An electric current generates a magnetic field, and magnetic fields cannot exist in the interior of type I superconductors. As a consequence of these two facts, electric currents can only flow near the surface of a type I superconducting wire so that the self-field vanishes in the interior. Here we examine how an electric current flowing through the entire cross-section of a normal conducting wire becomes a surface current when it enters a superconducting portion of the wire. This geometry provides insight into the dynamics of magnetic flux expulsion that is not apparent in the Meissner effect involving expulsion of an externally applied magnetic field. It provides clear evidence that the motion of magnetic field lines in superconductors is intimately tied to the motion of charge carriers, as occurs in classical plasmas (Alfven's theorem) and as proposed in the theory of hole superconductivity \cite{holesc}, in contradiction with the conventional London-BCS theory of superconductivity.