IDEALIZATIONS, was textbook touchstones

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

Doug Craigen wrote:

> > ...charges. They promised to address the issue "in the next edition".
> > That was five five years ago...
>
> Which makes ten years ago?  Or perhaps, you mean five years each ;->

The time span is actually longer. The "crusading" started at the AAPT
summer meeting in Maine. The article in TPT (A Myth About Capacitors
in Series, The Physics Teacher, 1988, 286) appeared after that meeting.
The active crusading ended with the thread (under the same title) on this
list, about five years ago.

I still consider the non-equality of charges to be different from common
idealizations, such as thin lenses, resistless leads, dependence of R on
temperature, etc. We can always choose the current which is sufficiently
small to make R (of a wire) nearly constant. The deltas and epsilons of
calculus are good analogies.

But Q1 and Q2 on two capacitors in series are never equal, unless
leakage resistances happen to coincide. Everybody knows this but
for some strange reason the authors of textbook keep repeating that
Q1=Q2. It is not a relation which becomes more and more correct
as a function of DOP or of some other factor.

I agree with Rick that idealizations are necessary. But in physics
un idealization is not what we wish to be true, it is something which
is approximately true under certain conditions. A valid idealization
can be refined. Doug is correct by writing:

> If I understand Ludwik's capacitor issue correctly this is not quite a
> correction so much as an important "real world" consideration.  Ideal
> capacitors such as those discussed in textbooks would have equal charges
> when connected in series.  However, such capacitors don't exist - and
> perhaps talking about them is a worse idealization than talking about
> "frictionless surfaces, perfectly elastic collisions" etc.