Isentropic temperature-volume derivative ($(\partial T/\partial V{)}_S=-(T/C_V)(\partial p/\partial T{)}_V$)

For a simple compressible system at fixed particle number, assume the internal energy has continuous second partial derivatives and $T$ is a locally invertible function of entropy at fixed volume. Its exact differential $dU=TdS-pdV$ gives the Maxwell relation $T_V{|}_S=-p_S{|}_V$. Meanwhile the constant-volume heat capacity is $C_V=T{S}_T{|}_V$. The chain rule therefore givesFor one mole of an ideal gas with constant molar $C_V>0$, this becomes $T_V{|}_S=-RT/(C_{V}V)$, whose integral curves satisfy $T{V}^{R/C_V}=\text{constant}$. It expresses cooling on reversible adiabatic expansion in terms of measurable thermal and mechanical derivatives.