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

Explanation

See the hint of example one of section solved exmples for miscellanesous examples. we have
`phi=((1-costheta)Q)/(2epsilon_0)`
But, `Q/epsilon_0=phi_("total")`
`:. phi=(phi_("total"))((1-costheta)/2)`…….i
`given `phi=1/4(phi_("total"))` a.
`Substituting in eqn i
We get`theta=60^@`
`R/b=tan60^@=sqrt3`
`:. R=sqrt3b`