How does current flow inside a grounded coax shield?

Question

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.

This diagram tries to illustrate the issue:

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?

Thanks, Ian

jay · Answer

That is a very interesting observation, Ian. The waveform in the drawing represents the measurement from a reference at the middle of signal and ground.
However the shape is, whichever side gets grounded, current has to return to the source through the paired wire or any other provided path (leak). For a transmission line, distributed-element model may help understanding.

Campbell · Answer

simulate this circuit &ndash; Schematic created using CircuitLab

Campbell · Answer

I think the varying voltage actually travels along the dielectric between the conductors.

As for current, current can flow in a loop regardless of voltage at a given point (e.g. for one loop with a 5V source and a 5 ohm load, the current will be 1 amp both before and after the load).

How does current flow inside a grounded coax shield?

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