I have been trying to reconcile the fact that current flows on the inside of a coax shield with the common practice of grounding the shield. These ideas seem contradictory at the moment. For a 2-wire transmission line it seems intuitive that both wires have a varying voltage wave along them, creating an electric field between the wires and also a potential difference along the wires, due to the peaks and troughs of the wave. Hence, current flows backwards and forwards in each wire.
For coax the argument is the same, until we connect the shield to ground. Now it seems that I have locked the shield potential, so now it seems that all the voltage variation is on the centre conductor, while the shield remains at a fixed potential. If the potential is fixed, how can there be current flow on the inside of the shield?
I know there is a high impedance between the inside and outside of the shield, but I can't see how that solves anything. Clearly I am misunderstanding the electromagnetic effects in some way. Appreciate any advice.
At a given moment, the shield has a sinusoidal current distribution, which to me suggests a potential difference along the shield. Yet we have the shield grounded?