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Re: superposition



On Wed, 15 Jan 1997, Donald E. Simanek wrote:

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.


Well... I'm afraid this simply won't work. With the conductive paper we
effectively set up boundary value problems--e.g., V is fixed on some
painted spots, grad V is perpendicular to the edges of all painted spots,
and grad V is parallel to the edges of the paper and to the edges of any
regions in which we scrape off the conductive coating. The paper solves
the PDE's for us. But superposing the fields that result from two
different sets of boundary conditions is not at all the same thing as
solving for the fields that result when we somehow "combine" boundary
conditions. (Frankly, I don't even understand how you would propose to do
the latter.)

.... and later ...

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.

Hmm, I can believe that the experimental results might not strongly
support a 1/r field strength variation, but I don't believe that "charge
in transit" could be successfully prosecuted as the culprit. As long as
the resistivity of the paper is uniform, the current density--and
therefore the field--must drop off as 1/r, mustn't they? Am I missing
something?

I could imagine the paper being slightly warmer near the inner circle due
to the larger current density. This might lead to an elevated electric
field at small r, but I doubt that it is a large effect at commonly used
currents.

John
----------------------------------------------------------------
A. John Mallinckrodt email: mallinckrodt@csupomona.edu
Professor of Physics voice: 909-869-4054
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web: http://www.sci.csupomona.edu/~mallinckrodt/