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Coaxial cable with infinite return conductor

Physics Asked by electro_h on January 24, 2021

If a coaxial cable has a coaxial return conductor with infinite outer radius, will the return conductor experience a voltage build-up due to current flowing through it, or will it stay on ground potential? Here I’m taking infinite to be the zero potential.

I would appreciate if both the d.c. and a.c. case are discussed, if the answer is dependent on type of excitation.

Edit: I think the specific problem where my intuition lets me down is whether there is a scalar potential build-up axially along the return conductor, such that the electric field in the return conductor is given by

$$
mathbf{E} = -nabla varphi – frac{partial mathbf{A}}{partial t}
$$

or, if the electric field is only due to the changing magnetic field, i.e. $mathbf{E} = – frac{partial mathbf{A}}{partial t}$.

My reason for thinking it’s the latter is that since the return conductor certainly is a good conductor, then $varphi$ must be constant and equal to zero, or else currents would flow radially to cancel the charge/potential build-up. Is this correct?

One Answer

AC current, due to the skin effect, tends to flow on the inner surface of the outer conductor of a coax cable. The higher the frequency, the thinner the skin depth. For instance, at $1$MHz, most of the current will flow inside a layer of a couple of hundreds of microns.

DC current will spread out much more, but most of the current will flow within the radius comparable with several lengths of the cable, since the resistance of a path beyond that would substantially exceed the resistance of the direct path.

So, in both cases, the return current will cause some voltage drop, but, in the DC case, it would be much smaller.

Answered by V.F. on January 24, 2021

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