= Isobaric ideal-gas internal-energy density
{title2=$U/V=p/({\gamma-1})$}
An <ideal gas> with constant <heat capacity ratio> $\gamma$ has $U=n c_{V,m}T$ up to a consistent thermal-energy reference, and $c_{V,m}=R/(\gamma-1)$. Combining this with $pV=nRT$ gives the displayed thermal internal-energy <density>. At fixed <pressure> it does not grow with <temperature>: the particle <density> falls as $1/T$. A fixed-volume space exchanging gas with the atmosphere therefore differs from a sealed fixed-particle system. Composition changes, activated internal modes or a <pressure> change require a modified comparison.
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