Two equal negative charge –q are fixed at point (0, –a) and (0, a) on y-axis. A positive charge Q is released from rest at the point (2a, 0) on the x-axis. The charge Q will :

move to infinity

Due to electrostatic repulsion the charges will move as farthest as possible and the angle between the two strings will be 180° as shown in figure. Tension in each string will be equal to the electrostatic repulsion between the two charges. Thus,

$T =F_e=\frac{1}{4 \pi \in_0} \frac{Q \times Q}{(2 L)^2}$

$=\frac{Q^2}{16 \pi \in_0 L^2}$