- #1
Loonuh
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Homework Statement
Homework Equations
/The Attempt at a Solution
[/B]I am trying to solve problem 2-13 from my textbook "Principles of Electrodynamics" (see image below).
I believe that I should be solving the potential as
[tex]
\varphi(r,\theta) = \sum_{n=0}^\infty (A_n r^n + \frac{B_n}{r^{n+1}})P_n(\cos\theta),
[/tex]
where [tex]A_n[/tex] and [tex]B_n[/tex] are determined by the boundary condition of the conductor. For [tex]r < R[/tex] we should have that [tex]B_n = 0[/tex] to make sure the potential is finite and for [tex]r > R[/tex] we should have that [tex]A_n = 0[/tex] to ensure that the potential goes to 0 at infinity. That being said, at [tex]r=R[/tex], we can apply the boundary conditions such that we
[tex]A_n^{in} = \frac{2n+1}{2a^n}\int_{-1}^{1}\varphi(R,\theta)P_n(\cos\theta)d\cos\theta[/tex]
and
[tex]B_n^{out} = \frac{2n+1}{2}a^{n+1}\int_{-1}^{1}\varphi(R,\theta)P_n(\cos\theta)d\cos\theta[/tex].
Now, my problem comes when I want to solve these two integrals because it appears to me that if I apply the boundary condition, namely that
[tex]
\varphi = 0 \;\;\;\; \frac{3}{2} < \cos\theta < 1\\
\varphi = \varphi_0 \;\;\;\; \frac{-\sqrt{3}}{2} < \cos\theta < \frac{\sqrt{3}}{2}\\
\varphi = 0 \;\;\;\; -1 < \cos\theta < -\frac{3}{2}
[/tex]
then the above two integrals reduce to
[tex]A_n^{in} = \frac{2n+1}{2a^n}\int_{-\frac{\sqrt{3}}{2}}^{\frac{\sqrt{3}}{2}}\varphi_0P_n(\cos\theta)d\cos\theta[/tex]
and
[tex]B_n^{out} = \frac{2n+1}{2}a^{n+1}\int_{-\frac{\sqrt{3}}{2}}^{\frac{\sqrt{3}}{2}}\varphi_0P_n(\cos\theta)d\cos\theta[/tex]
respectively.
Now, since P_n are odd functions for odd n, [tex]A_n^{in}[/tex] and [tex]B_n^{out}[/tex] appear to be 0 for all of the odd values of n. However, the problem arises now when I am trying to evaluate these integrals in general for even values of n, in this case the results seem intractable because I am trying to integrate the Legendre Polynomials over a symmetric interval where the limits are not -1 and 1.
From my understanding, this problem should be as straightforward as applying the boundary conditions as I have done to solve for the potential, is there something that I am overlooking?