A Carnot engine takes in 1000 J of thermal energy from a source at 127°C and expels 600 J of energy to the surroundings in each cycle. What is the engine's efficiency and the temperature of the cold reservoir?

20% and −43°C
40% and −33°C
50% and −20°C
70% and −10°C

Correct answer: 40% and −33°C

Solution

Efficiency = 1 - 600/1000 = 40%. For a Carnot engine Qc/Qh = Tc/Th, so Tc = (600/1000)*(400 K) = 240 K = -33 C. The correct pairing is 40% and -33 C, option 1; the stored answer (20%, -43 C) is wrong.

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