The electric potential energy of a system of fixed point charges is equal to the work that must be done
by an external agent to assemble the system, bringing each charge in from an infinite distance.
U is a scalar quantity.
Dimension of [U] = ML2T-2
Unit of [U] = joule
For two charges
U = {(kq1q2)/r}
U = q2V1
U = kq1q2 / r12)
For three charges
U = U12 + U23 + U13
= (kq1q2 / r12) + (kq2q3 / r23) + (kq1q3 / r13)
For n-charge
U = åall pairs (kqiqj /rij)
(number of pairs = nC2 )
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