Second law of thermodynamics

The second law of thermodynamics states that heat flows spontaneously from hot to cold and that not all heat can be converted into work in a cyclic process. It establishes entropy as a physical property that cannot decrease in isolated systems, leading to the irreversibility of natural processes. Key formulations include Clausius's statement (heat cannot pass from cold to hot without external work), Kelvin-Planck (a cyclic device cannot convert all heat from a single reservoir into work), and Carathéodory's principle (near any state there are adiabatically inaccessible states). The law sets an upper limit on heat engine efficiency via Carnot's theorem. In statistical mechanics, the second law emerges from probability: systems evolve toward equilibrium because that state has the most microstates. The law applies to macroscopic systems; fluctuations occur but are negligible at large scales.

The second law does not prevent local decreases in entropy when coupled with entropy increases elsewhere (e.g., in living organisms or refrigerators). It underpins the concept of exergy and provides criteria for spontaneous chemical reactions via Gibbs free energy. The law's arrow of time is linked to low-entropy initial conditions of the universe.