Thermal efficiency

Summary

The thermal efficiency of a heat engine is the fraction of the heat it takes in that it turns into net work. It is a number between $0$ and $1$, and by the second law it is always strictly less than $1$. Carnot's theorem shows that between reservoirs at temperatures $T_H$ and $T_C$ it can be at most $1 - T_C/T_H$. For refrigerators and heat pumps the analogous measure is the coefficient of performance.