# A point charge Q is located on axis.Find flux

mentalguy
A point charge Q is located on the axis of a disk of radius R at a distance b from the plane of the disk Show that if one fourth of the electric flux from the charge passes through the disk, then R= √3 b

I have looked it through all angles possible for me but it seems to evade me. Got this solution here http://www.askiitians.com/forums/Electrostatics/13/7495/electrostatics.htm

3. Looking at the above solution...i looked at the figure and thought that it was a cone so electric flux through the disc is area of cone * electrif field. But still i get no answer.In the above page it says that " flux throug a segment " is known and the formula is given but i dont know the derivation of that formula. Can someone help me out?

## The Attempt at a Solution

Gold Member
2021 Award
The electric flux through a surface is defined by
$$\Phi_S=\int_S \mathrm{d}^2 \vec{F} \cdot \vec{E}.$$
Now given the fact that everywhere except at the charge's loacation you have
$$\vec{\nabla} \cdot \vec{E}=0,$$
you can argue (do it!) that you can evaluate the flux through a part of a spherical shell which combines with the disk to a closed surface. Then it's very easy to evaluate this flux.

Hint: You don't even need to solve an integral but only need the area of the spherical surface!

mentalguy
The right question i should ask is that how to calculate the flux when it makes theta at center?