Entropy

Abstract

Abstract Entropy has several definitions and interpretations, about which there is no consensus. Entropy behaves differently from energy. The entropy of a freely expanding isolated ideal gas increases, whereas its energy does not change. The entropy of an ideal gas in a reversible adiabatic compression is constant, whereas its energy increases. The entropy of an isolated incompressible solid depends only on its temperature, and hence, an initially arbitrary temperature distribution evolves to isothermal, which maximizes its entropy. Because the entropy of a gas depends on temperature and pressure, which vary with height in Earth’s atmosphere, the equilibrium temperature profile of an isolated air column is not isothermal. Constant potential temperature maximizes entropy. Real processes are irreversible, and hence, increase the entropy of a system and its surroundings. Notions that the equilibrium temperature profile of an isolated column of air is the same as that in a solid exemplify the solid atmosphere fallacy.