Specific entropy
= Specific entropy
{title2=$s=S/M$}
Specific entropy is <entropy> per unit mass. For a <perfect gas> with constant <specific heat capacity> $c_v$ at constant volume and <specific-heat ratio> $\gamma$, differences obey
$$
s_2-s_1=c_v\log\left[\frac{p_2}{p_1}\left(\frac{\rho_1}{\rho_2}\right)^\gamma\right],
$$
where $p$ is <pressure> and $\rho$ is <mass density>. Thus a <homentropic flow> has constant $p/\rho^\gamma$.