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Homework Help: Surface charge density on conducting cone with point charge inside

  1. Feb 21, 2012 #1
    1. The problem statement, all variables and given/known data
    Calculate the surface charge density on a thin insulated and uncharged cone, which has a point charge inside of it on the cone axis. Furthermore, calculate the force between the point charge and the cone.

    2. Relevant equations
    The relevant equation is the Poisson equation
    [tex]\nabla^2 \phi = \delta(\mathbf{r}-\mathbf{r_0})[/tex]
    I'm not so sure about the appropriate boundary conditions although three things are certain:
    1. The potential on the cone's surface is uniform (since the surface is conducting):
    [tex]\phi|_{surface} = \phi_0[/tex]
    2. The tangential component of the electrical field on the surface is zero
    3. Since the cone was uncharged in the beginning, the total charge on the surface must remain zero:
    [tex]\oint_S \rho dS = 0[/tex]
    Since the Poisson equation is not separable in a way that the coordinate surfaces would coincide with the surfaces of the cone and since the method of images works only with plane or spherical surfaces, I presume this is a numerical problem.

    3. The attempt at a solution
    I see no other solution than numerical computing of the poisson equation via the finite element method.
    [tex]\Delta f(x,y) \approx \frac{f(x-h,y) + f(x+h,y) + f(x,y-h) + f(x,y+h) - 4f(x,y)}{h^2}[/tex]

    I can imagine computing the potential inside the cone this way but I have two problems:
    1. What are the correct boundary conditions for the potential?
    2. How should I obtain the surface charge distribution from the numerical data of the potential?

    Thank you
  2. jcsd
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