Potential energy stored in a system of charges is the work done (by an
external agency) in assembling the charges at their locations Potential
energy of two charges q1, q2 at r1, r2 is given by
1
2
0
12
1
4
q q
U
r
ε
=
π
where r12 is distance between q1 and q2 7 The potential energy of a charge q in an external potential V(r) is qV(r)