Two pith balls, each carrying charge $+q$ are hung from a hook by two strings. It is found that when each charge is tripled, angle between the strings double. What was the initial angle between the strings?

A negative charge is placed at the midpoint between two fixed equal positive charges, separated by a distance $2d$. If the negative charge is given a small displacement $x(x\ll d)$ perpendicular to the line joining the positive charges, how the force $\left(F\right)$ developed on it will approximately depend on $x$?

Eleven equal point charges, all of them having a charge $+Q,$ are placed at all the hour positions of a circular clock of radius $r$ except at the $10 \mathrm{h}$ position. What is the electric field strength at the centre of the clock?

The bob of a pendulum of mass $m$ suspended by an inextensible string of length $L$ as shown in the figure carries a small charge $q$. An infinite horizontal plane conductor with uniform surface charge density $\sigma$ is placed below it. What will be the time period of the pendulum for small amplitude oscillations?