K7ITM wrote:
I'm asking this because calls of 'troll' and 'loony' aren't working for me.

It's fairly straightforward, actually, if you believe in Faraday's law
of magnetic induction. That law says that for any closed loop (through
air, through a conductor, through anything), there is an electromotive
force (a voltage source, if you will) whose magnitude is proportional
to the rate of change of magnetic flux enclosed by the loop. As there
is no voltage drop along a perfect conductor, if your closed loop
follows the path of a perfect conductor, there is no voltage drop
around that loop, and therefore the rate of change of the total
magnetic flux enclosed by that loop must be zero. If the perfect
conductor is a closed box, then you can draw loops anywhere through
that conductor and you will never see a changing magnetic field
enclosed by that loop. Thus, the inside of the box and the outside are
magnetically independent; things happening on one side (magnetically)
are not sensed on the other side.

You can understand how this works if you realize that a changing
magnetic field outside the box that would penetrate the box if it
weren't there will induce currents in the conducting box (or even just
in a closed loop of wire). Those currents will (in a perfect
conductor) be exactly the right magnitude to cause a magnetic field
that cancels the external one everywhere inside the closed box (or the
net flux enclosed by a loop of wire). An example: if you short the
secondary of a mains transformer, the primary will draw lots of current
at its rated voltage: it's very difficult for the primary to change the
magnetic flux in the core.

Does the electric field shielding from a perfect conductor need any
explanation?

Of course, an imperfect conductor will be an imperfect magnetic shield.
But a perfect conductor won't let any change of field through, no
matter how slow (no matter how low an EMF it generates), so a perfect
conductor works as a shield all the way down to DC. A box made with an
imperfect conductor is essentially a perfect shield if the box's wall
thickness is at least many skin-depths thick at the frequency of interest.

That's a quick beginning. You can find lots more about this in E&M
texts. There's even useful stuff about it on the web. ;-)


Here is a link to a generalized proof of the skin effect:

http://www.ifwtech.co.uk/g3sek/misc/skin.htm

This is exactly equivalent to Tom's explanation above. The detailed
proof is quite mathematical but it is solidly based in classical physics
- Faraday's Law and Ampere's theorem (both of which are embodied in
Maxwell's equations). This derivation produces the well-known equations
for current density as a function of depth, conductivity and
permeability.

The special feature of this particular proof is that it's much more
general than the ones you see in better-known textbooks - and therefore
much more powerful. It shows that if RF current is flowing in/on *any*
conducting surface, for *any* reason, then the skin effect will be
present.

The possible reasons why RF current may be flowing can be divided into
two main groups:

* "Circuit conditions" - the conductor is part of a circuit that makes
RF current flow.

* "Electromagnetic induction" - the conductor is intercepting an
incident electromagnetic wave which induces a current.

In either case, an RF current flows... and wherever that happens, there
you will also find the skin effect.



--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek