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shielding



OTOH, I'm being an idiot, overlooking what's staring me in the face.

Put a charge outside a neutral shell. There's no field inside because
of shielding.

Now put the charge inside the shell instead. This time there is a
field outside the shell, so sounds like we'd say there's no shielding.

But as I just argued in my last message, if you move the inside
charge around in the cavity, the outside field is unaffected because
the outer surface charge density doesn't rearrange, only the inner
surface charge density does.

So the outside *is* shielded from the all-important ac fields causing
electromagnetic interference. There's a constant (static) dc field,
but you can always compensate appropriately for that.

Duh! Carl