Free expansion

A free expansion means expansion against zero opposing force. Such expansion occurs in vacuum. The work done by a system during such expansion is given by Eq. (4.5), $W = -P_{ext}\,\Delta V$. When the gas expands in vacuum there is no opposing force, that is, $P_{ext} = 0$, and hence $W = 0$. In other words, no work is done when the gas expands freely in vacuum. …