Re: Comments: txtbk misconceptions

["Donald E. Simanek" <dsimanek@eagle.lhup.edu>]

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

> Some of the later items strike me as being more of the nature of 
> disagreement concerning terminology and aren't misconceptions.
>
> item 36 is an example of this.
>
>  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.

I disagree on this one. The charged two-plate capacitor does indeed have
just the same total charge (zero) as an uncharged one. The Q in C = Q/V is
the magnitude of the charge on one plate, not the total charge. 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. 

A related misconception is the student notion that capacitance is the
amount of charge a device can hold, like the capacity of a bucket. (A
faulty analogy, here?) I've even heard teachers put it in that way, in
those words. 

And still another. Ask students this. Charge a capacitor with 1
microcoulomb to 2 volt. Calculate the capacitance. Discharge the
capacitor, so Q = 0. What is it's capacitance now? Many students say
"zero", (plugging Q=0 blindly into C = Q/V ?) when, of course, the
capacitance of an uncharged capacitor is the same as it is when charged. 

	-- Donald

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Dr. Donald E. Simanek                            Office: 717-893-2079
Professor of Physics                                FAX: 717-893-2048
Lock Haven University, Lock Haven, PA. 17745
dsimanek@eagle.lhup.edu                 http://www.lhup.edu/~dsimanek
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