Consider the configuration of a system of four charges each of value $+q$ . The work done by external agent in changing the configuration of the system from figure $(1)$ to figure $(2)$ is
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$\mathrm{W}_{\mathrm{ext}}=\mathrm{U}_{\mathrm{f}}-\mathrm{U}_{\mathrm{i}}$
$\mathrm{U}_{\mathrm{i}}=\frac{4 \mathrm{kq}^{2}}{\mathrm{a}}+\frac{2 \mathrm{kq}^{2}}{\sqrt{2 \mathrm{a}}}$
$\mathrm{U}_{\mathrm{f}}=\frac{4 \mathrm{kq}^{2}}{\sqrt{2 \mathrm{a}}}+\frac{2 \mathrm{kq}^{2}}{2 \mathrm{a}}$
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