Heat pumps, air conditioners and refrigerators all move heat transfer from a cold reservoir to a hot one, which is the opposite of a heat engine's natural direction. How does the source describe this relationship to heat engines?
AThey are unrelated devices, not heat engines
BThey are heat engines simply run backward
CThey are heat engines needing no work input
DThey are heat engines that skip the hot reservoir
Answer & Solution
Correct answer: B. They are heat engines simply run backward
1. A heat engine ordinarily lets heat transfer flow from hot to cold in order to produce work.
2. Heat pumps, air conditioners and refrigerators instead push heat transfer from cold to hot, the reverse of that natural direction.
3. This is described directly: these devices are heat engines run backward.
4. Running backward requires driving the process with work input rather than none, ruling out option C, and the devices are closely related to heat engines rather than unrelated to them, ruling out option A.
5. All of these devices are built specifically to deliver heat transfer into a hot reservoir, so option D contradicts their basic purpose.
_Source: OpenStax College Physics (CC BY 4.0), Ch 15 "Thermodynamics", section 15.5 Applications of Thermodynamics: Heat Pumps and Refrigerators_
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