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

On Feb 9, 8:32 am, Timo Nieminen <t...@xxxxxxxxxxxxxxxxx> wrote:
On Feb 2, 7:25 am, blackhead <larryhar...@xxxxxxxxxxxx> 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
would answer no, and the cavity must be held at a constant potential.

=====Can an ungrounded conductive cavity provide electrostatic
shielding?=====

This comes from Electromagnetic Fields and Waves by Lorrain et al,
page 77 on a Hollow, ungrounded conductor enclosing a charged body:
------
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 to keep E=0  inside the
cavity conductor.

The external charge distribution won't depend on where the charge is
internally. The charge -Q on the interior surface will be such that
the field in the conductor is zero. So, beyond conservation of charge
giving +Q as the total exterior charge, the interior surface has no
effect: E=0 is the conductor means that the inside charges exert no
force on the exterior charges, no matter where on the exterior surface
the charge might be.

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.

Maybe one can show that the energy stored in a polarized electric
field within a closed conductor enclosing a constant charge in a
cavity, is independent of where that charge is; giving a constant
potential V inside the conductor and therefore constant E at any point
outside.

-----
Inversely, the field inside the cavity is independent of the field
outside the conductor. The conductor then acts as an *electrostatic
shield.
-----
2) I don't believe this either for similar reasons. The external
fields will affect the distribution of +Q  on the outer surface, which
must affect the distribution of -Q  on the inner surface and therefore
the electrostatic field inside the cavity, to maintain E=0  inside the
conductor.

This one goes back to Franklin for the experimental observation, and
Priestly for the conclusion that it implies an inverse square force
law (equivalently, Gauss's law). It's the same thing as the first
case: the charge on the exterior surface re-arranges itself to shield
the conductor from the external field. Again, in the conductor E=0.
So, no effect on the interior surface, so no re-arrangement of charge
on the inside.

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

No, but it is a surprising result. Certainly surprised Ben Franklin
(and it's worth reading his letter to Priestly).- Hide quoted text -

- Show quoted text -

.

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