= Two-dimensional ideal gas
{title2=$q=A/\Lambda^2$}
An <ideal gas> whose particles move in an <area> $A$ with two independent translational momentum components. In its dilute classical limit the one-particle <translational partition function> is $q=A/\Lambda^2$, where $\Lambda$ is the <thermal de Broglie wavelength>, and $Z_N=q^N/N!$. The translational <internal energy> is $Nk_{\rm B}T$, and the surface pressure conjugate to <area> is $Nk_{\rm B}T/A$. A mobile dilute adsorbed gas is an example; a fully occupied array of fixed <adsorption sites> has different state counting.
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