Thermodynamic state

The thermodynamic state of a thermodynamic system is its macroscopic condition at an instant, characterized (within a chosen level of description) by the values of a complete set of state variables together with the relevant constraints (fixed boundary, fixed composition, specified external parameters, etc.).

In classical thermodynamics one most often restricts attention to equilibrium states , where macroscopic observables are time-independent and the system simultaneously satisfies thermal , mechanical , and (when composition can change) chemical equilibrium . Many microscopic configurations (microstates) typically correspond to a single thermodynamic state because they share the same macroscopic state variables.

Physical interpretation and statistical mechanics. In equilibrium statistical mechanics, an equilibrium thermodynamic state is represented by an ensemble: a probability measure over microstates. Macroscopic observables are modeled as random variables , and their thermodynamic values correspond to expectations (or other typical values) under that measure.

States and processes. The set of admissible states forms a state space. A thermodynamic process is a path through this space, while a cyclic process is a closed path returning the system to the same state. A quasistatic process idealizes a process as a continuous sequence of equilibrium states, so state variables remain well-defined along the path.

State postulate (common form). For a simple compressible single-component system of fixed composition, an equilibrium state can be specified by two independent state variables (for example, temperature and volume , or pressure and temperature), together with an equation of state and a fundamental relation . Quantities determined solely by the state are state functions , while transfers such as heat and work are path functions .