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Laplace equation in spherical coordinates

  1. Nov 8, 2015 #1
    1. The problem statement, all variables and given/known data

    Solve the Laplace equation inside a sphere, with the boundary condition:

    \begin{equation}
    u(3,\theta,\phi) = \sin(\theta) \cos(\theta)^2 \sin(\phi)
    \end{equation}
    2. Relevant equations
    \begin{equation}
    \sum^{\infty}_{l=0} \sum^{m}_{m=0} (A_lr^l + B_lr^{-l -1})P_l^m(\cos \theta) [S_m\sin(m\phi) + C_m\cos(m\phi)]
    \end{equation}
    3. The attempt at a solution
    I have derived the general solution for the Laplace equation in spherical problems and that went okej. The problem is when I try to match it with the boundary conditions. If I use the method of identification it is imminently obvious that:
    \begin{equation}
    C_m=0, m=1 \text{ and that for} \quad l\neq1 \rightarrow A=0
    \end{equation}
    , m has to equal one so the arguments of sin is correct and A=0 so the solution does not blow up, when r approaches 0. Which means I have two different solutions: one where l=1 and another solution for all the rest of l´s. When l equals one the Legendre polynomials dose not match the boundary conditions so that solution can be discarded.

    Which means that I have an equation that looks like:


    \begin{equation}
    \sum^{\infty}_{l=0} ( B_l3^{-l -1})P_l^1(\cos \theta) S_1\sin(\phi) = \sin(\theta) \cos(\theta)^2 \sin(\phi)
    \end{equation}


    When I look at table's for Legendre polynomials I do not find any that looks like it can solve my equation. I would really appreciate some guidance in the right direction.
     
  2. jcsd
  3. Nov 8, 2015 #2

    TSny

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    Homework Helper
    Gold Member

    Typo: what is the upper limit of the summation over ##m##?

    You are dealing with the interior of a sphere. So, you don't need to worry about ##r## going to infinity. However, what happens to ##r^{-l-1}## at the center of the sphere?
     
  4. Nov 9, 2015 #3
    The upper limit of the summation is suppose to be l.

    I have singularity there, so the constant B has to be zero. Thanks.
     
  5. Nov 10, 2015 #4
    No one that can help me?
     
  6. Nov 10, 2015 #5

    TSny

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    Homework Helper
    Gold Member

    Looks like now you just need to find the values of the ##A_l## coefficients by considering the boundary condition at r = 3. Look at a table of ##P_l^1(\cos \theta)## to see which ones can be used to obtain ##\sin \theta \cos^2 \theta##.
     
  7. Nov 10, 2015 #6
    That is what I have trying to do, quite unsuccessfully so far. But I know now at least that this is the way to do it, so thank you.
     
  8. Nov 10, 2015 #7

    TSny

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    Homework Helper
    Gold Member

    Hint: Look at your boundary condition at r = 3 and compare to a table of associated Legendre polynomials.
     

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    Last edited: Nov 10, 2015
  9. Nov 11, 2015 #8
    I think I solved it. Thanks for all the help!
     
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