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

• mentalguy
In summary, the conversation is discussing how to calculate the electric flux through a disk with a point charge located on its axis. The solution provided involves using the formula for flux through a segment and considering the flux through a part of a spherical shell that combines with the disk to form a closed surface. The key is to utilize the fact that the divergence of the electric field is zero everywhere except at the charge's location. This allows for an easy evaluation of the flux without needing to solve an integral.

#### 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 don't know the derivation of that formula. Can someone help me out?

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!

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