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Re: Resistor paper



I hadn't described the details of the experiment, but I think you did
what I did. To make sure the field lines were symmetric, I connected the
outer ring at two places, on each end of a diameter, and then measured
the potential as a function of distance along that diameter.
Even without this precaution, I got relatively circular equipotential
lines between the inner dot and the outer ring.

joe

On Thu, 21 Feb
2002, Roger Haar wrote:

HI,
We do this with the terminals at (7,10) and
(23,10) and our equipotentials look similar to
yours.

I am absolutely sure that the situation is like a
slice of two infinite rods. (The concentric rings
is enough experimental proof, and looking at how
the field lines can diverge is enough theory)
That said the finite size of the sheet of paper
causes the difference between what is observed on
the resistor paper and two infinite rods is
infinite space. If you think in terms of field
lines first, every field line (Current path on
the resistor paper) that would in infinite space
extended beyond the paper, is constrained to be on
the sheet.

Try my suggested "image charges" suggestion to
calculate the potentials, I am sure it will
provide good agreement with your experiment.

Thanks
Roger Haar

*****************************************************************************
kowalskil wrote:

We say that surface charges are necessary to create local
electric fields in wires. Are they also necessary to produce
local electric fields when a steady current flows in a sheet of
carbon-impregnated paper? Where would they be located?
How would they be distributed (in the case of two silver
circles painted on a conducting sheet)?

Is it reasonable to blame the observed discrepancy on surface
charges? I am referring to equipotential lines on carbon sheets;
they do not agree with predictions that many have made in the
"I need help" and "Calculating resistance" threads. A discrepancy,
like a bleeding wound, must be cured, in one way or another.
I hope somebody confirms or contradicts my observations by
using the same geometry. Two students expressed interest in
working with me on this project. Please keep helping.
Ludwik Kowalski


Joseph J. Bellina, Jr. 219-284-4662
Associate Professor of Physics
Saint Mary's College
Notre Dame, IN 46556