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Homework Help: Electric field inside a slab of with constant charge density.

  1. Feb 5, 2012 #1
    1. The problem statement, all variables and given/known data
    I have attached the question in a picture. I also attached a picture a drew as my attempt at understanding the question.

    2. Relevant equations
    flux = EA = Qenclo

    3. The attempt at a solution
    So I am not sure how to interpret this question. Is the charge distributed uniformly? If it is then it can be very difficult to solve as you can see in my diagram the electric field goes every where.

    Instead of a cylinder I was thinking of using a sphere as my surface... Does that even make sense?

    Here is an answer I found online:

    Do you guys agree? I notice the solution only used the top and bottom area of the gaussian surface...

    Attached Files:

  2. jcsd
  3. Feb 5, 2012 #2


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    Homework Helper

    usually, the slab is taken to be infinitely (or at least, very) wide in the x and y planes. So using symmetry, what do you think would be the electric field component in the direction of the cylinder's curved side?
  4. Feb 5, 2012 #3
    It seems like it should be zero, if it isn't it's extremely difficult to solve.... But I don't know why...?
  5. Feb 5, 2012 #4


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    Yeah, its zero. When I was doing undergraduate physics, the explanation they offered was that due to symmetry, it will be zero. If you think about the situation, it would be hard to imagine how the electric field in this direction would be non-zero, while still keeping the symmetry of the problem.

    I guess to do this problem properly, you could use rigorous mathematical arguments about symmetry groups. But that is stuff beyond most physics undergraduates, so an intuitive explanation is OK.
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