State variable

A state variable is one of the set of variables that are used to describe the mathematical "state" of a dynamical system. Intuitively, the state of a system describes enough about the system to determine its future behaviour in the absence of any external forces affecting the system. Models that consist of coupled first-order differential equations are said to be in state-variable form.

In thermodynamics, state variables are defined as large-scale characteristics or aggregate properties of a system which provide a macroscopic description of it. In general, state variables have the following properties in common:

  • They don't involve any special assumptions concerning the structure of matter, fields or radiation.
  • They are few in number needed to describe the system.
  • They are fundamental, as suggested by our sensory perceptions.
  • They can be, in general, directly measured.
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