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Homework Help: Induced Charge on a Conducting Disk

  1. Feb 10, 2014 #1
    1. The problem statement, all variables and given/known data

    A large, thin plastic disk with radius R = 1.1 meter carries a uniformly distributed charge of Q = -7e–5 C. A circular piece of aluminum foil is placed d = 3 mm from the disk, parallel to the disk. The foil has a radius of r = 6 cm and a thickness t = 1 millimeter.

    a. Find the net electric field at the center of the foil.
    b. Calculate the magnitude q of the charge on the left circular face of the foil.

    2. Relevant equations

    E = 1/(4*∏*ε)*q/r^2

    3. The attempt at a solution

    I know that the answer to part (a) is 0, because a conductor will not have an electric field inside of it.

    I also know that the charge on one side of the foil disk will be equal and opposite to the charge on the other side, since the disk has a neutral total charge. I'm just unsure of exactly where to go from there. Should I use force equations, since I know that the charges are at rest? Or is there something I know about the electric field that I can use?
  2. jcsd
  3. Feb 10, 2014 #2
    So you have a zero field inside the conductor, and non-zero right outside. Determine the field outside, remember the formula for induced surface charge density, then integrate over the surface...
  4. Feb 10, 2014 #3
    Thanks, Goddar. I guess my problem is calculating the electric field. Could you send me in the right direction?
  5. Feb 10, 2014 #4
    Method of images, since the disk is "large"...
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