5
(c) Suppose that the same system of charges is now placed in an
external electric field E = A (1/r 2); A = 9 × 105 NC–1 m2 What would
the electrostatic energy of the configuration be Solution
(a)
12
9
1
2
0
1
7
( 2)
10
9
10
4
0 18
q q
U
r
ε
−
× −
×
=
=
×
×
π
= –0