23) and Eq (2 25)],
U
rq q
rq q
q q
r
=
+
+




41
0
1
2
12
1
3
13
2
3
23
πε
(2 26)
Again, because of the conservative nature of the
electrostatic force (or equivalently, the path
independence of work done), the final expression for
U, Eq