Work (thermodynamics)

Work is energy transferred to or from a thermodynamic system through organized forces acting across its boundary. Its sign and magnitude depend on the chosen system and sign convention.

Pressure–volume work: Work transferred when a system’s volume changes against an external pressure. It is the standard boundary-work form for expanding or compressing a gas.
Thermodynamic system: A specified quantity of matter or region of space chosen for thermodynamic analysis. Work is defined by energy crossing the boundary of this chosen system.
Heat engine: A device that converts heat absorbed from a hot source partly into work while rejecting remaining heat. Its useful output is work produced during a thermodynamic cycle.
Adiabatic process: A thermodynamic process in which no heat is transferred across the system boundary. An adiabatic change can still involve work, so zero heat does not mean zero energy transfer.
Reversible process: An idealized process that can be reversed by an infinitesimal change without leaving net changes in system and surroundings. For a reversible volume change, pressure–volume work is calculated from the system’s equilibrium pressure.
Thermodynamic boundary: The real or imagined surface separating a thermodynamic system from its surroundings. Work transfer occurs across this boundary through forces and displacements.
Carnot cycle: An ideal reversible cycle consisting of two isothermal and two adiabatic processes. It establishes the maximum work output for engines operating between fixed-temperature reservoirs.
Isothermal process: A thermodynamic process that maintains constant temperature. Temperature staying fixed does not imply that work or heat transfer is absent.
Quasistatic process: A process slow enough that a system remains arbitrarily close to internal thermodynamic equilibrium. It lets pressure be defined throughout a path used to calculate boundary work.
Thermodynamic state: The condition of a system described by variables such as pressure, temperature, and volume. State endpoints alone do not determine work, because work depends on the intervening process.

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