  An expression for the internal energy of a fluid element in a weakly coupled,
magnetized, anisotropic plasma is derived from first principles. The result is
a function of entropy, particle density and magnetic field, and as such plays
the role of a thermodynamic potential: it determines in principle all
thermodynamic properties of the fluid element. In particular it provides
equations of state for the magnetized plasma. The derivation uses familiar
fluid equations, a few elements of kinetic theory, the MHD version of Faraday's
law, and certain familiar stability and regularity conditions.
