1.

Two point charges q_(1), and q_(2) are kept at a distance of r_(12) in air. Deduce the expression for the electrostatic potential energy of this system.

Answer»

Solution :Let a point charge 4, be situated at a given point A. Electric potential due to it at another point B, situated at a distance `r_(12)` will be
`V= (1)/(4pi in_(0)).(q_(1))/(r_(12))`
If a point charge `q_(2)` is brought from infinity to point B, then work done to bring this charge against the field of charge `q_(1)` will be
`W= q_(2)V= (1)/(4pi in_(0)). (q_(1)q_(2))/(r_(12))`
This work done is stored in the SYSTEM of TWO point charges as their electrostatic potential energy U. Thus
`U= (1)/(4piin_(0)).(q_(1)q_(2))/(r_(12))`


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