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Homework Help: EM 2 parallel plate

  1. Sep 23, 2007 #1
    2 infinite parallel plates separated by a distance s are at the potentials zero and V0.
    find the surface charge densities on the plates.

    I got the answer that it first calculate E and then as

    1. x=o, Ex = σ / ε, then could know σ
    2. x=s, Ex = - σ / ε ---> here's what I don't know. why there's a minus sign here ?

    Thank you so much ~
  2. jcsd
  3. Sep 23, 2007 #2
    There's another question in EM.

    the original question is like below :

    consider a spherically symmetric charge distribution ρ = ρ(r). By dividing the charge distribution into spherical shells, find the potential and the eletrica field strength in terms of ρ(r)

    I got the answer that it first consider it as charge only on a shell with radius r', get E and V in terms of σ, then infintely many this kind of shells build a sphere with charge density ρ(r), and then it shows that :

    σ =[ 4π (r')^2 dr' ρ(r) ] / 4π (r')^2 = ρ(r') dr' = σ(r')

    I don't get this. how come would have this equation ?
    it seems σ(r') should be : total charge / surface area, and shouldn't total charge be
    (4/3) π (r')^3 ρ(r) ?

    Thank you.
    σ =
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