Two charges, each equal to q, are kept at x=- a and x= a on the x -axis. A particle of mass m and charge q_0=q/2 is placed at the origin. If charge…

Two charges, each equal to q, are kept at $\displaystyle x=-\mathrm{a}$ and $\displaystyle x=\mathrm{a}$ on the $\displaystyle x$-axis. A particle of mass m and charge $\displaystyle q_0=\frac{q}{2}$ is placed at the origin. If charge $\displaystyle \mathbf{q}_{\mathbf{0}}$ is given a small displacement $\displaystyle (y \ll \mathrm{a})$ along the $\displaystyle y$ - axis, the net force acting on the particle is proportional to :