Consider the charges q

_{1}and q_{2}initially at infinity and determine the work done by an external agency to bring the charges to the given locations.
Suppose, charge q

_{1}is brought from infinity to the point r_{1}. There is no external field against which work needs to be done, so work done in bringing q_{1}from infinity to r_{1}is zero. This charge produces a potential in space given by
where r

_{1P}is the distance of a point P in space from the location of q_{1}.
From the definition of potential, work done in
bringing charge q

_{2}from infinity to the point r_{2 }is q times the potential at r_{2}due to q_{1}:
where r

_{12}is the distance between points 1 and 2.
If q

_{1}q_{2}> 0, Potential energy is positive. For unlike charges (q_{1}q_{2}< 0), the electrostatic force is attractive.
Potential energy of a system of three charges q

_{1}, qand q located at r_{1}, r_{2}, r, respectively. To bring q first from infinity to r_{1}, no work is required. Next bring q_{2}from infinity to r_{2}. As before, work done in this step is
The total work done in assembling the charges at
the given locations is obtained by adding the work done in different steps,

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