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fluidistic
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Homework Statement
I'm stuck in evaluating an integral in a problem. The problem can be found in Jackson's book page 135 problem 3.1 in the third edition. As I'm not sure I didn't make a mistake either, I'm asking help here.
Two concentric spheres have radii a,b (b>a) and each is divided into two hemispheres by the same horizontal plane. The upper hemisphere of the inner sphere and the lower hemisphere of the outer sphere are maintened at potential V. The other hemispheres are at zero potential.
Determine the potential in the region [itex]a \leq r \leq b[/itex] as a series in Legendre polynomials. Include terms at least up to [itex]l=4[/itex]. Check your solution against known results in the limiting case [itex]b \to \infty[/itex] and [itex]a\to \infty[/itex].
Homework Equations
Below.
The Attempt at a Solution
I know I could solve this problem via the Green function for 2 spheres, I will try once I solve it via a less tricky way to see if I can get the same result.
Attempt: I realize it's an azimuthal symmetric problem. Therefore the solution is of the form [itex]\Phi (r, \theta )= \sum _{l=0}^{\infty } [U_l^+ r^l + U_l^- r ^{-(l+1)}] P_l (\cos \theta )[/itex]. My goal is to determine these 2 "U" coefficients. In order to do this, I check out what the boundary conditions have to tell me.
1)[itex]\Phi (a, \theta )= \sum _ {l=0}^{\infty } [U_l^+ a^l +U_l ^- a^{-(l+1)}]P_l (\cos \theta )[/itex]. I know that this expression can be worth either 0 or V depending upon the value of theta, I'll use this a step later. Now I use a trick, I multiply by [itex]P_{l'} (\cos \theta ' )[/itex] and I integrate as to use the orthogonality of the Legendre polynomials.
[tex]\int _{-1}^1 \Phi (a,\theta )P_{l'} (\cos \theta ') d \cos \theta =\left ( \frac{2}{2l+1} \right ) [U_l^+ a^l +U_l ^- a^{-(l+1)}] [/tex]. But from cos theta worth -1 to 0, the integral vanishes because Phi is 0 there. Hence I'm left with [itex]\int _{-1}^0 \Phi (a,\theta )P_{l'} (\cos \theta ') d \cos \theta =V \int _{-1}^0 P_{l'} (\cos \theta ' ) d \cos \theta =-V \int _{\pi} ^{\pi/2} P_{l'} (\cos \theta ' ) \sin \theta d\theta [/itex]. I must solve this integral. I'm not sure it is "right", I probably have to consider [itex]-V \int _{\pi} ^{\pi/2} P_{l} (\cos \theta ) \sin \theta d\theta [/itex].
I've been searching lots of relations/properties of the Legendre polynomials to see if one could help me. For example I can rewrite [itex]P_l (x)[/itex] as [itex]\frac{1}{(2l+1)} \left [ \frac{d}{dx} P_{l+1 }(x)-\frac{d}{dx} P_{l-1} (x) \right ][/itex]. But it doesn't seem to help me in any way.
I'm out of ideas. If I had this integral calculated, I'd have one expression of the [itex]U_l ^+[/itex] and [itex]U_l ^-[/itex] in terms of "a".
By looking at the other boundary conditions, 2)[itex]\Phi (b, \theta )[/itex], I guess I could do a similar job and find another expression of these U's in terms of b this time. This would give me 2 equations with 2 unknown, so I could solve it... and thus the problem.
Any help is greatly appreciated.