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Homework Help: Electric field between two large sheets

  1. Feb 5, 2012 #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 δ. You want to use these sheets to hold stationary in the region between them an oil droplet of mass 374 micrograms that carries an excess of five electrons. Assume that the drop is in vacuum.
    What should δ be?

    2. Relevant equations
    qE = F

    3. The attempt at a solution
    I've solved for the charge on the oil droplet to be q=8.01x10^-19 C and I know that δ=q(sheet)/A.
    Also, since the problem implies that gravity is involved, F=mg=qE

    I can't get an expression for E that includes δ. I have a feeling this involves an integral but I don't know how exactly to set it up. Can I get some advice?

    Last edited: Feb 5, 2012
  2. jcsd
  3. Feb 5, 2012 #2
    You're definitely doing well so far. For the E-field, try starting with Gauss's law and Gaussian surface on one of the plates. You can always use superposition to get the overall E-field.
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