B.4.6 (HL)—Second law of thermodynamics

Second law

The entropy of an isolated system never decreases:

$$\Delta S_{\mathrm{isolated}}\ge 0$$

Real spontaneous processes usually increase it.

Local decreases are allowed

A non-isolated subsystem can decrease in entropy, such as the contents of a refrigerator, but the surroundings must increase in entropy by at least as much. Judge the total isolated system.

Direction and engines

The second law gives the net direction of thermal transfer from hot to cold and prevents a heat engine from converting all input thermal energy into work. It leads to the Carnot efficiency limit.

Common trap

“Entropy never decreases” applies to an isolated system or the universe, not necessarily to every local subsystem.