ABSTRACT
This chapter considers the switch from the normal to the superconducting state as a thermodynamic phase transition, and explores the free energy, entropy and heat capacity of the superconductor as functions of temperature and external field. It assumes a perfect Meissner effect–that on entering the superconducting state any applied magnetic field is completely and reversibly expelled from the material. Superconductivity may always be destroyed by a sufficiently large applied magnetic field. The chapter also considers the thermodynamic critical field, the field required to drive normal a sample of type I superconductor which is uniform and long in the direction of the field. The superconductor can find the energy needed to expel the flux by making a phase transition to a condensed state of lower internal free energy.
