Re: New capacitor problem

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

On Mon, 31 Mar 1997, brian whatcott wrote:

> At 10:12 AM 3/31/97, Prof. John P. Ertel (wizard) wrote:
> > This is a common problem when students forget the "true" meaning of the
> > elements of a formula.  The defining equation for the linear capacitor is
> > 	Q = CV
> > This can be read (and should be read) as the magnitude of the charge, Q,
> > (the EFFECT) that resides on either plate of a capacitive device is
> > directly proportional to the potential difference, V*, (the CAUSE) between
> > the plates and the proportionality CONSTANT is the capacitance, C, of the
> > device.
> > ...
> >                     / Prof. John P. Ertel \
>
> This seems a little eccentric to me.
> I prefer to say that charge may be stored in a capacitor.
> The charge carrier is the electron.
> The rate of charge  is the rate of electron flow in a conductor.
> This rate is given as (a large number of) electrons flowing per second.
> This unit is called the ampere.
> The integral of this charge rate is given as I.t
> It represents the accumulated charge.
> Hence Q = I.t

And this seems murky to me. You speak of [taking an] 'integral' yet you
don't use I.dt, and don't acknowledge that in charging a capacitor by
attaching it to a power supply of fixed output potential, the charge rate
isn't constant, but has an exponential dependence on time. Now if you get
yourself a constant-current-source (not at all out of the question these
days) your description would make some sense, but do your students know
about regulated current sources and regulated potential sources?

> An electric field may be used to deduce the amount of charge on a capacitor.
> This charge may be represented by a 'condensed'  or reduced voltage, if the
> air dielectric is replaced by a dielectric of higher permittivity.

Now we have a new element thrown into this--the dielectric. Is this the
explanation of the word 'condenser' which someone requested? If so, I'd
like to see some historical documentation that that is indeed the reason
for the term 'condenser'.

> This sort of description makes no special appeal to' true' meanings, 'first' 
> causes, and proportionality constants.

Here I heartily agree with the first part. No description or explanation
in a physics course should make special appeals to true meanings and first
causes. But I don't see why you toss in 'proportionality constants',
for I thought they were benign and useful, in fact I don't see how we can
do much in physics without them.

	-- Donald

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Dr. Donald E. Simanek                            Office: 717-893-2079
Prof. of Physics                    Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745          CIS: 73147,2166 
Home page: http://www.lhup.edu/~dsimanek            FAX: 717-893-2047
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