Obtain an expression for potential energy of a system of two charges in an external field. A system consisting of two charges 7μ C and –2 μC are placed at (-9 cm, 0, 0) and (9 cm,0, 0) respectively in an external electric field E = A (1/r2) where A = 9 × 105 C m2. Calculate the potential energy of this system.
Electric field due to charge q at distance r is given by 
… (i)
Now, the potential due to this charge is given by
… (ii)
Therefore, potential energy of a charge q’ in this potential is given by 
… (iii)
In the given problem, q = 7μC = 7 x 10-6 C
q’ = -2μC = -2 x 10-6 C
Therefore, potential energy of the system
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