Thermodynamic equilibrium

Lore & Background

Thermodynamic equilibrium is an axiom of thermodynamics, stating that there exist states of thermodynamic equilibrium. In a system that is in its own state of internal thermodynamic equilibrium, not only is there an absence of macroscopic change, but there is an 'absence of any tendency toward change on a macroscopic scale.' Systems in mutual thermodynamic equilibrium are simultaneously in mutual thermal, mechanical, chemical, and radiative equilibria. Systems can be in one kind of mutual equilibrium while not in others. In thermodynamic equilibrium, all kinds of equilibrium hold at once and indefinitely, unless disturbed by a thermodynamic operation. In a macroscopic equilibrium, perfectly or almost perfectly balanced microscopic exchanges occur; this is the physical explanation of the notion of macroscopic equilibrium.

Reader's Guide

Thermodynamic equilibrium is central to classical thermodynamics, which deals with states of dynamic equilibrium. The state of a system at thermodynamic equilibrium is the one for which some thermodynamic potential is minimized (in the absence of an applied voltage), or for which the entropy is maximized, for specified conditions. For a completely isolated system, entropy is maximum at thermodynamic equilibrium. For a closed system at controlled constant temperature and volume, Helmholtz free energy is minimum. For a closed system at controlled constant temperature and pressure without an applied voltage, Gibbs free energy is minimum. The second law of thermodynamics states that, in an isolated system, when partitions are removed between equilibrated subsystems in different states, the system spontaneously equilibrates, accompanied by an increase in the system's entropy. The concept also underlies contact equilibria, where systems with specific permeabilities reach common intensive variables such as temperature, pressure, or chemical potential.

Did You Know?

• Thermodynamic equilibrium is an axiom of thermodynamics, though not a widely named 'law.'
• In thermodynamic equilibrium, there are no net macroscopic flows of mass nor of energy within a system or between systems.
• For a completely isolated system, entropy is maximum at thermodynamic equilibrium.
• A system in thermodynamic equilibrium has a spatially uniform temperature, though other intensive properties may be driven to spatial inhomogeneity by an unchanging long-range force field.