Effusion

Effusion is the process in which a gas escapes through a hole of diameter considerably smaller than the mean free path of the molecules. Under these conditions, essentially all molecules which arrive at the hole continue and pass through the hole, since collisions between molecules in the region of the hole are negligible.

By the kinetic theory of gases, the kinetic energy for a gas at a temperature ${\displaystyle T}$,

where ${\displaystyle v_{\rm {rms}}}$ is the root-mean-square speed of the molecules and ${\displaystyle k_{\rm {B}}}$ is the Boltzmann constant. The average molecular speed <v> can be calculated from the Maxwell speed distribution as 0.921 ${\displaystyle v_{\rm {rms}}}$ (or ${\displaystyle v_{\rm {rms}}}$ = 1.085 <v>). The rate at which a gas of molar mass M effuses (typically expressed as the number of molecules passing through the hole per second) is then

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