From: "JACK L. URETSKY (C) 1996; HEP DIV., ARGONNE NATIONAL LAB, ARGONNE, IL 60439" <JLU@hep.anl.gov>
Date: Thu, 20 Feb 1997 20:17:06 -0600 (CST)
Hi Ludwik-
In response to:
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Yes, many students are trained to accept; they do not feel uncomfortable.
How do you deal with Ampere and 4*PI*eps in Socratian dialoques, Jack?
Ludwik Kowalski
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I follow the Mechanical Universe approach. Way back in
mechanics we learned the three (classical) forces of nature all
obeyed the same law, namely:
F = Gm1xm2/r^2.
This is a teaching of nature for which we do not yet have
an explanation. Here m1 and m2 are the "charges", r is the distance,
and G is a constant that makes the units come out right.
The issue now, is how do we measure the "charges"?
If the force is gravity, then the m's can be determined from
an inertia experiment (such as the frequency of an m dangling from
a standard spring). If the m's are magnetic or electric poles, then
we can define the value of G (K_m or K_e) and that then defines the
"unit of charge". If asked "why the particular SI value", the answer
can be historical reasons that can be learned as we progress in the
course.
The unit of current is obviously going to be one unit of
charge/s. The brilliance of the SI choices for K_m and K_e becomes
apparent when we learn about the "unification" of electric and
magnetic forces, namely electromagnetic induction. The induction
law takes its simplest form in SI units.
The identification of the gravitational charges with the
inertial masses is an arbitrary choice (rescaling G merely rescales
the definition of "gravitational mass"). The principle that makes
the choice useful is that the gravitational force is independent
of the composition of the mass (the Eotvos experiment), so all
bodies with the same inertial mass will give rise to the same
acceleration field.
Thanks (I guess) for rattling my chain.
Regards,
Jack