Thermodynamic Geodesics of a Reissner Nordstr\"om Black Hole
  Starting from a Geometrothermodynamics metric for the space of thermodynamic equilibrium states in the mass representation, we use numerical techniques to analyse the thermodynamic geodesics of a supermassive Reissner Nordstr\"{o}m black hole in isolation. Appropriate constraints are obtained by taking into account the processes of Hawking radiation and Schwinger pair--production. We model the black hole in line with the work of Hiscock and Weems. It can be deduced that the relation which the geodesics establish between the entropy $S$ and electric charge $Q$ of the black hole extremises changes in the black hole's mass. Indeed, the expression for the entropy of an extremal black hole is an exact solution to the geodesic equation. We also find that in certain cases, the geodesics describe the evolution brought about by the constant emission of Hawking radiation and charged-particle pairs. 
Christine R. Farrugia and Joseph Sultana
gr-qc