8.
(a) We use Eq. 26-17:
C = 4πε
0
ab
b
− a
=
(40.0 mm)(38.0 mm)
8.99
× 10
9 N·m
2
C
2
(40.0 mm
− 38.0 mm)
= 84.5 pF .
(b) Let the area required be A. Then C = ε
0
A/(b
− a), or
A =
C(b
− a)
ε
0
=
(84.5 pF)(40.0 mm
− 38.0 mm)
8.85
× 10
−12 C
2
N
·m
2
= 191 cm
2
.