A point charge Q is located on the axis of a disc of radius R at a distance b from the plane of the disc (figure). Show that if one-fourth of the electric flux from the charge passes through the disc, then `R=sqrt3b`.

Answer

`a=R/sqrt(3)`

Explanation

Flux`=q/(4 in_(0))=q/in_(0)((1-cos theta))/2`
where `theta` is the semi-vertex angle
`rArr cos theta=1/2=a/sqrt(a^(2)+R^(2)), a=R/sqrt(3)`