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Homework Help: Electric Field/ infinite charged sheet

  1. Feb 11, 2012 #1
    1. The problem statement, all variables and given/known data
    There are three charged sheets with various charge densities. The following link shows a diagram, and solutions (not mine; these are some professor's solutions but I don't understand them yet):

    http://www.phy.syr.edu/courses/PHY212.08spring/HW/WHW-3.pdf [Broken]

    Draw vectors at each of the points A-D to show the direction and relative magnitude at the point due to a) sheet 1 b) sheet 2 c) sheet 3.

    2. Relevant equations

    E field due to infinite sheet = σ/2ε
    (where ε = permittivity constant and σ = charge density)

    Flux = ∫ EdA = E*A = |E||A|cos(θ)

    Gauss' Law: Flux = q(enclosed)/εo

    3. The attempt at a solution

    I know which direction the vectors go, but I don't know what their magnitudes are. Surfing the internet gave me this (scroll to last page):

    http://www.phy.syr.edu/courses/PHY212.08spring/HW/WHW-3.pdf [Broken]

    I know that the magnitudes of each vector of the electric field from each individual sheet from points A-D should be equal, because the electric field from a charged sheet doesn't depend on radius (for example, sheet 1's field could give vectors of magnitude |a| at every point).

    However, I don't understand why sheet 2 (σ = -1), according to the above solutions, has vectors of twice the magnitude of sheets 1 and 3 (σ= +1). What am I missing?
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Feb 12, 2012 #2


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    I agree with you.

    His solution is consistent with the center sheet having a surface charge density of -2σ0 .

    (I have taught this subject for more than 20 years.)
  4. Feb 12, 2012 #3
    Thanks SammyS-- I'm glad I wasn't misunderstanding something.
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