Heat-flow reversal along a thermodynamic path
= Heat-flow reversal along a thermodynamic path
{title2=$\delta Q=dU+p\,dV$}
For a quasistatic simple compressible system with only pressure-volume <work>, the direction of <heat> transfer is determined by $dU+p\,dV$. A reversal occurs where this vanishes and changes sign. It need not coincide with a maximum or minimum of <temperature>, because the substance can still do <work> when its internal <energy> is stationary.