Since the discovery of the Meissner effect the superconductor to normal (S-N) phase transition in the presence of a magnetic field is understood to be a first order phase transformation that is reversible under ideal conditions and obeys the laws of thermodynamics. In particular, this implies that the kinetic energy of the supercurrent is not dissipated as Joule heat in the process where the superconductor becomes normal and the supercurrent stops. In this paper we analyze the entropy generation and the momentum transfer from the supercurrent to the body in the S-N transition as described by the conventional theory of superconductivity. We find that it is impossible to explain the transition in a way that is consistent with the laws of thermodynamics unless the momentum transfer from the supercurrent to the body occurs with zero entropy generation, for which the conventional theory of superconductivity provides no mechanism. Instead, we point out that the alternative theory of hole superconductivity provides a way out of this conundrum.