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Re: [Phys-l] RC Discharge



In the ongoing discussion I, for one, was limited to the common case where
the plates of each capacitor are oppositely charged (the overall capacitor
is neutral).
Otherwise, a different q(t) needs to be defined for each plate, not just +/-
a common q(t).

For V=q/c, consider a capacitor made of two concentric spheres. Here the
inner sphere is V volts above the outer sphere V = Qi/4*PI*epsilon*a,
where Qi is the absolute charge only on the inner sphere and a is the radius
of the outer sphere. This V is unaffected by the charge on the outer
sphere.

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
http://www.winbeam.com/~trebor/
trebor@winbeam.com
----- Original Message -----
From: "Edmiston, Mike" <edmiston@bluffton.edu>
To: "Bob Sciamanda" <trebor@winbeam.com>; "Forum for Physics Educators"
<phys-l@carnot.physics.buffalo.edu>
Sent: Monday, February 20, 2006 9:27 AM
Subject: Re: [Phys-l] RC Discharge


|I am confused by phrases such as: the Q plate, where the Q actually
| lives, the plate whose absolute charge is represented by q, etc.
|
| Certainly we can imagine starting with a capacitor in a field-free
| region with shorted and grounded lead wires. This capacitor will not
| have a potential difference between its leads, and by convention we
| would say neither plate has any net charge.
|
| We can disconnect from ground, un-short the leads, move charge from one
| plate to the other, and end up with positive charge on one plate,
| negative charge of equal magnitude on the other plate, and a potential
| difference between the plates. The charge magnitude on each plate would
| be |q| = (C)*|(deltaV)|. I suspect this is what most students think of
| when thinking of a charged capacitor.
|
| But we can also remove, say, negative charge from each plate, such that
| both plates end up with positive net charge. If we remove different
| magnitudes of negative charge from the two plates, both plates have net
| positive charge, but there is a potential difference between the plates.
| One plate would be at higher potential than the other even though both
| are positive. I would designate the higher-potential plate with a plus
| and the lower-potential plate with a minus where these signs are used to
| indicate which plate is at higher/lower potential.
|
| Now... If deltaV = q/C, which plate has charge q? I would say neither.
| I would say q in the capacitor equation represents half the difference
| in the net charges on the plates. I would say deltaV = q/C with q = (Q+
| minus Q-)/2.
|
| Am I missing something here?
|
|
| Michael D. Edmiston, Ph.D.
| Professor of Chemistry and Physics
| Bluffton University
| Bluffton, OH 45817
| (419)-358-3270
| edmiston@bluffton.edu
| _______________________________________________
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|