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Help with Electric Field on a Square Insulating Sheet?

  1. Sep 18, 2011 #1
    1. The problem statement, all variables and given/known data

    A square insulating sheet whose sides have length L is held horizontally. The sheet has a charge of Q spread uniformly over its area.

    a) Estimate the electric field at a point located a distance r_2 above the center of the sheet. Use the appropriate approximations based on the fact that r_2 >> L.

    2. Relevant equations

    E * dA = Q_encl/epsilon_0

    3. The attempt at a solution

    The area of the square sheet is L^2, which can go into the equation above. From that, I know that the electric field for r_2 << L is E = Q /(2*epsilon_0*L^2), but I can't seem to figure out the field for r_2 >> L.

    Any suggestions?
  2. jcsd
  3. Sep 18, 2011 #2


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    From far away a sheet of paper looks like a spot or a point. The approximation I think they want is to treat the sheet of charge as a point charge? You got it from there?
  4. Sep 18, 2011 #3
    Not quite. Do you mind explaining a little further?
  5. Sep 18, 2011 #4


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    If you are far from a sheet of charge it does not matter what the orientation of the sheet of charge is the electric field still points towards you. Far from a distribution of charge the charge distribution can be approximated as a point charge.
  6. Sep 18, 2011 #5
    If you have any object that has a uniform charge density; as your sheet does, your object can be treated as a point particle if the radius is MUCH MUCH greater than the overall size of the initial object. That's what the r_2>>>L is telling you. You should know how to calculate the E-field for a point particle.
  7. Sep 18, 2011 #6
    Oh okay, now I understand. Thank you for the help!
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