Free expansion of an ideal gas by kinetic gas theory

Abstract

We consider the adiabatic free expansion of an ideal gas. According to classical thermodynamics, the internal energy of the gas is not affected by the expansion, but kinetic gas theory predicts an average energy difference between escaping and remaining gas. This difference is transient if the gas escapes into a finite volume, consistent with the equilibrium prediction of classical thermodynamics. For escape into an infinite volume, classical thermodynamics is not applicable. Numerical calculations of the magnitude of the predicted effect are provided.