surface may feel
colder than another simply
because it is a better conductor.
Teaching Tip Explain that
the physics of the phenomenon
of walking harmlessly on redhot wooden coals with bare feet
is the same as the physics that
allows one to momentarily place
one’s hand in a very hot oven
without harm—not because
the temperature is low but
because air is a poor conductor
of heat. Conductivity, not only
temperature, must be considered.
Explain that since wood has low
heat conductivity, it is used for
handles on cooking utensils.
Wood is a poor conductor,
even when it’s red hot. After
the surface of a red-hot coal of
low-conductivity wood gives up
its heat, perhaps to a bare foot
that has just stepped on it, more
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
with a cooler surface. The physics
of hot-coal walking! The result
would be very different indeed
should a person try to walk over
red-hot pieces of iron. Caution:
Warn your students not to try
either of these themselves!
431
Demonstration
Place blobs of wax or butter
on rods of various metals.
Place each rod a similar
distance from a hot flame with
the blob of wax or butter at
the end of each rod farther
from the flame. Notice how
the heat is conducted along
the rods at different rates. This
demonstration illustrates the
relative conductivities of the
different metals.
FIGURE 22.3 A “warm” blanket
does not provide you with
heat; it simply slows the
transfer of your body heat
to the surroundings.
Teaching Tip Discuss the
poor conductivity of air, and
its role in insulating materials,
e.g., down-filled sleeping bags
and sportswear, spun glass and
Styrofoam insulation, fluffy
blankets, and even snow.
Insulators Liquids and gases generally make poor conductors—
they are good insulators. An insulator is any material that is a poor
conductor of heat and that delays the transfer of heat. Air is a very
good insulator. Porous materials having many small air spaces are
good insulators. The good insulating properties of materials such
as wool, fur, and feathers are largely due to the air spaces they contain. Birds vary their insulation by fluffing their feathers to create air
spaces. Be glad that air is a poor conductor, for if it were not, you’d
feel quite chilly on a 25°C (77°F) day!
Snowflakes imprison a lot of air in their crystals and are good insulators. Snow slows the escape of heat from Earth’s surface, shields
Eskimo dwellings from the cold, and provides protection from the
cold to animals on cold winter nights. Snow, like the blanket in
Figure 22.3, is not a source of heat; it simply prevents any heat from
escaping too rapidly.
FIGURE 22.4 Snow lasts longest on the
roof of a well-insulated
house. Thus, the snow patterns reveal the conduction,
or lack of conduction, of
heat through the roof. The
houses with more snow on
the roof are better insulated.
CHECK
Teaching Resources
• Reading and Study
Workbook
• PresentationEXPRESS
• Interactive Textbook
think!
You can place your hand
into a hot pizza oven for
several seconds without
harm, whereas you’d
never touch the metal
inside surfaces for even a
second. Why?
Answer: 22.1.2
• Next-Time Question 22-1
• Conceptual Physics Alive!
DVDs Heat Transfer
432
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......
......
In conduction,
collisions between
particles transfer thermal energy,
without any overall transfer of
matter.
CONCEPT
Heat is energy and is tangible. Cold is not; cold is simply the
absence of heat. Strictly speaking, there is no “cold” that passes
through a conductor or an insulator. Only heat is transferred. We
don’t insulate a home, such as some of those in Figure 22.4, to keep
the cold out; we insulate to keep the heat in. If the home becomes
colder, it is because heat flows out.
It is important to note that no insulator can totally prevent heat
from getting through it. An insulator just reduces the rate at which
heat penetrates. Even the best-insulated warm homes in winter will
gradually cool. Insulation slows down heat transfer.
CONCEPT
CHECK
How does conduction transfer heat?
22.2 Convection
22.2 Convection
Conduction involves the transfer of energy from molecule to molecule. Energy moves from one place to another, but the molecules
themselves do not. Another means of heat transfer is by movement
of the hotter substance. Air in contact with a hot stove rises and
warms the region above. Water heated in a boiler in the basement
rises to warm the radiators in the upper floors. This is convection,
a means of heat transfer by movement of the heated substance itself,
such as by currents in a fluid.
Key Term
convection
think!
You can hold your fingers
beside the candle flame
without
harm, but
not above
the flame.
Why?
Answer: 22.2
FIGURE 22.5
When the test tube is heated at
the top, convection is prevented
and heat can reach the ice by conduction only. Since water is a poor
conductor, the top water will boil
without melting the ice.
In convection, heat is transferred by movement of the hotter substance from one place to another. A simple demonstration
illustrates the difference between conduction and convection. With a
bit of steel wool, trap a piece of ice at the bottom of a test tube nearly
filled with water. Hold the tube by the bottom with your bare hand
and place the top in the flame of a Bunsen burner, as shown in
Figure 22.5. The water at the top will come to a vigorous boil while
the ice below remains unmelted. The hot water at the top is less dense
and remains at the top. Any heat that reaches the ice must be transferred by conduction, and we see that water is a poor conductor of
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, carrying its energy with it to the ice.
Convection ovens are
simply ovens with a fan
inside, which speeds up
cooking by circulating
the warmed air.
Demonstration
Do the activity in Figure 22.5,
with ice wedged at the
bottom of a test tube. Some
steel wool will hold the ice
at the bottom of the tube. It
is impressive to see that the
water at the top is brought
to a boil while the ice below
barely melts! (Convection, or
better, the lack of convection,
is illustrated here. If heating
were at the bottom and the
ice cube at the top, the ice
would quickly melt.)
discover!
beaker, water, heat
source, dark dye
MATERIALS
EXPECTED OUTCOME Though the
dye disperses quite rapidly, if
they watch carefully, students
will see that it follows the
convection flow pattern.
In smoke, steam, and in
the air over a hot stove
THINK
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
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433
Teaching Tip Explain that
the lack of convection in orbiting
vehicles such as the space shuttle
has interesting consequences.
For example, in orbit, one cannot
light a match without it snuffing
out very quickly. This is because
of the absence of convection in
orbit. Much of the convection
in fluids depends on buoyancy,
which in turn depends on gravity.
In orbit the local effects of
gravity are not there (because
the shuttle and everything in the
shuttle are freely falling around
Earth). With no convection, hot
gases are not buoyed upward
away from a flame but remain
around the flame, preventing
the entry of needed oxygen. The
flame burns out.
Teaching Tip Discuss the role
of convection in climates. Call
attention to the shift in winds as
shown in Figure 22.7.
Ask Why does the direction
of coastal winds change from
day to night? Land warms faster
than water, and in the day the
land and the air above it are
warmer than the water and the
air above it. The air rises and
results in a sea breeze from water
to land. At night, the reverse
happens.
Convection occurs in all fluids, whether liquid or gas. Whether we
heat water in a pan or heat air in a room, the process is the same, as
shown in Figure 22.6. When the fluid is heated, it expands, becomes
less dense, and rises. Warm air or warm water rises for the same
reason that a block of wood floats in water and a helium-filled
balloon rises in air. In effect, convection is an application of
Archimedes’ principle, for the warmer fluid is buoyed upward by
denser surrounding fluid. Cooler fluid then moves to the bottom, and
the process continues. In this way, convection currents keep a fluid
stirred up as it heats. Convection currents also have a large influence
on the air in the atmosphere.
b
Moving Air Convection currents stirring the atmosphere produce
FIGURE 22.6 Convection occurs in all
fluids. a. Convection
currents transfer heat in
air. b. Convection currents
transfer heat in liquid.
winds. Some parts of Earth’s surface absorb heat from the sun more
readily than others. The uneven absorption causes uneven heating of
the air near the surface and creates convection currents. This phenomenon is often evident at the seashore. In the daytime the shore warms
more easily than the water. Air over the shore rises, and cooler air from
above the water takes its place. The result is a sea breeze, as shown in
Figure 22.7.
At night the process reverses as the shore cools off more quickly
than the water—the warmer air is now over the sea. If you build a
fire on the beach you’ll notice that the smoke sweeps inward in the
day and seaward at night.
FIGURE 22.7 Convection currents are produced by uneven heating.
Ask Is fog a low-altitude
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
air, and a sea breeze results.
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434
b. At night, the land is cooler than the water,
so the air flows in the other direction.
discover!
discover!
Is Your Breath Warm or Cold?
1. With your mouth open wide, blow on your hand.
Note the temperature of your breath.
2. Now pucker your lips to make a small opening
with your mouth and blow on your hand again.
Does the temperature of your breath feel the same?
3. Think In which case does your exhaled breath expand more—
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.
On a much larger
scale, convection due
to uneven solar heating of Earth’s surface
combines with the
effects of Earth’s
rotation to contribute
to overall global wind
patterns.
Because less atmospheric pressure squeezes on it at higher altitudes.
As the air expands, it cools—just the opposite of what happens when
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 opposite happens when air expands. Expanding air cools.
Ask Since warm air rises,
why are mountain tops cold
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
air, not a contradiction!
FIGURE 22.8 When a molecule collides with a target molecule that is
receding, its rebound speed after the collision is less than
it was before the collision.
Demonstration
......
How does convection transfer heat?
CHAPTER 22
Hold your fingers beside a
flame. Ask students why you
cannot do the same with your
fingers above the flame. (The
air above the flame is hotter
than the air beside it because
of the convection flow.)
In convection, heat
is transferred by
movement of the hotter
substance from one place to
another.
......
We can understand the cooling of expanding air by thinking of
molecules of air as tiny balls bouncing against one another. Speed
is picked up by a ball when it is hit by another that approaches with
a greater speed. When a ball collides with one that is receding, its
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
hits an approaching paddle, but loses speed when it hits a receding
paddle. This also applies to a region of air that is expanding; molecules collide, on the average, with more molecules that are receding
than are approaching, as shown in Figure 22.9. Thus, in expanding
air, the average speed of the molecules decreases and the air cools.22.2
CHECK
The warm breath
expands more when blown
through a narrow gap.
Expanding air cools and so
feels cooler when on the hand.
THINK
Teaching Tip Explain that
when a portion of air is heated, it
expands and becomes less dense
than the surrounding air. The
buoyancy force becomes greater
than the weight and the warm
air rises. When it rises, it expands
and cools.
Cooling Air Rising warm air, like a rising balloon, expands. Why?
CONCEPT
EXPECTED OUTCOME When the
student blows on his or her
hand through the smaller gap
in the lips, the air feels cooler.
CONCEPT
CHECK
FIGURE 22.9 Molecules in a region of
expanding air collide more
often with receding molecules than with approaching
ones.
HEAT TRANSFER
435
Teaching Resources
• Transparency 42
• Next-Time Question 22-2
435
22.3 Radiation
Key Terms
radiation, radiant energy
Teaching Tip Discuss the
radiation one feels from redhot coals in a fireplace and
how the intensity of radiation
decreases with distance. Consider
the radiation one feels when
stepping from shade to sunshine.
The heat one feels is not so much
because of the sun’s temperature,
but because the sun is big!
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
the rays of sunlight striking
Earth with rain that strikes
two pieces of paper—one held
horizontally and the other held
at an angle in the rain. Dispel
the misconception that the paper
held horizontally must get wetter
than the paper held at an angle
because it is closer to the clouds!
FIGURE 22.10 Radiant energy is transmitted as electromagnetic waves.
a. Radio waves send signals
as heat.
energy is light waves.
FIGURE 22.11 Most of the heat from a fireplace goes up the chimney
by convection. The heat that
warms us comes to us by
radiation.
......
How does the sun warm Earth’s surface? It can’t be through conduction, because there is 150 million kilometers of virtually nothing
between Earth and the sun. Nor can it be by convection, because there
is nothing between the sun and Earth to expand and rise. The sun’s
heat is transmitted by another process—by radiation.22.3.1 Radiation
is energy transmitted by electromagnetic waves, as shown in Figure
22.10. What is being radiated from the sun is primarily light.
Radiant energy is any energy that is transmitted by radiation.
In radiation, heat is transmitted in the form of radiant energy,
or electromagnetic waves. Radiant energy includes radio waves,
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
CONCEPT
CHECK
heat source, pair of
How does radiation transmit heat?
glasses
EXPECTED OUTCOME Students
will find that the effects of the
heat are less when they put on
the glasses.
discover!
Why Do Glasses Keep You Cool?
1. Sit close to a fire in a fireplace and
feel the heat on your closed eyelids.
2. Now slip a pair of glasses over your
eyes. How do your eyes feel?
3. Think Why did the glasses cause your
eyes to feel a different temperature?
THINK The lenses do not
transmit the infrared waves (or
heat) from the fire.
......
In radiation, heat is
transmitted
in the
CHECK
form of radiant energy, or
electromagnetic waves.
CONCEPT
436
c. A visible form of radiant
22.3 Radiation
discover!
MATERIALS
b. You feel infrared waves
through the air.
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22.4 Emission of
22.4 Emission of Radiant Energy
Radiant Energy
All substances continuously emit radiant energy in a mixture
of wavelengths. Objects at low temperatures emit long waves, just
as long, lazy waves are produced when you shake a rope with little
energy as shown in Figure 22.12. Higher-temperature objects emit
waves of shorter wavelengths. Objects of everyday temperatures emit
waves mostly in the long-wavelength end of the infrared region, which
is between radio and light waves. Shorter-wavelength infrared waves
absorbed by our skin produce the sensation of heat. Thus, when we
speak of heat radiation, we are speaking of infrared radiation.
Everything around
you both radiates
and absorbs energy
continuously!
Key Terms
steller radiation, terrestrial
radiation
If time is short, Sections
22.4 and 22.5 may be omitted
without consequence.
Common Misconception
Only hot things radiate energy.
FIGURE 22.12
FACT All objects continually emit
radiant energy in a mixture of
wavelengths.
Shorter wavelengths
are produced when
the rope is shaken
more rapidly.
The fact that all objects in our environment continuously emit
infrared radiation underlies infrared thermometers such as the one in
Figure 22.13. How nice it is that you simply point the thermometer at
something whose temperature you want, press a button, and a digital
temperature reading appears. The radiation emitted by the object
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 proportional to the Kelvin temperature T of the emitter:
FIGURE 22.13 An infrared thermometer
measures the infrared
radiant energy emitted by
a body and converts it to
temperature.
f T
People, with a surface temperature of 310 K, emit light in the lowfrequency 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 radiant 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 whitehot 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
is called stellar radiation.
CHAPTER 22
HEAT TRANSFER
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......
All substances
continuously emit
radiant energy in a mixture of
wavelengths.
CONCEPT
CHECK
Teaching Resources
• Reading and Study
Workbook
think!
Why is it that light radiated by the sun is yellowish, but light radiated by
Earth is infrared?
Answer: 22.4
• PresentationEXPRESS
• Interactive Textbook
22.5 Absorption of
Radiant Energy
......
Teaching Tip Explain that
some materials absorb better
than others. The good absorbers
are easy to spot, because they
absorb visible radiation and so
appear black.
CONCEPT
CHECK
438
What substances emit radiant energy?
22.5 Absorption of Radiant Energy
Make the distinction that
objects don’t absorb because
they’re black, but are black
because they absorb so well.
Cause precedes effect.
Teaching Tip Explain that
though there are various colors
of eyes, all have one thing in
common: The pupils are black.
This is because the light that
enters the eyes through the
pupils is absorbed. (An exception
to this is that flash photography
can sometimes produce photos
that show people with red eyes.
This happens because the bright
flash can be reflected from the
retina of the eye if the eye does
not have time to adjust to the
bright light. Some cameras have
a “red-eye reduction” setting.
This setting produces multiple
flashes that give the eyes time to
adjust before the photograph is
taken.)
The surface of the sun has a high temperature (5500°C) and
therefore emits radiant energy at a high frequency—much of it in
the visible portion of the electromagnetic spectrum. The surface of
Earth, by comparison, is relatively cool, and so the radiant energy it
emits consists of frequencies lower than those of visible light. Radiant
energy that is emitted by Earth is called terrestrial radiation, which
is in the form of infrared waves—below our threshold of sight. The
source of the sun’s radiant energy involves thermonuclear fusion
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
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.)
When radiant energy encounters objects, it is partly reflected
and partly absorbed. The part that is absorbed increases the internal
energy of the objects.
If everything is emitting energy, why doesn’t everything finally run
out of it? The answer is that everything also absorbs energy from its
environment.
A hot pizza placed outside on a winter day is
a net emitter. The same
pizza placed in a hotter
oven is a net absorber.
438
Absorption and Emission For example, a book sitting on your
desk is both absorbing and radiating energy at the same rate. It is in
thermal equilibrium with its environment. Imagine that you move the
book out into the bright sunshine. If the book’s temperature doesn’t
change, it radiates the same amount of energy as before. But because
the sun shines on it, the book absorbs more energy than it radiates.
Its temperature increases. As the book gets hotter, it radiates more
energy, eventually reaching a new thermal equilibrium. Then it radiates as much energy as it receives. In the sunshine the book remains
at this new higher temperature.
If you move the book back indoors, the opposite process occurs.
The hot book initially radiates more energy than it receives from
its surroundings. So it cools. In cooling, it radiates less energy. At a
sufficiently lowered temperature it radiates no more energy than it
receives from the room. It stops cooling. It has reached thermal equilibrium again.
Good emitters of radiant energy are also good absorbers;
poor emitters are poor absorbers. For example, a radio antenna constructed to be a good emitter of radio waves is also, by its very design,
a good receiver (absorber) of them. A poorly designed transmitting
antenna is also a poor receiver.
A blacktop pavement and dark automobile body may remain
hotter than their surroundings on a hot day. But at nightfall these
dark objects cool faster! Sooner or later, all objects in thermal contact
come to thermal equilibrium. So a dark object that absorbs radiant
energy well emits radiation equally well.22.5
think!
If a good absorber of
radiant energy were a
poor emitter, how would
its temperature compare
with its surroundings?
Answer: 22.5
FIGURE 22.14
Even though the interior of
the box has been painted
white, the hole looks black.
Absorption and Reflection Absorption and reflection are
opposite processes. Therefore, a good absorber of radiant energy
reflects very little radiant energy, including the range of radiant
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,
because the radiant energy that enters is reflected from the inside
walls many times and is partly absorbed at each reflection until very
little or none remains to come back out. You can see this illustrated
in Figure 22.15.
Demonstration
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
none) comes back out.
......
Good emitters of
radiant energy are
also good absorbers; poor
emitters are poor absorbers.
CONCEPT
CHECK
FIGURE 22.15 Teaching Resources
Radiant energy that enters an
opening has little chance of leaving
before it is completely absorbed.
CHAPTER 22
Teaching Tip Emphasize that
everything emits radiation—
everything that has any
temperature—but everything
does not become progressively
cooler because everything also
absorbs radiation. We live in
a sea of radiation, everything
emitting and everything
absorbing. When emission
rate equals absorption rate,
temperature remains constant.
Some materials, because of their
molecular design, emit better
than others.
• Concept-Development
Practice Book 22-1
• Next-Time Question 22-3
HEAT TRANSFER
439
439
22.6 Newton’s Law
FIGURE 22.16
of Cooling
Anything with a mirrorlike surface
reflects most of the radiant energy
it encounters. That’s why it is a poor
absorber of radiant energy.
Key Term
Newton’s law of cooling
Teaching Tip Relate the rate
of cooling to the black and silver
containers that are cooling and
warming. We see the difference
between a proportionality
sign and an equals sign for
the formula here, for the
rate of cooling or warming is
proportional not only to the
difference in temperatures but
also to the differences in the
“emissivities” of the surfaces.
Good reflectors, on the other hand, are
poor absorbers, like the toaster in Figure
22.16. Light-colored objects reflect more
light and heat than dark-colored ones. In
summer, light-colored clothing keeps
people cooler.
On a sunny day Earth’s surface is a net absorber. At night it is
a net emitter. On a cloudless night its “surroundings” are the frigid
depths of space and cooling is faster than on a cloudy night, where
the surroundings are nearby clouds. Record-breaking cold nights
occur when the skies are clear.
The next time you’re in the direct light of the sun, step in and
out of the shade. You’ll note the difference in the radiant energy you
receive. Then think about the enormous amount of energy the sun
emits to reach you some 150,000,000 kilometers distant. Is the sun
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 primarily because it is big. Really big!
Teaching Tip Point out that D
means “the change in.”
Teaching Tip Relate Newton’s
law of cooling to Think and
Explain 34 (cream in the coffee),
35 (cooling a beverage in the
fridge), and 37 (thermostat on a
cold day). These questions make
excellent discussion topics.
Demonstration
......
CONCEPT How does an object’s emission rate compare with its
Fill a beaker with warm
water and a similar beaker
with boiling water. Record
the temperatures of the
two beakers at regular
intervals as they cool to
room temperature. Note the
different rates of cooling.
Ask Does Newton’s law of
cooling apply to the warming
of a cold object in a warm
environment? Yes
......
The colder an object’s
surroundings, the
faster the object will cool.
CONCEPT
CHECK
Teaching Resources
• Laboratory Manual 59
• Probeware Lab Manual 10
440
CHECK
absorption rate?
22.6 Newton’s Law of Cooling
think!
Since a hot cup of tea
loses heat more rapidly
than a lukewarm cup of
tea, would it be correct to
say that a hot cup of tea
will cool to room temperature before a lukewarm
cup of tea will? Explain.
Answer: 22.6
440
An object hotter than its surroundings eventually cools to match the
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.
The rate of cooling of an object depends on how much hotter
the object is than the surroundings. The colder an object’s surroundings, the faster the object will cool. The temperature change
per minute of a hot apple pie will be more if the hot pie is put in a
cold freezer than if put on the kitchen table because the temperature
difference is greater. A warm home will lose heat to the cold outside
at a greater rate when there is a larger difference between the inside
and outside temperatures. Keeping the inside of your home at a high
temperature on a cold day is more costly than keeping it at a lower
temperature. If you keep the temperature difference small, the rate of
cooling will be correspondingly low.
22.7 Global Warming
This principle is known as Newton’s law of cooling. (Guess who is
credited with discovering this?) Newton’s law of cooling states that
the rate of cooling of an object—whether by conduction, convection,
or radiation—is approximately proportional to the temperature difference DT between the object and its surroundings:
Newton’s law of cooling is an empirical relationship and not
a fundamental law like
Newton’s laws
of motion.
rate of cooling T
......
Newton’s law of cooling also holds for heating. If an object is
cooler than its surroundings, its rate of warming up is also proportional to DT. Frozen food warms up faster in a warmer room.
CONCEPT
CHECK
22.7 Global Warming and the
Greenhouse Effect
Causes of the Greenhouse Effect The first concept has been
previously stated—that all things radiate, and the frequency and
wavelength of radiation depends on the temperature of the object
emitting the radiation. High-temperature objects radiate short waves;
low-temperature objects radiate long waves. The second concept we
need to know is that the transparency of things such as air and glass
depends on the wavelength of radiation. Air is transparent to both
infrared (long) waves and visible (short) waves, unless the air contains excess carbon dioxide and water vapor, in which case it absorbs
infrared waves. Glass is transparent to visible light waves but absorbs
infrared waves. (This is discussed later, in Chapter 27.)
Now to why that car gets so hot in bright sunlight: Compared with
the car, the sun’s temperature is very high. This means the wavelengths
of waves the sun radiates are very short. These short waves easily pass
through both Earth’s atmosphere and the glass windows of the car. So
energy from the sun gets into the car interior, where, except for some
reflection, it is absorbed. The interior of the car warms up.
CHAPTER 22
Key Term
greenhouse effect
Common Misconception
The greenhouse effect on Earth is
undesirable.
FACT The greenhouse effect
provides a temperature that
supports life as we know
it. Without it, the average
temperature of Earth would be
about 218ºC. What is undesirable
is an increase in this effect.
What causes an object to cool faster?
An automobile sitting in the bright sun on a hot day with its windows rolled up can get very hot inside—appreciably hotter than
the outside air. This is an example of the greenhouse effect, so
named for the same temperature-raising effect in florists’ glass
greenhouses. The greenhouse effect is the warming of a planet’s
surface due to the trapping of radiation by the planet’s atmosphere. Understanding the greenhouse effect requires knowing
about two concepts.
and the Greenhouse
Effect
Physics on the Job
Ecologist
The greenhouse effect
is of particular concern to the ecologist.
Ecologists study the
relationship between
the living and nonliving
factors in an ecosystem. Ecologists need
to use physics when
they analyze changes
in atmospheric temperatures over time.
Understanding the
relationships between
energy, temperature,
and greenhouse gases
enables ecologists to
identify processes that
interfere with Earth’s
natural processes.
Ecologists can find
opportunities in government and privately
funded projects.
HEAT TRANSFER
441
Teaching Tip Discuss the
greenhouse effect, first for
florists’ greenhouses, and then
for Earth’s atmosphere. The key
idea is that the medium (glass
for the greenhouse, atmosphere
for Earth) is transparent to highfrequency (short wavelength)
electromagnetic waves but
opaque to low-frequency (long
wavelength) electromagnetic
waves.
Teaching Tip Point out that
Earth’s atmosphere is primarily
warmed by terrestrial radiation,
not solar radiation. That’s why air
near the ground is warmer than
air above. The opposite would
be the case if the sun were the
primary warmer of air!
Teaching Tip Explain that
terrestrial radiation also cools
Earth, especially on clear nights.
Clouds reradiate terrestrial
radiation. Farmers sometimes
use smudge pots in orchards
to create a cloud close to the
ground. This enables terrestrial
radiation (absorbed by the
smoke) to be reradiated to the
ground resulting in a longer
cooling time for the ground.
This helps crops survive nights
without freezing.
441
Teaching Tip Briefly discuss
the idea of wave frequency.
Review Figure 22.12, showing the
relationship of wave frequency
to wavelength. The origin
of electromagnetic waves is
vibrating electrons in matter.
Explain that the frequency
of electromagnetic radiation
emitted by a source increases
with the temperature of the
source. Electrons vibrate at
greater frequencies in hot matter
than in cold matter. The sun
is so hot that the frequency of
electromagnetic waves it emits
is high enough to activate our
visible receptors. Write f , T
in big letters to indicate large
values of both frequency and
temperature. This radiation is
visible light. It is absorbed by
Earth, which in turn emits its own
radiation. Write f , T in small
letters to indicate low values of
both frequency and temperature.
FIGURE 22.17 Earth’s temperature
depends on the energy
balance between incoming
solar radiation and outgoing
terrestrial radiation.
The car interior radiates its own waves, but since it is not as
hot as the sun, the radiated waves are longer. The reradiated long
waves encounter glass windows that aren’t transparent to them. So
most of the reradiated energy remains in the car, which makes the
car’s interior even warmer. (That is why leaving your pet in a car on a
hot sunny day is a no-no.) As hot as the interior gets, it won’t be hot
enough to radiate waves that can pass through glass (unless it glows
red or white hot!).
The same effect occurs in Earth’s atmosphere, which is transparent to solar radiation, as shown in Figure 22.17. The surface of Earth
absorbs this energy, and reradiates part of this at longer wavelengths, as
shown in Figure 22.18. Energy that Earth radiates is called terrestrial
radiation. Atmospheric gases (mainly water vapor, carbon dioxide,
and methane) absorb and re-emit much of this long-wavelength
terrestrial radiation back to Earth. So the long-wavelength radiation that cannot escape Earth’s atmosphere warms Earth. This global
warming process is very nice, for Earth would be a frigid –18°C otherwise. Our present environmental concern is that increased levels of
carbon dioxide and other atmospheric gases in the atmosphere may
further increase the temperature and produce a new thermal balance
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 balance 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,
it prevents terrestrial radiation
from leaving.
442
442
FIGURE 22.19
Shorter-wavelength radiant energy
from the sun enters through the
glass roof of the greenhouse. The
soil emits long-wavelength radiant energy, which is unable to pass
through the glass. Income exceeds
outgo, so the interior is warmed.
......
CONCEPT
CHECK
Volcanoes put more
particulate matter into
the atmosphere than
industries and all
human activity. But
when it comes to
carbon dioxide, the
impact of humans is
big enough to affect
climate.
The near unanimous
view of climate
scientists is that human activity is
a main driver of global warming
and climate change.
......
Adding materials such as those from the burning of fossil fuels
to the atmosphere changes the absorption and reflection of solar
radiation. Except where the source of energy is solar, wind, or water,
increased energy consumption on Earth adds heat. These activities can
change the radiative balance and change Earth’s average temperature.
The near unanimous view of climate scientists is that human
activity is a main driver of global warming and climate change. This
view is the outcome of a long, painstaking road of successively more
sophisticated climate models.
Confidence in the models, run by more and more sophisticated
computers, is bolstered by an intriguing outcome: data gathered earlier 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
well. The greater concern is the combination of growing amounts
of both these greenhouse gases.
An important credo is “You can never change only one thing.”
Change one thing, and you change another. Burn fossil fuels and you
warm the planet. Increase global temperature and you increase storm
activity. Changed climate means changed rainfall patterns, changed
coastal boundaries, and changes in insect breeding patterns. How
these changes upon changes will play out, we don’t know.
What we do know is that energy consumption is related to population size. We are seriously questioning the idea of continued growth.
(Please take the time to read Appendix E, “Exponential Growth and
Doubling Time”—very important stuff.)
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
trees (while decreasing the
rate at which they are cut
down)! This would not be an
end-all to the problem, however,
because the carbon returns to
the biosphere when the trees
ultimately decay.
CONCEPT
CHECK
Teaching Resources
• Reading and Study
Workbook
For: Links on global warming
Visit: www.SciLinks.org
Web Code: csn – 2207
• Laboratory Manual 60
• Transparency 43
• PresentationEXPRESS
• Interactive Textbook
• Next-Time Question 22-4
How does human activity affect climate change?
• Conceptual Physics Alive!
DVDs Heat Radiation
CHAPTER 22
HEAT TRANSFER
443
443
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Teaching Resources
REVIEW
• TeacherEXPRESS
• Conceptual Physics Alive!
DVDs Heat Transfer; Heat
Radiation
Concept Summary
•
•
•
•
•
•
•
In conduction, collisions between particles transfer thermal energy, without any
overall transfer of matter.
In convection, heat is transferred by
movement of the hotter substance from
one place to another.
In radiation, heat is transmitted in the
form of radiant energy, or electromagnetic waves.
All substances continuously emit radiant
energy in a mixture of wavelengths.
Good emitters of radiant energy are also
good absorbers; poor emitters are poor
absorbers.
The colder an object’s surroundings, the
faster the object will cool.
The near unanimous view of climate
scientists is that human activity is a main
driver of global warming and climate
change.
Key Terms
444
think! Answers
22.1.1 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.
22.1.2 Air is a poor conductor, so the rate of heat
flow from the hot air to your relatively cool
hand is low. But touching the metal parts
is a different story. Metal conducts heat
very well, and a lot of heat in a short time
is conducted into your hand when thermal
contact is made.
22.2
Heat travels upward by convection. Air is a
poor conductor, so very little heat travels
sideways.
22.4
The answer is that the sun has a higher
temperature than Earth. Earth radiates
in the infrared because its temperature is
relatively low compared to the sun.
22.5
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.
Things around us approach a common
temperature only because good absorbers
are, by their very nature, also good emitters.
22.6
No! Although the rate of cooling is greater
for the hotter cup, it has farther to cool to
reach thermal equilibrium. The extra time
is equal to the time the hotter cup takes to
cool to the initial temperature of the lukewarm cup of tea. Cooling rate and cooling
time are not the same.
••••••
conduction (p. 431)
conductors (p. 431)
insulator (p. 432)
convection (p. 433)
radiation (p. 436)
radiant
energy (p. 436)
444
••••••
stellar
radiation (p. 437)
terrestrial
radiation (p. 438)
Newton’s law of
cooling (p. 441)
greenhouse effect
(p. 441)
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 22.1
1. What is the role of “loose” electrons in
heat conductors?
Section 22.3
9. Dominoes are placed upright in a row, one
next to another. When one is tipped over, it
knocks against its neighbor, which does the
same in cascade fashion until the whole row
collapses. Which of the three types of heat
transfer is this most similar to?
2. Why does a piece of room-temperature
metal feel cooler to the touch than paper,
wood, or cloth?
5. Cold is the absence of heat.
6. Warmed air is less dense and
is buoyed upward.
8. Increases; decreases, if
adiabatic
9. Conduction
3. What is the difference between a conductor
and an insulator?
10. The energy in electromagnetic
waves
Section 22.4
11. How does the predominant frequency of
radiant energy vary with the absolute temperature of the radiating source?
12. Is a good absorber of radiation a good emitter or a poor emitter?
5. What is meant by saying that cold is not
a tangible thing?
4. They have many air spaces
and air is a good insulator.
7. The land is warmer than the
water during the day, so
the air rises. The opposite
happens at night.
10. What is radiant energy?
4. Why are materials such as wood, fur,
feathers, and even snow good insulators?
3. A conductor moves heat
quickly, whereas an insulator
moves heat slowly.
13. Which will normally cool faster, a black pot
of hot tea or a silvered pot of hot tea?
11. Higher temperature sources
produce waves of higher
frequencies.
12. Good; otherwise there would
be no thermal equilibrium.
13. Black is a better emitter, and
so will cool faster.
14. It absorbs rather than reflects
light.
15. Light entering is absorbed.
Section 22.2
6. How does Archimedes’ principle relate to
convection?
7. Why does the direction of coastal winds
change from day to night?
8. How does the temperature of a gas
change when it is compressed? When
it expands?
Section 22.5
14. Why does a good absorber of radiant
energy appear black?
15. Why do eye pupils appear black?
CHAPTER
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445
445
16. Cold room; greater DT
17. Yes
18. Radiant energy emitted by
Earth
REVIEW
ASSESS (continued)
19. Earth’s temperature is lower,
so it produces waves of longer
length.
20. a. Only short wavelengths
pass back out.
b. Earth
Think and Explain
21. Same temperature as your
hand
22. No, energy flows from your
hand via the rod to the snow.
23. Fiberglass is a good insulator
because of trapped air.
24. Heat from warm ground
conducted by stone melts
snow in contact.
25. Iron transfers internal energy
very fast.
26. No convection; the CO2
around the candle cuts off the
oxygen supply.
27. Agree; at thermal equilibrium,
gases have same temperature,
which means same average
KE.
28. Disagree; having same KE
doesn’t mean same speed,
unless all molecules have
equal masses.
29. H2 molecules are faster.
KE 5 1/2 mv2. For fixed KE,
less mass means more speed.
Section 22.6
16. Which will undergo the greater rate of cooling, a red-hot poker in a warm oven or a
red-hot poker in a cold room (or do both
cool at the same rate)?
24. Visit a snow-covered cemetery and note that
the snow does not slope upward against the
gravestones but, instead, forms depressions
around them, as shown. Make a hypothesis
explaining why this is so.
17. Does Newton’s law of cooling apply to
warming as well as to cooling?
Section 22.7
18. What is terrestrial radiation?
19. Solar radiant energy is composed of short
waves, yet terrestrial radiation is composed
of relatively longer waves. Why?
20. a. What does it mean to say that the greenhouse effect is like a one-way valve?
b. Is the greenhouse effect more pronounced
for florists’ greenhouses or for Earth’s
surface?
Think and Explain
••••••
21. At what common temperature will both
a block of wood and a piece of metal feel
neither hot nor cool when you touch them
with your hand?
22. If you stick a metal rod in a snowbank, the
end in your hand will soon become cold.
Does cold flow from the snow to your hand?
23. Wood is a better insulator than glass. Yet
fiberglass is commonly used as an insulator
in wooden buildings. Explain.
446
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446
25. Wood is a poor conductor, which means
that heat is slow to transfer—even when
wood is very hot. Why can firewalkers safely
walk barefoot on red-hot wooden coals, but
not safely walk barefoot on red-hot pieces of
iron?
26. When a space shuttle is in orbit and there
appears to be no gravity in the cabin, why
can a candle not stay lit?
27. A friend says that, in a mixture of gases in
thermal equilibrium, the molecules have the
same average kinetic energy. Do you agree
or disagree? Defend your answer.
28. A friend says that, in a mixture of gases in
thermal equilibrium, the molecules have
the same average speed. Do you agree or
disagree? Defend your answer.
29. In a mixture of hydrogen and oxygen gases
at the same temperature, which molecules
move faster? Why?
30. Less mass means higher
speed, so the U-235 has
a greater average speed.
Lighter and slightly faster
U-235 diffuse better.
ASSESS
31. They allow convection.
32. Heat received is from
radiation.
30. Which atoms have the greater average speed
in a mixture, U-238 or U-235? How would
this affect diffusion through a porous membrane of otherwise identical gases made
from these isotopes?
31. Notice that a desk
lamp often has
small holes near
the top of the metal
lampshade. How
do these holes keep
the lamp cool?
32. Turn an incandescent lamp on and off
quickly while you are standing near it. You
feel its heat, but you
find when you touch
the bulb that it is not
hot. Explain why you
felt heat from the lamp.
33. In Montana, the state
highway department spreads coal dust on
top of snow. When the sun comes out, the
snow rapidly melts. Why?
34. Suppose that a person at a restaurant is
served coffee before he or she is ready to
drink it. In order that the coffee be hottest when the person is ready for it, should
cream be added to it right away or just
before it is drunk?
35. Will a can of beverage cool just as fast in the
regular part of the refrigerator as it will in
the freezer compartment? (What physical
law do you think about in answering this?)
36. Is it important to convert temperatures to
the Kelvin scale when we use Newton’s law
of cooling? Why or why not?
37. If you wish to save fuel on a cold day, and
you’re going to leave your warm house for
a half hour or so, should you turn your
thermostat down a few degrees, down all the
way, or leave it at room temperature?
38. Why is whitewash sometimes applied to the
glass of florists’ greenhouses? Would you
expect this practice to be more prevalent in
winter or summer months?
33. The dust absorbs solar energy
and melts the snow.
34. Right away, because whiter
coffee won’t radiate and
cool so quickly; also, the
higher the temperature of
the coffee compared with
its surroundings, the greater
will be the rate of cooling.
And, increasing the amount
of liquid for the same surface
area slows the cooling.
35. No, it cools faster in the
freezer because its rate of
cooling is proportional to the
difference in temperature.
36. Not important; either gives
same differences.
37. Off altogether; the amount
of heat energy, and thus
fuel, required to raise the
temperature inside again
is small compared with the
amount of heat energy that
continually escapes.
38. Whitewash reduces incoming
radiant energy by reflection;
good in summer.
39. Earth’s temperature would
decrease and cooling of
the climate would result.
Conversely, warming of
Earth’s climate would result.
39. If the composition of the upper atmosphere
were changed so that it permitted a greater
amount of terrestrial radiation to escape,
what effect would this have on Earth’s climate? Conversely, what would be the effect
if the upper atmosphere reduced the escape
of terrestrial radiation?
CHAPTER
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TRANSFER
447
447
Think and Solve
40. 12 L is 12 kg 5 12,000 g. Q 5
mcDT 5 (12,000 g)(1.0 cal/g°C)
3 (70°C 2 20°C) 5
600,000 cal.
41. Yes; mcDTball lost by ball 5
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
cDT 5 (800 J/kg°C)(500°C) 5
400,000 J/kg. Time required
is (400,000 J/kg)/(0.03 J/kg?yr)
5 13.3 million years.
43. a. Q 5 mcDT 5 (50.0 g) 3
(1.0 cal/g C°)(50°C 2 22°C) 5
1400 cal. At 40% efficiency
0.4 3 energy from peanut
raises water temperature.
Heat content is 1400 cal/0.4 5
3500 cal (3.5 Cal).
b. Food value is 3.5 Cal/0.6 g
5 5.8 C/g.
44. Work done by hammer is
F 3 d; temp change of nail
from Q 5 mcDT. (5 grams 5
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.
REVIEW
ASSESS (continued)
Think and Solve
••••••
40. An automobile cooling system holds
12 liters of water. Show that when its
temperature rises from 20°C to 70°C, it
absorbs 60 kilocalories.
41. Austin places a 50-g aluminum ball into an
insulated cup containing 75 g of water at
20°C. The ball and water reach an equilibrium temperature of 37°C. Austin makes
some calculations and reports that the
initial temperature of the ball must have
been slightly more than 155°C. Do your
calculations agree? (Ignore heat transfer to
the cup.)
42. Decay of radioactive isotopes of thorium
and uranium in granite and other rocks in
Earth’s interior provides sufficient energy to
keep the interior molten, heat lava, and provide warmth to natural hot springs. This is
due to the average release of about 0.03 J per
kilogram each year. Show that 13.3 million
years are required for a chunk of thermally
insulated granite to increase 500°C in temperature. (Use 800 J/kg°C for the specific
heat capacity of granite.)
448
448
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43. In a lab you burn a 0.6-g peanut beneath
50 g of water. Heat from the peanut increases
the water temperature from 22°C to 50°C.
a. Assuming 40% efficiency, show that the
food value of the peanut is 3500 calories
(3.5 Calories).
b. What is the food value in Calories per
gram?
44. Pounding a nail into wood makes the nail
warmer. Suppose a hammer exerts an average force of 500 N on a 6-cm nail whose
mass is 5 grams when it drives into a piece
of wood. Work is done on the nail and it becomes hotter. If all the heat goes to the nail,
show that its increase in temperature
is slightly more than 13°C. (Use 450 J/kg°C
for the specific heat capacity of the nail.)
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
45. At a certain location, the solar power per
unit area reaching Earth’s surface is
200 W/m2, averaged over a 24-hour day.
Consider a house with an average power
requirement of 3 kW with solar panels on
the roof that convert solar power to electric
power with 25 percent efficiency. Show that
a solar collector area of 60 square meters
will meet the 3 kW requirement.
Activities
Activities
47. If you live where there is snow, do as Benjamin Franklin did more than two centuries
ago and lay samples of light and dark cloth
on the snow. (If you don’t live in a snowy
area, try this using ice cubes.) Describe differences in the rate of melting beneath the
cloths.
48. Wrap a piece of paper around a thick metal
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.)
46. This is a good demo to show.
Steel wool can be used to
wedge the ice at the bottom
of the test tube. Be sure to
put the top part of the waterfilled tube in the flame.
47. The snow under the dark
cloth melts faster. The dark
cloth absorbs more energy
from the sun.
48. The metal must reach 230°C
for the paper to do the same.
••••••
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
Appendix F
Teaching Resources
• Computer Test Bank
• Chapter and Unit Tests
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