Tilamäärä

Tilamäärä, in thermodynamics and statistical mechanics, is a physical quantity that depends only on the current state of a system and not on the path taken to reach that state. In English, it is often referred to as a state quantity or state variable. Tilamäärät are used to describe the conditions of a system and to express relationships through equations of state that relate properties such as pressure, volume, and temperature.

Tilamäärät can be classified as intensive or extensive. Intensive tilamäärät, such as temperature and pressure, do not depend on the size or amount of material. Extensive tilamäärät, such as volume and internal energy, scale with the amount of material present. Together, these quantities characterize the thermodynamic state and allow prediction of how the system responds to changes in external conditions.

Examples of common tilamäärät include temperature (T), pressure (P), volume (V), internal energy (U), entropy (S), and Gibbs free energy (G). These quantities are defined for a system in a given state and are related by equations of state, such as PV = nRT for an ideal gas. Heat (Q) and work (W), in contrast, are typically not tilamäärät because they depend on the specific process path between states.

In statistical mechanics, tilamäärät correspond to macroscopic observables whose average values characterize a macrostate. Changes in tilamäärät (for example, ΔT, ΔS) depend only on the initial and final states, not on the detailed trajectory between them.