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Re: Electrostatic shielding

On Thu, 1 Feb 2001, Leigh Palmer wrote:

Now we're getting somewhere! Let's continue with some fundamental

Perhaps I live in an abnormal conceptual world. When I see a charged
object, I know that somewhere else there must be an OPPOSITLY charged
object, since the "flux lines" cannot just end abruptly. I imagine that
all electric charge everywhere was "created" by being pulled away from an
opposite charge. Consequently, when I see a charged object inside a
Faraday cage, I automatically imagine that it was placed there (that at
some time in the past it had to penetrate the wall of the cage.) I don't
just see a faraday cage with a charged object inside. Instead I see an
unbreakable chain of past events connected to that system. This makes it
immediately obvious that the charge on the surface of the cage has
increased by the same value as the charge on the object within, and that
the latter caused the former. In the case where the charge on the cage
was initially zero, the value of the surface charge must be the same as
the value of the charge on the object within.

Here's another consequence of this sort of thinking: the charged object
does not have to enter the Faraday cage in order to be shielded! If the
object is very small relative to the size of the cage, and the distance
between the cage and the object is very small relative to the size of the
cage, then the charged object is shielded. Why? Because the charged
object attracts an equal and opposite image charge, and these opposite
charges produce a zero e-field at distances far greater than their
separation. It just forms a very-very tiny dipole. At the same time, the
entire surface of the cage aquires an alike charge equal to the charge of
the tiny object, as if the charge on the object has already spread to the
whole cage surface. In other words, the tiny charged object never has to
actually touch the cage in order to "deliver" its charge to the surface.
Just being very close to the cage is good enough (as long as the cage is
far, far larger than the charged object.) If the charged object actually
enters the cage, the invisibly small dipole does actually vanish entirely.

Weird, eh? For those who don't believe it, try sketching the field lines
for a system where a tiny charged object approaches a very large neutral
metal sphere. You end up with a uniform surface charge spread over the
whole sphere, and a very tiny dipole at one spot on the sphere's surface.
The tiny object never loses it's charge though.

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