Re: electric charge

[Michael Bowen <fizzbowen@MINDSPRING.COM>]

At 19:47 2001/12/25, Ludwik Kowalski wrote:
> The comments of this thread prompted me to sketch an
> introduction to electricity. Is it acceptable? I plan to
> distribute it as a handout to students, after performing
> standard demonstrations with rods and pith balls.
> Ludwik Kowalski

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.

Also, isn't the Coulomb constant k about nine times greater in magnitude
than suggested? I thought the force was closer to nine billion newtons. If
you feel compelled to round to the nearest power of ten, I think you need
one more zero. Unless this is a very beginning class, I think we should not
be afraid to use scientific notation here.

I'll leave it to the more erudite members of the list to pick the nits, but
these were some of the tall nails that I spotted sticking out from your
conceptual framework; hope I whacked 'em good for you. :-)

> <snip>
> It turns out that the magnitude of an electric force
> between two charges (q1 and q2) is proportional to
> the product q1*q2 and inversely proportional to the
> square of the distance (d^2) between their centers. This
> observation, made by Coulomb, is known of Coulomb's
> law. It can be written as:
>
>             F = k*q1*q2 / d^2
>
> where k is the proportionality constant. The value of
> that constant can be chosen arbitrarily in order to
> define a unit of electric charge. For the purpose of this
> introduction the unit of electric charge, one coulomb, C,
> we will defined by declaring that k=1,000,000,000. This
> is equivalent to saying that the electric charge is one
> coulomb if it attracts or repels an identical charge with
> a force of one billion newtons when the distance between
> the centers of two charges is one meter. One coulomb is
> a very large charge; charges produced on robbed rods
> and plates are usually expressed in microcoulombs or in
> nanocoulombs. Ignoring sign differences we can say
> that the charge of one electron and the charge of one
> proton are identical (1.6*10^-19 C).