SI and electric charge

[Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU>]

More I think about SI more convinced I am that a change in the
way of presenting this system to students will be beneficial. The
old status quo tolerated four systems of units, practical and three
absolute (CGSE, CGSM and Gaussian). This was not good and
SI was an improvement. But there are many ways of introducing
SI units in teaching. Conceptually any electric unit can be the
first (in a sequence of definitions); starting with coulomb does
not change anything in SI, provided the number of electrons in
coulomb does not change. I am not trying to invent another
system of units. One system for all is highly desirable and it
should be used in all introductory physics courses.

SI resulted from merging the idea of rationalization (1/4*PI in
Coulomb's law), the idea of using four basic units instead
of three, and the idea of making electromagnetic units absolute
(rather than standard-based). Non of the above changes when
C is introduced before A, when epsilon_o and mu_o are
combined into one experimentally measurable (?) constant
called speed of light, or when 4*PI is dissolved in the
numerical definition of the first unit, such as coulomb.

So why should coulomb, and not ohm or farad, be the
first unit in a teaching sequence? Because the first electrical
concept of that sequence is electric charge. If another (?)
pedagogical sequence the first unit would change without
changing the SI. What is wrong with this? How many of
us think that defining C before A is harmless? How many
think it is worth promoting? No, I am not asking for a vote;
these questions are rhetorical. But please share your opinion.
Ludwik Kowalski

> Let me add that the main reason for making A fundamental,
> rather than C, was practical, not conceptual. Forces between
> macroscopic currents can be measured more accurately than
> forces between macroscopic charges. Those who promoted
> SI ignored pedagogical considerations. That is what I think.