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Re: Electrostatic shielding



On Wed, 31 Jan 2001, Leigh Palmer wrote:

Michael Edmiston, Bill Beaty, et al are confusing themselves. They are
talking about a process in which the cage has no net charge, a condition
which is never mentioned in the description of the Faraday cage shielding
effect.

True. It doesn't matter what the initial charge on the cage might be.

Grounding the cage has no relation to the net charge on the cage.

A connection to ground provides a path by which net-charge of the cage can
change as the charged objects are placed inside. An electrically-floating
cage behaves differently than one which has a ground wire.

Bill said:

If you put a coulomb of charge inside a faraday cage, the cage itself
seems to be charged with 1c, as if the flux lines go right through the
walls of the cage. However, if you move your 1c of charge to different
places within the cage, the field outside the cage does not change. The
cage does prevent us from seeing the charge distribution from outside.
But it doesn't prevent us from seeing the flux from any net charges which
we place inside.

If you put a coulomb of charge inside a Faraday cage I'll be impressed,
but the charge on the cage itself will depend upon other factors as well,
particularly it initial charge and its state of isolation from the rest
of the world. It is very important to specify these things!

Certainly. Is your "cage" a small metal box sitting on a formica desktop?
Or is it built into lab walls, and connected to a ground stake?
"Grounding" need not be done through a wire. Suppose the room-sized cage
has 10,000pF capacitance to the earth, and also suppose that there is a
million-ohm leakage-path through the concrete floor to the earth. If a
net-charge should be deposited upon the cage, it will decay away
exponentially with a .01 second time constant. The 1M resistance serves
as a good "ground wire" as far as static fields are concerned. To isolate
that cage, so that the charge only decays with a 1000 sec time-constant,
the leakage resistance would have to be increased to 100 giga-ohms. Just
put the cage on tall styrofoam blocks, and turn on the dehumidifier!


There are no
default values. Bill's picture is almost correct if the cage is isolated
and initially uncharged.

Even if it was initially charged, the e-field outside will change to a
different value whenever a charged object is placed inside. In a real
experiment the demonstrator must be careful not to touch the cage, even
momentarily, or the surface charge on the cage which was induced by the
object inside it will greatly decrease as it spreads to their body.

I think Pasco sells a charged-measuring electrometer which is based on
this effect. If you measure the potential of a metal cup with respect to
the earth, then any charged objects placed into the cup will create a
proportional e-field outside on the cup. Recalibrate your kilovolt/meter
field measurments in terms of microcoulombs.



It is a real stretch to say that we are "seeing
the flux from any net charges" inside, one I certainly would not make.

The flux is disconnected from the charged objects inside, since they can
be moved about without altering the fields outside. In the Faraday Ice
Pail experiment, a charged conductive object is touched to the inner
surface of the "cage", and loses all of its charge. Even this does not
affect the fields outside the cage: they act as if the small object is
still charged.

Side note: it seems amazing that Faraday didn't invent the VandeGraaff
electrostatic generator. This is probably because the VDG machine is so
obvious in hindsight.


David Bowman is correct; I should have made it clear that my "inside" and
"outside" referred to the gaussian surface, and that inside included the
inner surface of the cage.

Leigh


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