Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

Entropy isn’t changed by a Carnot engine

The efficiency of a heat engine is defined by

efficiency = 1 - QL/QH ,

and the efficiency of a Carnot engine is

efficiency = 1 - TL/TH ,

so for a Carnot engine we have QL/QH = TL/TH, which can be rewritten as QL/TL = QH/TH. The entropy lost by the hot reservoir is therefore the same as the entropy gained by the cold one.

Last Update: 2009-06-21