Abstract: It is not a sufficient equilibrium condition that the pressures are equal on both sides of a smooth adi- abatic movable piston. We consider a system consisting of ideal gas which is partitioned into two parts by an adiabatic movable piston. From the microscopic point of view, even if the two parts are with equal pressures that different temperatures, energy can still transfer from higher temperature gas to lower temperature gas through the piston vibration,which is difficult to observe in macroscopic systerm but very significant in mesoscopic system.

Key words: ideal gas, movable piston, equilibrium