 In other words, the graphs of formula_83 and formula_87 are slices of the surface defined by the state equation, cut by planes of constant formula_50 and constant formula_39, respectively.Then, from the fundamental thermodynamic relation it follows that
This equation can be rewritten as
where
both depending on the state formula_80.The heat capacity ratio, or adiabatic index, is the ratio formula_101 of the heat capacity at constant pressure to heat capacity at constant volume.