Equation of state
From Glossary of Meteorology
equation of state
(Also known, for an ideal gas, as the Charles–Gay–Lussac law, or Charles law, or Gay–Lussac's law.) An equation relating temperature, pressure, and volume of a system in thermodynamic equilibrium.
The equation of state for an ideal gas of N molecules in a volume V is
where p is pressure, T is absolute temperature, and k is Boltzmann's constant. After division by the mass of the volume, this may also be written as
where α is the specific volume and R is an individual gas constant. For a mixture of gases, a similar equation may be written with the aid of Dalton's law, where R becomes a weighted average of the individual gas constants.