Second Law of Thermodynamics — Unit test

Question 1 • Crowley Credit Recovery Physics A

Heat engine X has an efficiency of 67%. Heat engine Y has an efficiency of 82%.

Answer

Heat engine X moves more thermal energy from cold to warm areas than heat engine Y.

Heat engine X releases more energy to the surroundings than heat engine Y.

Heat engine Y releases more energy to the surroundings than heat engine Y.

Heat engine Y moves more thermal energy from cold to warm areas than heat engine X.

Explanation

Efficiency is the fraction of incoming thermal energy converted into useful work, so the fraction released to the surroundings is 100% minus efficiency. Engine X releases 33% of its input as waste heat, while engine Y releases 18%. For the same amount of input energy, X therefore releases more energy to the surroundings, matching choice B.

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