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Homework Help: Electric Fields of horizontal sheets

  1. Feb 6, 2010 #1
    1. The problem statement, all variables and given/known data

    Two very large horizontal sheets are 4.25 cm apart and carry equal but opposite uniform surface charge densities of magnitude [tex]\sigma[/tex]. You want to use these sheets to hold stationary in the region between them an oil droplet of mass 364 [tex]\mu[/tex]g that carries an excess of five electrons. Assume that the drop is in vacuum.

    2. Relevant equations

    E = FE / q

    Fg = mg

    E = [tex]\sigma[/tex] / 2[tex]\epsilon[/tex]0

    [tex]\epsilon[/tex]0 = 8.854 * 10 -12 C2/N*m2

    3. The attempt at a solution

    I calculated the Fg because I know the FE must be equal and opposite of gravity in order to be suspended.

    I also calculated the net charge of the oil drop which was 5xe- (the charge of one electron) = 8.01 * 10-19 C.

    I divided the FE by the net charge of the drop to find the Electric Field in N/C. I then multiplied E (electric field) by 2[tex]\epsilon[/tex]0 to solve for [tex]\sigma[/tex] but this is not the correct answer apparently.

    Where have I went wrong?
  2. jcsd
  3. Feb 6, 2010 #2


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    The expression E = σ/(2ε0) is the electric field due to a single plate. What is the electric field in the region between the two plates?
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