Thermodynamic equilibrium is the condition of a macroscopic system in which no net, spontaneous macroscopic changes occur in time. In this state all parts of the system exhibit uniform or balanced values of the relevant intensive variables. The concept belongs to thermodynamics and applies to a defined thermodynamic system whose microscopic details are not explicitly tracked.
Main conditions
Thermodynamic equilibrium can be decomposed into distinct requirements that are often stated together. Commonly listed types are:
- Thermal equilibrium — no net heat flow; temperatures are equal throughout and between bodies.
- Mechanical equilibrium — no unbalanced forces or flows; pressures (or generalized mechanical potentials) are balanced, e.g. hydrostatic balance.
- Chemical equilibrium — no net chemical reactions or species exchange; chemical potentials are equal across phases or regions.
- Radiative equilibrium — net radiative energy exchange is zero so radiative heating and cooling balance.
Intensive variables and potentials
The instantaneous macroscopic state at equilibrium is specified by intensive parameters such as pressure and temperature, together with composition and fields. Equilibrium can also be characterized variationally: for an isolated system the entropy is at a maximum, while under common constraints one of the thermodynamic potentials reaches an extremum. Examples include the Helmholtz free energy or the Gibbs free energy; the term thermodynamic potential is used for these state functions.
Microscopic and practical points
From a microscopic viewpoint equilibrium corresponds to the macrostate with the largest number of accessible microstates consistent with the constraints. In the thermodynamic limit fluctuations around equilibrium are negligible, which justifies treating macroscopic intensive variables as well defined. In finite or small systems fluctuations can be significant and strict equilibrium becomes a probabilistic notion.
Uses, examples and distinctions
Equilibrium concepts underpin phase diagrams, chemical reaction calculations, and engineering designs such as heat engines and refrigeration cycles. Distinct from a non-equilibrium steady state, an equilibrium state has no net flows of matter or energy. Metastable states may persist for long times but are not true equilibrium because a lower-energy (or higher-entropy) configuration is accessible.
In practical modeling, the assumption of local thermodynamic equilibrium allows use of equilibrium relations within small regions of a spatially varying system, an approach common in fluid dynamics, astrophysics, and atmospheric science. Recognizing when equilibrium assumptions hold is essential for accurate description and prediction of macroscopic behavior.
For further reading see introductory treatments of thermodynamics and textbooks on statistical mechanics and physical chemistry that discuss equilibrium criteria, thermodynamic potentials, and applications across engineering and the natural sciences. Additional resources include reviews of radiative and chemical equilibria in specialized contexts.
Key distinctions: equilibrium vs steady state, global vs local equilibrium, and true equilibrium vs long-lived metastability. Understanding these differences clarifies when classical equilibrium thermodynamics applies and when more general non-equilibrium methods are needed.