Tuesday, July 14, 2009

State function

In thermodynamics, a state function, state quantity, or a function of state, is a property of a system that depends only on the current state of the system, not on the way in which the system acquired that state. A state function describes the equilibrium state of a system. 

For example, internal energy, enthalpy and entropy are state quantities because they describe quantitatively an equilibrium state of thermodynamic systems. In contrast, mechanical work and heat are process quantities because they describe quantitatively the transition between equilibrium states of thermodynamic systems.

 A thermodynamic system is described by a number of thermodynamic parameters. These choices are equivalent. They are simply different coordinate systems in the two-dimensional thermodynamic state space. An analogous statement holds for higher dimensional spaces.

The path can be specified by noting the values of the state parameters as the system traces out the path, perhaps as a function of time, or some other external variable. The number of parameters needed to describe the system is the dimension of the state space of the system in the state space.

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