Re: I need help.

[kowalskil <kowalskil@MAIL.MONTCLAIR.EDU>]

Joseph Bellina wrote:

> Since the flow lines all lie in the plane of the paper, it seems to me
> that the situation is a 2 dimensional one.
> Let me describe an experiment I did some time ago which supports this
> claim.
> Construct a ring of conducting paint, and put a dot of conducting paint
> in the center.  Measure the potential as a function of distance from the
> center, plot the curve, differentiate it by drawing some tangent lines,
> and then plot the slopes as a function of distance on log-log paper.  Of
> course now you would do it with a computer.  The slope of the
> resulting line is -1, so the electric field falls off as 1/r, which is the
> characteristic of an infinite rod.   In this sense the paper can be
> seen as a perpendicular cut in the space around a charged rod.

This experiment is a convincing evidence that the electric field
in a flat conductor is the same as that produced by a uniformly
charged rod of infinite length. It is very different from the E
due to a point charge.

Taking this for granted, David Bowman used the superposition
principle to predict the shapes of equipotential lines. He wrote:

> Now since the "difference of the logarithms is the logarithm
> of the quotient" we can re-write the potential as
> V = V_0 - C*ln(r_A / r_B).

Unfortunately, experimental equipotential lines contradict
this expectation. Why is it so? Please help to solve the puzzle.
The most important contribution, at this time is to check
my conclusion; no experiment can be taken seriously
unless it is confirmed by others. Spend 15 minutes after
the lab is set for students and report your findings. (But
make sure R of the voltmeter is at least ten megaohms.)
Ludwik Kowalski