Second law of thermodynamics

The second law of thermodynamics describes the direction of natural processes through entropy, a quantity that tracks energy dispersal and the number of possible microscopic arrangements. In an isolated system, total entropy cannot decrease. It increases in irreversible processes and remains unchanged in the ideal reversible limit, so an increase is not inevitable in every case. This principle explains why heat flows spontaneously from a hotter body to a colder one and why converting heat into useful work faces fundamental limits.

A local decrease in entropy is possible when a system exchanges energy with its surroundings, provided the full accounting respects the law. Scientists and engineers apply it in chemistry and energy systems, while it also informs cosmological questions. Its distinctive role is to describe which processes can occur, beyond the energy balance required by the first law.