The source defines the efficiency Eff of a heat engine as a ratio of two quantities. Which ratio does it use?
ACold reservoir temperature divided by hot reservoir temperature
BHeat sent to the cold reservoir divided by heat taken from the hot reservoir
CNet work output divided by heat transfer into the engine
DHeat transfer into the engine divided by net work output
Answer & Solution
Correct answer: C. Net work output divided by heat transfer into the engine
1. Efficiency is meant to measure how much useful output an engine produces relative to what it costs to run.
2. The source defines this directly as the ratio Eff = W / Qh, the net work output divided by the heat transfer into the engine.
3. Option D inverts this ratio, describing 1/Eff rather than Eff itself.
4. The ratio of reservoir temperatures in option A belongs to the later Carnot efficiency formula, not this basic definition, and the ratio of the two heat transfers in option B is a related but different quantity used inside the alternate efficiency expression.
_Source: OpenStax College Physics (CC BY 4.0), Ch 15 "Thermodynamics", section 15.3 Introduction to the Second Law of Thermodynamics: Heat Engines and Their Efficiency_
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