What fundamental requirement dictates that the efficiency of any real heat engine must always be less than 100%?
Answer
The heat energy that is not converted into work ($Q_C$) must always be rejected to a lower-temperature sink and must be greater than zero.
The First Law of Thermodynamics ensures energy conservation, meaning any heat not converted to work ($Q_C$) must be rejected. Since $Q_C$ must always be greater than zero for a real process, efficiency must be less than one.
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