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Homework Help: Please help Electric Field Problem due tomorrow

  1. Oct 6, 2008 #1
    Please help!!! Electric Field Problem due tomorrow

    1. The problem statement, all variables and given/known data
    We want to analyze how a charged object picks up a neutral piece of metal. A small circular disk of foil is lying flat on a table. The foil has radius R and thickness t. A glass ball w/ positive charge Q is at height h above the foil.

    The ball's electric field polarizes the foil. The foil surfaces with charges +q and -q then act as the the plates of a parallel plate capacitor with separation t. But the foil is a conductor in electrostatic equilibrium so the electric field inside the foil must be zero. Write this condition as a mathematical statement and use it to find the charge on the upper surface of the foil.


    2. Relevant equations



    3. The attempt at a solution

    I know that the electric field of this sphere is Q/4[tex]\pi[/tex][tex]\epsilon[/tex]or^2

    Should the electric field from the ball equal the electric field from the top surface of the plate?

    Ecapacitor = [tex]\eta[/tex]/[tex]\epsilon[/tex]o

    in the case of the foil disk [tex]\eta[/tex]=q/4[tex]\pi[/tex]r2

    so the electric field is then q/4[tex]\pi[/tex]R2[tex]\epsilon[/tex]o

    If I set this equal to the electric field of the ball I get

    q/4[tex]\pi[/tex]R2[tex]\epsilon[/tex]o=Q/4[tex]\pi[/tex][tex]\epsilon[/tex]or^2

    q=QR2/h2

    The answer is actually q=QR2/4h2

    Could someone help me understand this better?
     
  2. jcsd
  3. Oct 6, 2008 #2
    Re: Please help!!! Electric Field Problem due tomorrow

    ok I found my mistake in the surface area of the foil... now for Q=50nC R= 1.0 mm and t=.01mm I need to find how close the ball must be to lift the foil
     
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