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Homework Help: A point charge Q is located on axis.Find flux

  1. Oct 15, 2013 #1
    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?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 15, 2013 #2


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    The electric flux through a surface is defined by
    [tex]\Phi_S=\int_S \mathrm{d}^2 \vec{F} \cdot \vec{E}.[/tex]
    Now given the fact that everywhere except at the charge's loacation you have
    [tex]\vec{\nabla} \cdot \vec{E}=0,[/tex]
    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!
  4. Oct 15, 2013 #3
    The right question i should ask is that how to calculate the flux when it makes theta at center?
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