JEE Main 2013 — Electrostatics Question with Solution

Two small equal point charges of magnitude $q$ are suspended from a common point on the ceiling by insulating mass less strings of equal lengths. They come to equilibrium with each string making angle $\theta$ from the vertical. If the mass of each charge is $m$, then the electrostatic potential at the centre of line joining them will be $$\left(\frac{1}{4\pi {\in }_0}=k\right)\text{. }$$