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Re: Charged disk; was electrostatic ...

I just performed a semi-quantitative demo exposing the
misconception recognized by Chuck Britton (see below).

A pear-shaped metallic object on a dielectric column was
connected to the terminal of an electrostatic machine
(Dirod 8600). An electrified pithball was located about
one inch from each pole and the angle with the vertical
was estimated. It was about 45 degrees near the pole
where R is larger and perhaps 25 degrees at the pole
where R is smaller.

It would be the other way around if a very large fraction
of the total Q were near the pole whose radius is smaller.
I did not measure the radii but one is about 4 or 5 times
larger than another. Chuck Britton wrote:

A fairly common misconception is that 'most' of the charge
will collect on the more sharply curved surface, (needle tip).
A closer analysis will indeed show that the charge DENSITY
is quite large on the highly curved surface, but the small
area involved does NOT lead to a preponderance of charge
being on the tip. Corona discharge has been ruled out in this
discussion of an 'isolated' object. Lightning rod tips are NOT
isolated and do indeed exhibit coronal discharge.