Thermodynamic equilibrium

A state in which a system’s macroscopic properties remain constant and no net heat, matter, or other flows occur. It requires thermal, mechanical, and chemical equilibrium.

Thermal equilibrium: A state in which bodies in thermal contact have the same temperature and exchange no net heat. It is the thermal condition required for full thermodynamic equilibrium.
Thermodynamic system: A quantity of matter or region of space chosen for thermodynamic analysis, separated from its surroundings by a boundary. Equilibrium is defined for a specified system and its boundary conditions.
Non-equilibrium thermodynamics: The study of systems with gradients, currents, or changing macroscopic properties away from thermodynamic equilibrium. Persistent gradients and currents distinguish non-equilibrium states from equilibrium.
Phase equilibrium: A condition in which two or more phases coexist with no net transfer of matter between them. Coexisting phases are in equilibrium when their relevant thermodynamic potentials match.
Statistical mechanics: A framework that derives macroscopic physical behavior from the statistical properties of microscopic states. It connects equilibrium thermodynamics to the distribution of molecular states.
Mechanical equilibrium: A state in which forces and pressure gradients balance, leaving no macroscopic mechanical motion. Pressure imbalances would drive motion and prevent equilibrium.
State variable: A macroscopic property determined by a system’s state, independent of the path used to reach it. Equilibrium is described by state variables such as pressure, temperature, and volume.
Steady state: A condition in which macroscopic properties remain constant despite ongoing flows through a system. A steady state can appear unchanging while heat or matter continues to flow.
Phase diagram: A map showing which phases of a material are stable under different conditions, such as temperature and pressure. Its boundaries mark conditions where competing phases are in equilibrium.
Boltzmann distribution: The probability distribution assigning energy states weights proportional to the exponential of negative energy divided by thermal energy. It describes equilibrium populations of microscopic energy states.

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