= Effusion
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= Molecular effusion
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<Effusion> is molecular passage through a small opening whose dimensions, including its thickness, are much smaller than the intermolecular <mean free path>. For an equilibrium <ideal gas>, the one-way particle rate through a thin hole of area $A$ is $\dot N=A p/\sqrt{2\pi m k_BT}$. This weights molecules by their wall-normal <velocity>, rather than just counting all molecules in the reservoir. https://farside.ph.utexas.edu/teaching/355/Surveyhtml/node224.html[Fitzpatrick, Molecular Effusion, equations 5.284–5.285]{ref}
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