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Re: B and electric charge



Thanks for good questions, Jack. I have no objection to what
you wrote (se the end of this message). But I would not say a
"standard charge"; any arbitrary charge, or a distribution of
charges, can be used to verify that two point-like charges are
equal. I will be away for a week. Perhaps the debate on the
sequence of electric concepts (in the first physics course) will
continue when the semester starts.

What I would like to discuss is the definition of B. Why do
we define it in terms of the EFFECT of the magnetic field
and not in terms of what CAUSES it? Instead of introducing
B via the Lorentz law (directly or indirectly) we can introduce
it via the Bio Savart law. Which way is pedagogically more
desirable and why? I have no opinion so far.

I also think that Chris Horton's idea of "circuits before
electrostatics" (see below) is worth debating. I think that a
minimum of electrostatics is necessary before circuits but
not everything we cover under this part of the course. In
other words, start with a minimum, introduce circuits and
return to both fields at the end.
Ludwik Kowalski

Chris Horton wrote:

IMHO the Coulomb is the most logical fundamental unit, once the technology
of counting electrons is fully developed. The fundamental conserved
quantity being measured is the charge on a certain number of electrons,
protons, etc. The use of current as the fundamental quantity is a
historical artifact.

Which one is introduced first is another matter. If we start with circuits,
the Ampere has to come first. If we start with electrostatics, the Coulomb
comes first. IMO starting with electrostatics and developing the concept of
current from there - perhaps following the path laid out by Chabay and
Sherwood (Sp?) - is conceptually sounder, but circuits do get the students'
attention and interest.

Student gains on some concept mastery instrument should of course be the
final arbiter. Has anyone tried to measure and compare the effectiveness of

Jack Uretsky wrote:

On Thu, 3 Jan 2002, Ludwik Kowalski wrote:

Jack asked (his full message is shown below again)

OK, so the force law gives the constant in terms of C. So I
need a measure of C that is independent of the force law.
Did I miss something?

You are assuming (as most textbook authors) that A is known
before C. In that case k is not used to define C, it must come
from a measurement (because all other units in the force law
already exist). In the sequence I plan to follow this spring the
Coulomb's law first appears as a proportionality equation.
How else can it be when Q has no units? The "fudge factor",
~9,000,000,000 will be then introduced to define coulomb.

I don't understand your answer. My question stands on its own,
and contains no such assumptions. You use the force law to define C.
Your construction must go something like this: I have a set of charges.
I pick a standard charge, which enables me to define equal charges (2
charges that have the same repulsive force from the standard, at the
same distance). Now I pick a "fudge factor", let's make it 25. Now
I have a definition of "Coulomb".
Let's go in baby steps, and this is step 1. Am I with you so
far? Importantly (using your words) the size of the fudge factor is
arbitrary.