UniversityPhysicsVolume2-Ch04. Old Dominion University PHYS 232N

Conceptual Questions
1. State an example of a process that occurs in nature that is as close to reversible as it can be.
Solution
Some possible solutions are frictionless movement; restrained compression or expansion; energy
transfer as heat due to infinitesimal temperature nonuniformity; electric current flow through a
zero resistance; restrained chemical reaction; and mixing of two samples of the same substance
at the same state.
2. Explain in practical terms why efficiency is defined as W Qh .
Solution
Efficiency is practically defined as “what you get” divided by “what you pay for.” You are
getting work and paying for it by heat from the hot reservoir.
3. If the refrigerator door is left open, what happens to the temperature of the kitchen?
Solution
The temperature increases since the heat output behind the refrigerator is greater than the cooling
from the inside of the refrigerator.
4. Is it possible for the efficiency of a reversible engine to be greater than 1.0? Is it possible for
the coefficient of performance of a reversible refrigerator to be less than 1.0?
Solution
Without additional work added, the efficiency of a reversible engine must be less than 1.0 and
the coefficient of performance of a refrigerator can never be less than 1.0.
5. In the text, we showed that if the Clausius statement is false, the Kelvin statement must also be
false. Now show the reverse, such that if the Kelvin statement is false, it follows that the
Clausius statement is false.
Solution
If we combine a perfect engine and a real refrigerator with the engine converting heat Q from the
hot reservoir into work W Q  to drive the refrigerator, then the heat dumped to the hot reservoir
by the refrigerator will be W Q  , resulting in a perfect refrigerator transferring heat Q from
the cold reservoir to hot reservoir without any other effect.
6. Why don’t we operate ocean liners by extracting heat from the ocean or operate airplanes by
extracting heat from the atmosphere?
Solution
It takes too much energy or work to extract heat from the large temperature differences.
7. Discuss the practical advantages and disadvantages of heat pumps and electric heating.
Solution
Heat pumps can efficiently extract heat from the ground to heat on cooler days or pull heat out of
the house on warmer days. The disadvantage of heat pumps are that they are more costly than
alternatives, require maintenance, and will not work efficiently when temperature differences
between the inside and outside are very large. Electric heating is much cheaper to purchase than
a heat pump; however, it may be more costly to run depending on the electric rates and amount
of usage.
8. The energy output of a heat pump is greater than the energy used to operate the pump. Why
doesn’t this statement violate the first law of thermodynamics?
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