74. We first need to find an expression for the energy stored in a cylinder of radius
R
and
length
L
, whose surface lies between the inner and outer cylinders of the capacitor (
a < R
< b
). The energy density at any point is given by
uE
=
1
2
0
2
ε
, where
E
is the magnitude of
the electric field at that point. If
q
is the charge on the surface of the inner cylinder, then
the magnitude of the electric field at a point a distance
r
from the cylinder axis is given
by
E
q
Lr
=
2
0
π
(see Eq. 2512), and the energy density at that point is given by
q
Lr
==
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 Spring '10
 Rebello,NobelS
 Energy, Fundamental physics concepts, Energy density, 0 l

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