Thermodynamic equilibrium
State with no net flows and no tendency toward change.
Thermodynamic equilibrium is a foundational axiom of thermodynamics, describing either the internal state of a single system or a relationship between multiple systems separated by walls of varying permeability. In this state, no net macroscopic flows of mass or energy occur within a system or between systems, and crucially, there is no macroscopic tendency toward change. Systems in mutual thermodynamic equilibrium are simultaneously in thermal, mechanical, chemical, and radiative equilibrium; however, they can be in one type of mutual equilibrium without being in others. When all these equilibria hold together and indefinitely, the system is in full thermodynamic equilibrium, unless disturbed by a thermodynamic operation. Macroscopic equilibrium is physically explained by perfectly or almost perfectly balanced microscopic exchanges.
A system in internal thermodynamic equilibrium has a spatially uniform temperature, though its other intensive properties may become spatially non-uniform due to an unchanging long-range force field from its surroundings. In non-equilibrium systems, net flows of matter or energy exist; a system that could undergo such changes but does not yet is in a "meta-stable equilibrium." The second law of thermodynamics states that in an isolated system, when partitions between equilibrated subsystems in different states are removed, the system spontaneously equilibrates, increasing its entropy.
Classical thermodynamics treats states of dynamic equilibrium. For specified conditions, the state at equilibrium minimizes a thermodynamic potential or maximizes entropy. For a completely isolated system, entropy is maximized. For a closed system at constant temperature and volume, Helmholtz free energy is minimized. For a closed system at constant temperature and pressure without applied voltage, Gibbs free energy is minimized. Various equilibria are achieved when specific intensive variables match: thermal equilibrium when temperatures are equal, mechanical equilibrium when pressures are equal, and diffusive equilibrium when chemical potentials are equal. All forces are balanced with no significant external driving force.
When the surroundings are treated as another system, the boundary between them has specific permeability properties. For a wall permeable only to heat, thermal equilibrium occurs when energy transfer as heat stops permanentl
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- Thermodynamics
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- Axiomatic state of no net macroscopic flows; minimization of thermodynamic potentials; condition for chemical 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.
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