Re: Comments: txtbk misconceptions

[William Beaty <billb@eskimo.com>]

On Thu, 12 Feb 1998, Rauber, Joel              Phys wrote:

> I may be about to be argueing semantics, but here goes; of course that was 
> my point.

> > >  36) 2-plate capacitors are charged with energy, not with electrical
> > >      charge. A charged capacitor contains just as many + and - charged
> > >      particles as a discharged capacitor.

> > > I don't think it is a misconception to say 2-plate capacitors are 
> "charged
> > > with electrical charge", the books I read that say this or similar
> > > statements; provide an operational definition of what those words mean
> > which
> > > is similar to the second sentence above.  And hence doesn't represent a
> > > misconception.  At worse its a poor choice of words (not my opinion 
> though,
> > > I like the words just fine); and therefore not a proper item for this 
> list.

Joel,

If an author states that a capacitor is charged with electrical charge,
but then goes on to clarify the real situation, I would say that the
author first made an intentional misstatement, then retracted the mistake
later on.  Just because a retraction or clarification is always included,
does not forgive the initial misstatement.  After all, a naiive student
will probably believe that "capacitors store charge" is totally accurate,
and will imagine that when the author tried to clarify, he/she was simply
trying to say the same thing in a different way.  Never mind that
"capacitors store charge" directly contradicts the statement that
"positive charge is on one plate, negative is on the other."  Our student
will accept both, and, not noticing the contradiction, be somewhat
confused about capacitors forever after.

<snip>
> never said otherwise, let me rephrase, the books are generally careful to 
> specify that "charging up a capacitor to charge Q", is really putting equal 
> and opposite net charge on each of the two conductors.  This is exactly what 
> you say above.  Therefore I don't think those words are a misconception, 
> when they are operationally defined to mean the correct situation.
>
> > One way to
> > charge such a capacitor is to move charge of one sign from one plate to
> > the other, thereby changing the energy of that configuration of charge.
> > The usual textbook explanations generally don't get across to students
> > what's actually happening.
>
> The fact that it doesn't get across to the students doesn't mean that its a 
> misconception; it just means that it may not be a vary good way of getting 
> the conception across to the students.  That is a different issue.  (also, 
> might mean the students really don't know how to read, but that is also 
> another issue).

Why not call a spade a spade?  "Capacitors store charge" is an error.  I
think that it is also a common student misconception.  Evidence?  I myself
had it.  My "charged" misconception survived undergrad physics courses and
four years of electrical engineering school.  I didn't expunge it until a
decade later, when I ran full tilt into the issue while working at Museum
of Science in Boston.  Now when I hear educators saying "I've charged up
this capacitor... now I've discharged it," I feel a bit nauseated.  I
think "AARRRRG! That person is spreading that damned misconception to
others and doesn't even realize it."

Textbooks state that "capacitors store charge."  And instructors say "I've
charged this capacitor... now I've discharged it."  These sorts of phrases
are repeated over and over.  The instructor knows what he/she REALLY
means, but the students hear only the words, not the instructor's
thoughts.  I suspect that students internalize those words, and at the
same time students may not internalize the "clairifications."  I cannot
recall clearly, but I'm pretty sure this is how I learned the
misconception myself.  No, the clarifications did not cancel it out.  My
brain happily supported "capacitors store charge" in one pigeonhole, and
"capacitor plates carry opposite charges" in another.  I could work with
capacitors, but my gut-level "feel" for them was crippled.  I was vaguely
aware that their physics was contradictory. 

Regardless of their later clarifications, some books really do state that
"capacitors store charge."  Therefor I added the above entry to the
misconception list. ( I do vaguely recall encountering K-12 texts or
encyclopedias which state that "capacitors store charge", and then have no
clarifying statements whatsoever.) 

It has been my experience that when I explain this whole issue to
electronics people, a great light often dawns.  It's one of the few "aha!" 
experiences I can directly and intentionally communicate to another
person.  Removing the decades-old contradiction from the mind of another
is like taking the ancient thorn from the lion's paw.  I even have a
couple of thank-you letters somewhere. "Capacitors store charge" is no
small issue.  It's my opinion that no textbook or educator should EVER say
those words.  The terms "charged" and "discharged" should be banned from
our vocabulary.  They are just too too habitually misused.  Maybe say
that capacitors are "energized" or something?  Is "separated the charges
of" too unweildy a replacement for "charged?"

If I haven't yet been convincing, maybe numbers will force some clarity
onto the concepts. If we place +1uC onto one capacitor plate, and -1uC
onto the other capacitor plate, should we say that 2uC has been stored in
the capacitor?  Or should we say that 0uC has been stored in the
capacitor?  When speaking of "charging" and "discharging" capacitors, how
do we define the word "charge"?  If we say that the capacitor is "charged" 
with 0uC, what will students think?  Or, if we say that capacitors are
"charged" 2uC, isn't that a blatent violation of the law of conservation
of charge?  It may seem to be less of a violation if no numbers are
involved, but just because the problem is hidden does not mean it has gone
away. 

> A book propagated misconception, IMHO, is not a poor choice of words that 
> doesn't get the point across; rather it is the use of the wrong physical 
> principle to explain something.
>
> I'm trying to seperate poor pedagogy from outright wrong physics being 
> propagated.  Therefore, I still standby what I said about item 36.
>
> We may have to agree on what a "misconception" is, in order to generate this 
> list.
>
> Joel

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