The basic thermodynamic potential is internal energy. In a simple fluid system, neglecting the effects of viscosity the fundamental thermodynamic equation is written: d U = T d S − − P d V + μ μ d M where U is the internal energy, T is temperature, S is entropy, P is the hydrostatic pressure, V is the volume, μ μ is the chemical potential, and M mass. In terms of the internal energy density, u, entropy density s, and mass density ρ ρ , the fundamental equation at fixed volume is written: d u = T d s + μ μ d ρ ρ