Re: The conceptual change process

[David Bowman <dbowman@tiger.gtc.georgetown.ky.us>]

I would like to thank Ludwik for collecting his list of textbook
misconceptions--especially because it motivated Bill Beaty to expound on
the "capacitors store charge" misconception.  I also thank Bill for making
his point so forcefully--including his personal "confession".  It had never
occurred to me over the too-many years that I had taught physics that this
could be a problem.  I am dismayed that the misconception to which Bill is
referring seems (to him at least) to be especially prevalent among those
who should, of all people, be taught to know better, i.e. the electrical
engineering and electronics technician communities.  Although I'm not sure
that banning the use of the phrases 'charge a capacitor' and 'discharge a
capacitor' are neccessarily the solution to the problem, I am grateful to
have been made aware of the seriousness and prevalence of the problem.
This is one area where I, for one, will be extra careful and vigilant when
discussing capacitors in the future.

Regarding some of Bill's discussions about the spring and rubber band
analogies to a capacitor:
>  ...               .  Insisting that capacitors store charge can confound
> the roles of energy and charge in the minds of students.  (This certainly
> is what happened to me as a student.)  It's analogous to stating that
> springs store steel.  Very confusing.  Springs store energy in a
> particular *configuration* of steel, but a configuration of steel is not
> steel.  When a spring is "charged" or compressed, no steel is injected
> into it, but energy is.  When a capacitor is "charged", no charge is
> injected into it, but energy is.  At least springs are visible, so some
> sorts of misconception are impossible.  Not so with invisible charge-flow
> and invisible EM energy.

I think Bill's point is taken (at least by me) here.  But I want to point
out (even though I'm sure Bill is well aware of this) that in the usual
analogy between a capacitor and a spring the spring's steel is *not* the
functional analog of the capacitor's "charge".  Rather, it is the relative
*displacement* of the spring's ends from their equilibrium position that is
the analog of the of the charge imbalance on each capacitor plate, just as
the spring's compliance is the analog of the capacitor's capacitance, the
relative velocity of the spring's ends is the analog of the current
through the capacitor, the compressive/tensile force acting across the
spring's ends is the analog of the potential difference across the
capacitor's plates, and the elastic energy stored in the spring is the
analog of the electrostatic energy stored in the capacitor.

David Bowman
dbowman@gtc.georgetown.ky.us