  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. The reverse (N-S) transition is the Meissner
effect. This implies in particular 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 between the supercurrent and the body in
the S-N transition and the N-S 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 between the supercurrent and 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 does not encounter such difficulties.
