Cases – Chapter 8
1. A solar panel is an electronic device that produces electric power when it’s exposed to sunlight. It’s also a type of heat engine.
a. What object supplies the heat that powers this heat engine?
b. What object receives the waste heat that passes through this heat engine?
c. If you try to run a solar panel by exposing it to light from an incandescent bulb, it will produce relatively little electric
power. Explain this result in terms of the temperature of the bulb’s filament.
d. Suppose that the solar panel could withstand the sun’s temperature. If you were to immerse the panel in the sun, it wouldn’t
produce any electricity. Why not?
e. A thermoelectric cell produces electricity when you heat one side of it and cool the other. How does this device resemble a
solar panel?
2. A refrigerator is a heat pump that cools the contents of a food locker. Like an air conditioner, it has an evaporator, a condenser,
and a compressor.
a. Where is the evaporator located in a refrigerator?
b. Where is the condenser?
c. What will happen to the food’s temperature if you wrap the refrigerator’s condenser and compressor in thermal insulation, so
that they can’t exchange heat with the room?
d. What will happen to the temperature of the room if you open the door to the refrigerator and let it run for a few hours?
e. Why does it take more electricity to operate a freezer than a refrigerator of similar size and quality?
3. Desalinating salt water is one way arid regions are able to obtain drinking water. However, it’s not an easy process.
a. Which has more entropy: a container of salt water or a container of fresh water plus a box of salt?
b. Why does your answer to part a indicate that the desalination process requires either the conversion of ordered energy into
thermal energy or the movement of heat from a hotter region to a colder region?
c. One technique for desalination is reverse osmosis, in which salt water is squeezed through a special membrane that permits
only pure water to pass. Show that squeezing water through this membrane takes work (ordered energy).
d. Another technique for desalination is distillation, in which salt water is boiled in a hotter region to form water vapor that is
then condensed to pure water in a colder region. This desalination process is a heat engine that separates the water from the
salt. Explain why it’s a heat engine.
e. Still another desalination technique is freezing the salt water into pure ice and salt crystals. In this case the salt water has to
be cooled to very low temperatures to freeze it, then warmed to a much higher temperature to melt it. Show that this technique
is also using a heat engine to drive the desalination process.
4. It’s your first day on the staff of the U.S. Patents Office and you’re excited about having some new ideas come across your
desk.
a. An inventor comes to you with a small box that is supposed to make batteries obsolete. The inventor claims that the box can
produce electricity forever without having to be recharged. Why can you be sure that this claim is nonsense?
b. Another inventor comes in with a small motorlike device that’s powered by a burning candle. The inventor claims that this
device takes the heat from the candle and turns it entirely into work. The device thus doesn’t warm the room at all. Your
knowledge of the laws of thermodynamics assures you that this claim, too, is nonsense. Why?
c. As though the entire population of inept inventors was released on you in one day, another character comes in claiming to
have a heat pump that can transfer heat out of a box of corn flakes for as long as you like. The cereal just gets colder and
colder. Once again, you know that this is impossible. Explain.
d. Just when you thought you were out of the woods, a person comes in with a coffee mug that automatically reassembles itself
if you accidentally break it. You’re supposed to put the broken mug in a box and wait. In a minute or two, the mug will be as
good as new. You agree with the inventor that this reconstruction trick doesn’t violate any of the laws of motion. However, you
can still be sure that it won’t work. Why?
5. On a sunny morning at the seashore, the sun warms the land quickly while the sea remains cool.
a. The air above the land also warms and expands. What happens to its density? Why?
b. Because of its expansion, the column of air over the land becomes taller than the column of air over the nearby water. High
altitude air begins to flow from over the land to over the water. What causes this flow?
c. With the weight of additional air pushing down on it, the air just above the water experiences a rise in pressure. It begins to
flow toward the lower pressure air over the land and a land breeze forms. If the land is the hotter region and the water is the
colder region, which way is heat flowing?
d. Explain why this mechanism for moving air back and forth near the seashore is a form of heat engine.
6. Decompression sickness was first observed in workers preparing underwater foundations or caissons for bridges. The work
had to be done in pressurized air to keep the water out. When the pressure was released, the workers would experience “caisson
disease,” a painful or even fatal disorder in which gas bubbles formed inside their tissues. Because they bent over in pain, their
illness was called “the bends.”
a. The workers would enter the caisson while it was at atmospheric pressure. The doors were then sealed and air was pumped
in. The temperature in the caisson rose rapidly. What caused this temperature rise?
b. A few minutes after the caisson was brought up to full pressure, the air inside the caisson would return to normal
temperature. What had happened to its thermal energy?
c. When the shift was over, the doors to the caisson were opened and the air was allowed to rush out. That outgoing air was ice
cold. The air inside the caisson was also so cold that sometimes it would begin to snow. What caused this temperature drop?
d. When during this cycle of pressurizing and depressurizing was work done on the air inside the caisson
e. When was work done by the air inside the caisson?
f. What heat transfer took place?
7. While it flies at high altitudes, a jet airplane must continually replace its cabin air with fresh air from outside. But because that
outside air is very cold and its pressure and density are both quite low, the plane must compress the air.
a. The airplane already has an excellent compressor on board: its jet engines. It uses some of the air that has passed through the
initial compressors of these engines, before fuel has been added. This compression process makes the air extremely hot. Why?
b. Has the air’s total energy changed as a result of its passage through the compressor? If it has changed, what provided or
absorbed some of the air’s energy?
c. Rather than baking its occupants, the airplane refrigerates the compressed air. It uses an air conditioner to pump heat from
the hot compressed air to the cold outside air. Why is it surprising that the airplane uses a heat pump to move heat in that
direction?
d. As fresh air flows into the cabin, stale air is released from the plane. Overall, cold air enters the plane’s engines and hot air
leaves the plane through air outlets. Heat also flows into the outside air from the air conditioner. How is the entropy of the
outside air affected by this process?
e. The only real change inside the airplane is that its fuel gradually diminishes. Since that fuel doesn’t contribute to the plane’s
entropy, the plane’s entropy isn’t changing significantly. How can the plane continue to operate without an increase in its
entropy? Doesn’t this violate the second law of thermodynamics?
8. When air in the atmosphere rises or falls it often experiences a change in temperature. Interesting examples of this behavior
are the chinook winds in the Rocky Mountains and the foehns in the Alps.
a. When a wind containing moist air blows up the side of a tall mountain, its altitude increases and its pressure drops. What
happens to this air’s density?
b. Why does this rising air experiences a drop in temperature?
c. The air’s dropping temperature causes its moisture to condense into rain while the air itself continues over the mountaintop
without the moisture. As the moisture condenses from a gas into a liquid, it warms the air. Which portion of an air conditioner
employs a similar process?
d. The warmed air then descends into the valley beyond the mountain and it warms up even more. The air reaching the valley
is unusually hot and dry. Explain this additional warming during its descent.