Figure shows two + ve charges q1 and q2 separated by a distance r. The electrostatic interaction energy of this system can be expressed as work done in bringing charge q2 from infinity to the given separation from q1.
It can be calculated as
[ – ve sign shows that x is decreasing]
If the two charges are of opposite signs, then potential energy will be negative as
When more than two charged particles are there in a system, the interaction energy can be given as the sum of interaction energies of all the different possible pairs of particles. For example if a system of three particles having charges q1, q2 and q3 is given as shown in figure.
The total interaction energy of this system can be given as