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On Tue, 14 Jan 1997 kowalskil@alpha.montclair.edu wrote:
Just thinking aloud about a superposition demo. Suppose you use carbon
paper with two silver-painted spots and you measure E (grad of V) at a
given location. Then you take another paper and the second spot is at
another location. You measure E at the same spot as before and it is
different. Finally you "superpose" by painting three spots on the third
paper and measure E again. This should, in principle, be an easy way
to show that E3=E1+E2 (vectors).
Ludwik Kowalski
Sounds like a practical demo, and is a neat use of the conductive
paper/paint apparatus. One of these days I must try it, for I'm always
looking for new ways to use this stuff.
How about studying a conductive pattern consisting of just one small spot
of conductive paint, and a large circle of conducting paint centered on
it, as large as the paper will allow. The field should be radial, and easy
to deal with. Now investigate the field strength as a function of radial
distance from the center spot. It is approximately 1/r. The potential goes
as ln r. But not exactly. There's charge in transit from the center
outward. At any distance, r, the charge enclosed by a Gaussian surface is
that of the center electrode plus that in the paper with the circle of
radius r. How does that affect the field and potential variation with r.