# Re: skin depth of zero thickness in DC circuits - Derkeiler.com

Electronics - Devices

Oct 7, 2013 (4 years and 9 months ago)

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Re: skin depth of zero thickness in DC circuits
Source: http://newsgroups.derkeiler.com/Archive/Comp/comp.dsp/2011-06/msg00286.html
From: "Joel Koltner" <zapwireDASHgroups@xxxxxxxxx>·
Date: Fri, 10 Jun 2011 14:47:20 -0700·
"fisico32" <marcoscipioni1@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:6LKdnUzg06I2FW_QnZ2dnUVZ_jednZ2d@xxxxxxxxxxxxxxx
1) if an EM wave of frequency f (any frequency) is incident on a perfect
metal wall (infinite conductivity sigma). The current is a surface current
that exists only on the surface of the ideal conductor;
I would buy this. In fact, I believe it's really the same case as #3.
2) If a AC battery is connected to a load via wires with finite
resistivity. The AC current only distributes itself on the periphery of the
conductor with the current density being largest near the surface of the
conductor;
This is true.
3) In the case of a DC battery connected to a load via ideal wires
(sigma=infinity). We read in books that DC current flows uniformly across
the cross-section of the conductor. However if the wires are perfect
conductors the electric field E inside is equal to zero! How is that DC
current generated then if E=0 and there is no F=q*E force to push charges
and make a current....
I believe the answer is that there's still an electric field on the *outside* of the conductor and this will affect
the electrons at the surface of your perfect conductor, moving them along.
I read that even in the DC case, if the wires have infinity conductivity
(as postulated in intro book) ,the current ends up flowing only on the skin
a not inside the conductor as most book say...
I could believe that, although it is a bit of a pathological case in that all regular conductors have resistance.*
(I'll avoid commenting on superconductors since, while I've read a tiny bit about them, they don't behave as
Re: skin depth of zero thickness in DC circuits
Re: skin depth of zero thickness in DC circuits 1
Maxwellian [classical] physics alone predicts for perfect conductors, and I'd likely say something incorrect.)
I thought that in the DC case there was no skin effect...
Take your formula for skin depth and plug in conductivities for real metals (e.g., copper or silver or
aluminum) and a frequency in the ballpark of, say, 1mHz or whatever it is you want to call "DC" (...since
there's no such thing as "real" DC anyway, right? -- Just really, really low AC... :-) ) You'll end up with a
skin depth that's typically in the ballpark of "many inches," which for most all intents might as well be "no
skin effect" (since most conductors are much smaller than this).
---Joel
* If you want more things that aren't quite as ideal as textbooks would suggest... a real-world coax cable can't
support a pure TEM mode because, due to the finite resistance of the metals, some of the e-field will be in the
same direction as that of propagation. Oops! ...but here again, what really happens is a very close
approximation of TEM mode.
.
Re: skin depth of zero thickness in DC circuits
Re: skin depth of zero thickness in DC circuits 2