Thermodynamic process

A change in a system’s thermodynamic state along a path between initial and final states. Its path determines quantities such as heat and work, even when the endpoint states are fixed.

Thermodynamic state: The condition of a thermodynamic system specified by variables such as pressure, volume, and temperature. A process traces changes between the system’s thermodynamic states.
Quasistatic process: A process carried out slowly enough that the system remains arbitrarily close to equilibrium throughout. It makes a continuous sequence of equilibrium states available for describing a path.
Isothermal process: A thermodynamic process that occurs at constant temperature. For an ideal gas, its internal energy stays constant while heat and work balance.
Thermodynamic cycle: A sequence of thermodynamic processes that returns a system to its initial state. A process connects states; a cycle strings processes together and closes the path.
State function: A property whose change depends only on a system’s initial and final states. State functions distinguish endpoint changes from path-dependent quantities.
Reversible process: An ideal process that can be reversed by an infinitesimal change without leaving net effects in system or surroundings. Reversibility sets the limiting case for work and efficiency along a path.
Adiabatic process: A process in which no heat crosses the system boundary. Its temperature can change through work even without heat transfer.
Path function: A quantity whose value depends on the path taken between states rather than endpoints alone. Heat and work are path functions, unlike state functions.
Thermodynamic equilibrium: A condition in which a system’s macroscopic properties do not change spontaneously with time. Equilibrium states define the endpoints of many idealized processes.
Irreversible process: A process that cannot be reversed without net changes in the system or its surroundings. Real processes generate entropy and proceed irreversibly.

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