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

On Feb 2, 9:28 pm, Jos Bergervoet <jos.bergerv...@xxxxxxxxx> wrote:
On 2/1/2012 10:25 PM, blackhead wrote:

This is another question posted on Physics Stack Exchange querying the
claims in a physics book that a floating conductive cavity shields the
inside and outside regions from one another's electrostatic fields. I

But it makes only little difference whether the cage
is floating or not.. Outside a floating cage the field
you observe will only tell you the total amount of charge
that is present inside.

All other information of the inside field is unobservable.
Only one number comes through.. (the charge).

It will also tell you how the charge on the inner surface is
distributed. Change this distribution, and you change the distribution
of the field outside, even though the total flux remains equal to the
charge inside.

and the cavity must be held at a constant potential.

Isn't that automatically the case since it is conducting?

In addition, a constant potential wrt say Earth.

The surface charge density at a given point on the outside surface of
the conductor is independent of the distribution of Q in the cavity.
It is the same as if the conductor were solid and carried a net charge
Q. "
-----
1) I don't believe this. As I move +Q around the inside of the cavity,
this will affect the distribution of -Q  on the inside surface, which
must also affect +Q  on the outside surface

Why must it? The claim is that distribution on the
outside is unaltered. Can you give a counterexample?
Or disprove it on theoretical grounds?

I claim that moving a charge Q inside the cavity will alter the
distribution of charge on the inner surface. This will change the
distribution of charge on the outer surface to maintain E=0 inside the
conductor.

So am I correct to disbelieve that an ungrounded conductive cavity can
provide electrostatic shielding for the reasons 1. and 2. above?

Hmm.. I would say that the shield always obtains a
constant potential (after some time) because if the
potential is not constant charge will continue to
redistribute itself. Doesn't that prove the shielding?

I don't think it does. You have to show that the distribution of
charge on the inner surface isn't affected by charge on the outer
surface and I don't see how E=0 inside the conductor proves this.

If one has a solid conductor of any shape with an arbitary charge
distribution on the outer surface, then E=0 inside. Neither can there
be charge inside a conductor. We can then remove the conductor inside
to create a cavity which doesn't change a thing. Therefore, this
proves that the distribution of charge on the outside doesn't affect
the distribution of charge on the inside and so the cavity is shielded
from external electric fields, and I was wrong.

Jos

.

## Relevant Pages

• Re: Can an ungrounded conductive cavity provide electrostatic shielding?
... and the cavity must be held at a constant potential. ... the conductor is independent of the distribution of Q in the cavity. ... It is the same as if the conductor were solid and carried a net charge ... giving +Q as the total exterior charge, the interior surface has no ...
(sci.physics.electromag)
• Re: Can an ungrounded conductive cavity provide electrostatic shielding?
... and the cavity must be held at a constant potential. ... the conductor is independent of the distribution of Q in the cavity. ... It is the same as if the conductor were solid and carried a net charge ... giving +Q as the total exterior charge, the interior surface has no ...
(sci.physics.electromag)
• Re: Can an ungrounded conductive cavity provide electrostatic shielding?
... and the cavity must be held at a constant potential. ... the conductor is independent of the distribution of Q in the cavity. ... It is the same as if the conductor were solid and carried a net charge ... the field in the conductor is zero. ...
(sci.physics.electromag)
• Re: Can an ungrounded conductive cavity provide electrostatic shielding?
... and the cavity must be held at a constant potential. ... the conductor is independent of the distribution of Q in the cavity. ... It is the same as if the conductor were solid and carried a net charge ... the charge on the exterior surface re-arranges itself to shield ...
(sci.physics.electromag)
• Re: Leyden Jar Question
... negative charge per unit area over its entire surface. ... conductor being a conductor is that it cannot support an uneven charge ... An uneven charge distribution is _usual_ for any charged ...
(sci.physics)