= Planar effusion flux
{title2=$\Gamma=w n_2\langle v\rangle/\pi$}
For an isotropic two-dimensional <velocity> distribution, a straight opening of width $w$ in a region of number per <area> $n_2$ loses particles at the displayed rate. The angular integral is $\int_{-\pi/2}^{\pi/2}\cos\theta\,d\theta/(2\pi)=1/\pi$. At fixed equilibrium <kinetic energy> the <speed distribution> scales as $m^{-1/2}$, so the rate has the same scaling. A fixed second moment alone does not determine the first moment without a distribution assumption.
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