Four charges are placed at four corners of a square as shown figure. The side of the square is a. Two charges are positive and two are negative, but their magnitudes are the same. Now, an external agent starts decreasing all the sides of the square slowly and at the
same rate. What happens to the electrical potential energy of the system and what will be the nature of work done by the agent?

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increases, positive

increases, negative

decreases, negative

decreases, positive

answer is C.

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Detailed Solution

Electrical potential energy of system

$U = - 2 [\frac{k q^{2}}{\sqrt{2} a}] = \frac{- \sqrt{2} k q^{2}}{a}$ (four pairs will cancel each other)

If a decreases, U also decreases and if U decreases, the agent will do negative work.

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