A charge $\mathit{q}\;$ is placed at the center of line joining two equal charges Q. The system of 3 charges will be in equilibrium if $\mathit{q}\;$ is equal to:

Question

\[(a)\;\frac{-Q}{2} \qquad (b)\;\frac{-Q}{4}\qquad (c)\;\frac{Q}{4}\qquad (d)\;\frac{Q}{2}\]