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Re: I need help.



Joseph Bellina asked:

... Finally, why in heavens name are you considering such
a complicated geometry to decide whether the conducting
sheet is 2D or 3D? Or have I missed something?

The problem developed when I realized that the equipotential
lines on Pasco sheets can not be explained. The discrepancy
between the data and field due to two long cylinders, should
be explained, one way or another. Is it a topic worth discussing
on Phys-L? I hope so, but I am not sure. I just read what David
posted and learned from it. The next step is to find an acceptable
explanation of the discrepancy. Going to more favorable and
less favorable geometries seems to be a good project for my
students. They will learn by helping me. I might show them
some Phys-L messages from this thread.

On Fri, 22 Feb 2002, Ludwik Kowalski wrote:

If it is true that surface charges are necessary in wires to
bend electric lines locally then they are also necessary to
bend the E lines in a conducting sheet. Is this a reasonable
expectation? If not then why not? It is a separate answer
worth discussing under the Chabay/Sherwood thread.
Ludwik Kowalski

I suspect there is some misscommunication going on here.
The reason lines of E bend is because of superposition...you
have two essentially point charges whose electric fields add.
The resulting tangent lines are curved. That is not at all the
issue in the current in a wire and the need for surface charges.
In that case, the electric field obtained by adding the two
contributions from the battery plates cannot account for the
complicated field configuration which most occur if you are
going to argue that electrons in the wire move because of the
electric field caused by the battery plates. Therefore there
most be other charges present to create the complex fields
configuration needed. Hence one appeals to the existence of charged regions which are induced by the original field
and the shape of the conductor. These are the
surface charges. One does not have to appeal to surface
charges to get curved field lines.

That was a very good answer to my question. Thanks.
Ludwik Kowalski