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Re: electric charge



Michael Bowen wrote:

Students might be misled by the following (quoted) paragraph
into believing that Coulomb's law works regardless of the shapes
of the charged objects. Something needs to be said about point
(small) charges versus extended charges; you might also want
to point out, just briefly, that spherical charged objects, under
the right circumstances, work almost as well as point charges.
Coulomb's law does not work very well with charges arranged
on closely-spaced pairs of flat conducting plates, for example.

Speaking of conductors, I see you've omitted any discussion of
them (versus insulators). This may or may not be intentional on
your part. ...

Thanks again. The most difficult part in teaching electricity, from
my point of view, is to start it. The electrostatics flows smoothly
after the Coulomb's law is established. Likewise d.c. circuits
become easy after Ohm's law (and the mystery of the battery cell)
are accepted by students. My little sketch was about the very
beginning. I agree that a distinction between conductors and
insulators should be made as soon as possible, most naturally
during the preliminary frictional demonstrations.

If I recall correctly, distances between the spherical balls used
by Coulomb were about one diameter or less. At such distances
the 1/r^2 law becomes a crude approximation. This is an
interesting topic but I would not discuss it on day one. (By
the way, I am ordering the Coulomb's law apparatus from
Pasco this year in order to explore this topic experimentally
with students. How reproducible are measurements made
with this piece of equipment?
Ludwik Kowalski