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Homework Help: Charge density as a function of r

  1. Sep 21, 2010 #1
    1. The problem statement, all variables and given/known data

    Sphere of radius R, Er=ER(r4/R4)
    Find the volume charge density (ρr) as a function of r

    2. Relevant equations

    Gauss's law

    3. The attempt at a solution

    I get that dQ=4πr2ρrdr, but have no idea how to solve for ρ
  2. jcsd
  3. Sep 21, 2010 #2


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    Have you seen the following form of Gauss's Law?

    [tex]\nabla\cdot E = \rho/\epsilon_0[/tex]
  4. Sep 21, 2010 #3
    like kuruman said, its application of gauss's law

    Flux = [tex]\oint[/tex]E (dot) dA = q/e

    you found q to be 4πr2ρ, but p = Charge / Volume

    but 4πr^2 isnt the volume of a sphere
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