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$

Question

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