I have been teaching E&M with SI units for many years, and I don't
find the kind of pedagogical difficulties with Coulomb's law that you
claim. The unit of force (the Newton) already has been determined by the
fact that arbitrary units have been chosen for mass, length, and time (kg,
m, s) so that 1 N = 1 kg*m/s. When I get towards the end of the semester,
we talk about Newton's "Universal" Law of Gravitation, where F = GmM/r^2.
Here you need to introduce a constant G to get the strength of the force
right (because the force unit has been DEFINED from F = ma, not from
GmM/r^2). So when we get to Coulomb's Law where F = kqQ/r^2, students are
"primed" to expect that k may not be 1, that it may depend on how we
choose the charge unit. When I tell them that it is much more convenient
to define the charge unit from a magnetic experiment than from Coulomb's
law itself they don't have a great deal of difficulty with that. After
all, we didn't define the mass unit from F = GmM/r^2....we defined it
arbitrarily to be convenient. The unit of charge that we define - the
Coulomb turns out to be a big one so k is small....no big deal!
Dr. Mark H. Shapiro
Physics Department
California State University, Fullerton
P.O. Box 6866
Fullerton, California 92834-6866