# Re: Can an ungrounded conductive cavity provide electrostatic shielding?

On 2/10/2012 3:27 AM, blackhead wrote:
...
I don't understand this. The sum of all the electric forces inside
might be zero, but those individual forces from the interior charges
still exist.

But if they sum up to zero isn't by definition their force
on the exterior charges zero?

Yes. But Timo suggested that because the sum of the interior-cavity,
interior-surface and exterior-surface electric forces are zero inside
the conductor, then this means the sum of the interior electric forces
must be zero on the exterior surface. I don't get this.

Well, I also would say that I'm not sure I understand what
Timo meant.. Two separate things can be stated:
1) The sum of the interior forces is zero inside the conductor.
2) The sum of the exterior forces is zero inside the conductor.
What you wrote above is the sum of all of them, which is of
course also zero, but that fact would not (by logic alone)
imply that both are zero individually..

...
I will admit one silly mistake I have made in this discussion:
Forgetting that the change in the cavity charge distribution can
compensate for the change in the interior-surface charge distribution.

That is of course in a way remarkable.. (A two-dimensional
distribution would seem to have insufficient degrees of freedom
to always be able to compensate a three-dimensional distribution!)
But as modern physicists we are familiar with to the holographic
principle (Susskind-'t Hooft), so yes, it was silly of you to
forget that! :-)

So it's possible to keep the exterior-surface charge distribution
constant while still maintaining E=0 inside the conductor even though
the interior-surface charge distribution has changed.

OK, so you believe it is possible. But you don't see the proof
that it actually happens?!

--
Jos
.

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