The maximum theoretical efficiency of a heat engine between two reservoirs depends on their temperatures; what quantity is this?

• A. Pressure
• B. Entropy
• C. Temperature
• D. Volume

Answer: (C)

The maximum theoretical efficiency is set by the temperatures of the two heat reservoirs. For an ideal reversible engine, the Carnot limit shows that efficiency depends only on those temperatures: η = 1 − Tc/ Th, with both temperatures measured in Kelvin. This means the bound is universal for any working substance and cycle between the same hot and cold reservoirs, and it isn’t determined by pressure, volume, or entropy. Real engines can’t reach this limit because of irreversibilities, but it still provides the ultimate ceiling. So the key quantity that determines the maximum theoretical efficiency is temperature.