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Re: charged insulators



On Fri, 3 Jul 1998, Darwin Z. Palima wrote:

"If an insulator allows so little flow of charges, how can an insulator
end up as uniformly charged (as we use in examples in electrostatics)?
Does it mean that we can achieve uniformly-distributed charges in an
insulator only after a very long time?"

Yes, this is a real problem in electrostatics. If we want to put a
uniform charge on an insulating surface, we can't just connect the surface
to a wire, since the wire can, at most, only create a microscopic patch of
excess charge on the surface. A much more severe and widespread problem:
how to REMOVE charges from an insulating surface which has inadvertantly
become electrified.

Some methods which spring to mind for "coating" an insulator with charge
(there may be others):

Make the insulator slightly conductive first (with 'anti-static' spray)
then connect the surface to a high-potential power supply supply.

Charge another substance, then spray the substance onto the insulating
surface (electrified gas, electrified oil mist, etc.) One example:
the sharp combs in a VandeGraaff machine.

Place the surface in intimate contact with a differing insulating
surface, then peel them apart (e.g., rub a rubber surface with rabbit
fur, or roll a polyethele roller across it.)

Illuminate the surface with high-velocity charged particles in a
vacuum chamber, so that the particles adhere. (ion implantation device)

Expose the surface to ionized gas (plasma,) while an electrode at high
DC potential is in close proximity, so the conductive plasma forms a
bridge between the surface and the charging source. (this is how the
photosensitive drum in a photocopier is "painted" with uniform charge.)


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