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Homework Help: Electric field problem - Plastic Slab

  1. Jan 20, 2005 #1
    Please consider this problem:
    A plastic slab of thickness t has a uniform free charge density, +rho, embedded inside, and also one surface has a surface charge of -sigma. Find the electric fields here and sketch as a function of distance from one surface. Also find the potential as a function of distance, sketch results. [Ignore issues of dielectric polarizability, the function of the plastic is simply to fix the charges in place]

    Here is what I did: I draw a Gaussian pillbox with area A. Only the top and bottom caps contribute where E and dA are parallel. The charge enclosed is: (rho)(A)(t) + (-sigma)(A) = A((rho)(t) - sigma)

    By Gauss's Law, we have: 2EA = [A((rho)(t) - sigma)] / epsilon, so therefore, E = ((rho)(t) - sigma) / 2epsilon

    I'm not sure what I did wrong here since the question asks to plot E as a function of r. In my case I would have a constant line. What did I do wrong? Thanks.
  2. jcsd
  3. Jan 20, 2005 #2
    Maybe I also have to find the E field within the slab... That answer would depend on r. Any thoughts?
  4. Jan 20, 2005 #3


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    Homework Helper

    Yes. You've found the field outside the slab. The field inside depends on distance from edge.
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