Quasistatic process

A quasistatic process is an idealized thermodynamic change slow enough for a system to pass through a continuous sequence of equilibrium states. Its equilibrium path makes thermodynamic properties well-defined throughout the change.

Thermodynamic state variable: A macroscopic property, such as pressure or temperature, that describes a system's thermodynamic state. State variables can be assigned at every point along the equilibrium path.
Pressure: The force exerted per unit area by a fluid or gas on a surface. A quasistatic piston process can be described by the changing pressure of the gas.
Isothermal process: A thermodynamic process in which the system's temperature remains constant. A quasistatic isothermal expansion follows equilibrium states at one fixed temperature.
Irreversible process: A process that cannot be reversed without leaving net changes in the system or its surroundings. Friction and finite temperature differences make a process irreversible even when it proceeds slowly.
Mechanical equilibrium: A condition in which forces and pressure differences produce no macroscopic acceleration or motion. Infinitesimal pressure imbalances can drive the slow boundary motion in a quasistatic expansion.
Temperature: A physical quantity that determines the direction of heat transfer between systems in thermal contact. Temperature must be well-defined at each equilibrium state along the process.
Adiabatic process: A thermodynamic process in which no heat enters or leaves the system. A quasistatic adiabatic path relates pressure, volume, and temperature through equilibrium states.
Thermodynamic process: A change in a system's thermodynamic state caused by interactions with its surroundings or internal evolution. Quasistatic processes are a restricted idealized class of thermodynamic changes.
Reversible process: An ideal process that can be reversed by an infinitesimal change, restoring both system and surroundings without net change. Reversibility requires quasistatic behavior plus the absence of dissipative effects.
Internal energy: The total microscopic kinetic and potential energy contained within a thermodynamic system. Its change links heat and work along a quasistatic path through the first law.

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