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Phys 3327 Fall 2010
HW 5
Due Wednesday 10/6/10 in lecture
Homework 5
Dynamic Electromagnetism
Ex. 5.1: Capacitor
Consider a parallelplate capacitor consisting of two large parallel perfectly conducting discs
a distance
d
apart and separated by a vacuum. The discs have radius
a
. At time
t
= 0, the
capacitor is connected to a battery, such that a constant current
i
starts to ±ow onto the
upper plate. (The same current ±ows out of the lower plate.) When answering the questions
below you may neglect edge eﬀects. You may give all your answers to leading order in the
small ratio
a/ct
.
a) What are the electric and magnetic ²elds in the space between the capacitor plates,
as a function of time? Be sure to specify magnitude and direction.
b) Calculate the total ²eld energy density for the space between the capacitor plates. Do
the electric and magnetic ²elds contribute equally to the energy density?
Hint: use the inequality
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This note was uploaded on 12/04/2010 for the course PHYS 3327 at Cornell University (Engineering School).
 '10
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 Magnetism, Work

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