Complete the account of why a heat engine cannot be perfectly efficient.
A cyclic engine must ______ some heat to a cold reservoir in order to close its cycle, so its efficiency is capped even when it is perfectly ______. This limit is imposed by the ______ law, and the maximum is set by the ratio of the two ______ temperatures.
Word bank: reject · frictionless · second · absolute · first
Hints
- The restriction survives even for an idealised engine with no dissipation at all.
- The cap depends only on the two reservoir temperatures, measured on an absolute scale.
Show the answer
A cyclic engine must reject some heat to a cold reservoir in order to close its cycle, so its efficiency is capped even when it is perfectly frictionless. This limit is imposed by the second law, and the maximum is set by the ratio of the two absolute temperatures.
The cap is imposed by the second law, not by friction: even a perfect Carnot engine must reject heat to return to its starting state. Friction makes real engines worse still, but removing it entirely would not lift the ceiling.
Practise Cyclic Processes and the Carnot Cycle
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