A non-conducting disc of radius a and uniform +ve surface charge density $\; \sigma\;$ is placed on the ground with it 's axis vertical . A particle of mass m and +ve charge q is dropped , along the axis of the disc from a height H with zero initial velocity . Find the value of H if the particle just reaches the disk .$\;\large\frac{q}{m}=4 \in_{0} g/\sigma$ . Find the position of particle at which the particle is in equilibrium

Question

$(a)\;\large\frac{2a}{\sqrt{3}}\qquad(b)\;\large\frac{a}{\sqrt{3}}\qquad(c)\;\large\frac{4a}{\sqrt{3}}\qquad(d)\;\large\frac{a}{3}$