Electrical Potential Energy of a Configuration of Charges
Two Point Charges
Consider a system of two charges q1 and q2 separated by distance r1. This configuration is associated with a potential energy U1 (say). When the separation is increased to r2, potential energy becomes U2.
The work done by electrostatic force can be calculated as follows.
Let the separation at any instant is r. Keeping q1 fixed, q2 is displaced by dr
dWE = 
. 
dWE = Fdr cos 0 =
dr
By definition of potential energy,
Taking infinity as reference i.e., r1 =
and U1 = 0
Two Point Charges
Consider a system of two charges q1 and q2 separated by distance r1. This configuration is associated with a potential energy U1 (say). When the separation is increased to r2, potential energy becomes U2.
The work done by electrostatic force can be calculated as follows.
Let the separation at any instant is r. Keeping q1 fixed, q2 is displaced by dr
dWE = . dWE = Fdr cos 0 =
drBy definition of potential energy,
Taking infinity as reference i.e., r1 =
and U1 = 0
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