Consider the configuration of a system of four charges each of value (+q). Find the work done by external agent in changing the configuration of the system from (Fig. 3.44 A) to (Fig. 3.4B).
a. ,

Explanation

`U_i` is the electrical potential energy in square arrangement. So
`U_i = (4 q^2)/(4 pi epsilon_0 a) + (2 q^2)/(4 pi epsilon_0 sqrt(2) a)`
`U_f` is the electric potential energy of circular arrangement of charge.
`U_f = (4 q^2)/(4pi epsilon_0 sqrt(2) a)+ (2q^2)/(4pi epsilon_0 (2 a))`
`w_("ext") = Delta U = U_f -U_i= - (q^2)/(4 pi epsilon_0 a) (3 - sqrt(2))`
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