First slide

Potential energy

Question

Three charges Q, (+q) and (+q) are placed at the vertices of an equilateral triangle of side I as shown in the figure. If the net electrostatic energy of the system is zero, then Q is equal to

Easy

A

$- \frac{q}{2}$
B

-q
C

+q
D

Zero

Solution

Potential energy of the system,
$U = k \frac{Qq}{l} + \frac{{kq}^{2}}{l} + \frac{kqQ}{l} = 0 \Rightarrow \frac{kq}{l} (Q + q + Q) = 0 \Rightarrow Q = - \frac{q}{2}$

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