Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: How many volts ?



On Mon, 24 Mar 1997, LUDWIK KOWALSKI wrote:

Two identical metallic plates (A1=A2 = area = 1 meter^2) are far away
from each other. Supported by isolated handles they are charged. The
first plate receives Q1=-10nC while the second receives Q2=+100nC.
Then the plates are moved toward each other and a capacitor is created.
The distance between parallel plates is 0.01 meter. Thus we know that
C=0.885 nF (vacuum). Predict the difference of potential, dV.
......................................................................
If the absolute values of Q1 and Q2 were equal then we would have
dV=Q1/C. But charges are different in this problem. How to calculate dV?

Assuming that you can make the standard infinite sheets approximation,
there would be +45 nC on the outer surfaces of both sheets, +55 nC on the
inner surface of one and -55 nC on the inner surface of the other. The
field inside would not depend on the charges on the outer surfaces, so the
result is that of a capacitor charged with 55 nC and the potential
difference would be 62 V.

This is a result of applying Gauss' law and using the condition that E = 0
inside the plates. In essence you have a 110 nC *difference* which leads
to the "55 nC total charge" result and a 90 nC *excess* which must be
shared equally between the outer plates of both capacitors.

John
----------------------------------------------------------------
A. John Mallinckrodt email: mallinckrodt@csupomona.edu
Professor of Physics voice: 909-869-4054
Cal Poly Pomona fax: 909-869-5090
Pomona, CA 91768 office: Building 8, Room 223
web: http://www.sci.csupomona.edu/~mallinckrodt/