Re: Charge Distribution around pointed areas

[William Beaty <billb@ESKIMO.COM>]

On Mon, 15 Nov 1999, Allen Miller wrote:

> It follows that the surface charge density is larger on the projection
> than it is on the rest of the plate.

Here's another 'intuitive' description:  charge on a conductive surface is
mobile and self-repelling, therefor the charged particles adjust
themselves until each particle is repelled by neighbors and the net
repulsion is zero.  If a flat surface has an edge, then the charges on the
edge will have more neigbors on one side than the other, therefor they
will be pushed towards the edge and packed closer together.  For a
protuberance on a flat surface, the same thing occurs but even more so.

I vaguely recall some phys-L discussion about a misconception involving
charge on sharp points: it is not JUST the radius which controls the
charge density.  Surrounding structures on the same object also affect it.
For example, a sharp edge which is down inside a hole in the object may
have zero charge density, even though its radius is very small.  Or, if
two adjacent needles extend from the same surface, and one is taller than
the other, the taller one can have a significantly higher charge density,
since the taller one creates a shielding effect (a sort of "shadow",) and
the shorter one might lie within this shadow.


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