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Archived Gauss Law and the wrong Gaussian surface.

  1. Oct 26, 2013 #1
    1. The problem statement, all variables and given/known data

    The problem was to calculate the electric field inside an infinite length cylinder (with radius R) with a non uniform charge density. The charge density depended on r. Its easy enough to solve using a gaussian cylinder with r less than R. But what if I wanted to complicate things and use a gaussian sphere inside the cylinder with r < R?

    2. Relevant equations

    ∫E[itex]\bullet[/itex]dA = q / ε° Gauss' Law

    ρ = ρ°(1 - r/R) This is charge density distribution

    q = ρ[itex]\bullet[/itex]dV

    V= 4/3 π r^3

    A= 4 π r^2

    3. The attempt at a solution

    Since the electric field is not the same everywhere, it can't be removed from the first integral. It is however constant over dθ when r and d[itex]\Phi[/itex] are held constant.
    q =∫ ρ*dV = ∫ ρ°(1- r/R) * A* dr
    q = ∫ρ°(1-r/R) * 4 π r^2 * dr
    q is easy enough to solve. So the problem lies in ∫E*dA
    I had a few ideas about this, First one: convert the problem into spherical coordinates and solve it that way. Second one: The electric field is a vector quantity, so could I say the total electric field at a point r is equal to the sum of the partial derivatives of the electric field at that point? In that case, I would need to find an expression for dE/d[itex]\Phi[/itex]. Any guidance on this would be appreciated. Thanks!
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Mar 10, 2016 #2
    As I understand r is the distance from axis, so you can not find the total enclosed charge inside a sphere by simply integrating ρ = ρ°(1 - r/R) you first need to convert r to spherical coordinates. After doing that you can try to solve equations for electric field but probably it will not be easy because taking a sphere as gaussian surface actually makes thing worse than finding electric field using Poisson's equation.
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