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Homework Help: Ap physics c

  1. May 14, 2014 #1
    1. The problem statement, all variables and given/known data


    #3 c ii)

    2. Relevant equations

    3. The attempt at a solution

    ##\oint E\cdot dA = \dfrac{q_{encl}}{\epsilon_0}##

    To find q_encl would this be correct;

    ## q_{encl} = \int \dfrac{-\beta}{r^2}\cdot e^{-r/\alpha}\cdot (4\pi r^2)dr##

    then integrating that, and substituting it into the expression for q_{encl}.

    ##E = \dfrac{q_{encl} }{\epsilon_0 \cdot 4\pi r^2}##
  2. jcsd
  3. May 14, 2014 #2

    Simon Bridge

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    The calculation do not appear to relate to E&M 3 c ii.
    You are given the charge density already. What is the relationship between that and the charge enclosed?
  4. May 14, 2014 #3
    Well the charge density is not the actual charge (q enclosed), so i integrated the expression of:
    density * surface area *dr to get a thin spherical shell of charge which accumulates. This would be q enclosed.

    Is this correct?
  5. May 14, 2014 #4

    Simon Bridge

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    The q-enclosed is the charge enclosed inside the gaussian surface.
    It sounds like you have the right approach to work it out.

    Note: your second line is the expression for q-enclosed.
    You then say "integrate that and substitute into the expression for q-enclosed" ... i.e. into itself.
    This is what threw me off.

    Presumably you mean - substitute into into another expression to find E.
  6. May 17, 2014 #5


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    Partially correct. Reread the very first sentence of the problem. Besides the equation for the electron charge density, what else does the model consist of?

    Yes, that is the idea.
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