Question

A charge $+q$ is placed at the centre of the line joining of two exactly equal positive charges $Q$. Charge $q$ can move only in $x-x^{\prime }$ direction as shown in figure. Assume $+Q$ charges to be fixed.

In which of the above cases charge $q$ is in stable equilibrium ?

• A. (I) and (II)
• B. (II) and (III)
• C. (III) and (IV)
• D. (I) and (IV)

Answer 1

The correct option is B (II) and (III)

If a charge is displaced by a small distance $dx$ from mean position $({\overrightarrow{F}}_{net}=0)$ and it experiences a restoring force opposite to the direction of displacement. Then such equilibrium is called stable equilibrium.

Force experienced by charge $q$ after displacement is given in the following figure.


So, option $\left(b\right)$ is the correct answer.

$\begin{array}{c}\text{Why this question ?}\\ \text{Tip: Restoring force is responsible for the}\\ \text{condition of stable equilibrium.}\\ \end{array}$