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fluidistic
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Homework Statement
I'm given a charge density rho ([itex]\rho (r) = r^2 \sin ^2 \theta e^{-r}[/itex]) and I'm asked to find the multipole expansion of the potential as well as writing explicitely all the non vanishing terms.
Homework Equations
Not sure and this is my problem.
The Attempt at a Solution
I notice that phi doesn't appear in the expression for rho, so that there's an azimuthal symmetry and I might be lucky.
I do not know what formula to use for the multipole expansion. In wikipedia I see tons of formulae, in Jackson's book I see [itex]\Phi (\vec x ) = \frac{1}{\varepsilon _0 } \sum _{l,m } \frac{1}{2l+1} \left [ \int Y^*_{lm (\theta ', \varphi ' ) r'^{l} \rho (\vec x ) d^3 x' \right ] \frac{Y_{lm} (\theta, \varphi)}}{r^{l+1}}[/itex].
While in Griffith's book (he does not mention when his formula is valid as far as I know, to me it looks like it's valid only in the case of azimuthal symmetry but I may be wrong. Any comment on his formula is welcome) I see [itex]V(\vec r ) = \frac{1}{4\pi \varepsilon _0 } \sum _{n=0}^{\infty } \frac{1}{r^{n+1 }} \int (r')^n P_ n (\cos \theta ' ) \rho (\vec r' )d \tau '[/itex].
Anyway I've followed a bit Jackson's formula and I reached for the integral to be evaluated:
[itex]\int Y^*_{lm} (\theta ' , \varphi ' ) r'^l r'^{l+4}e^ \sin ^3 \theta ' e^ {-r'} dr' d \theta ' d \varphi '[/itex]. (Hmm I do not see any latex error but this text won't compile for some reason.)
I realize that I almost have an integral of [itex]Y^*_{lm}[/itex] and [itex]Y_{2,-2}[/itex] over a sphere of radius 1. Would the integral be worth [itex]\int r' ^6 dr[/itex]?
If so, then I don't really understand the mathematical justification. It's like saying that [itex]\int gfh dV =\int h dr[/itex] because [itex]\int fg d \Omega=1[/itex] where g,f and h are functions (h is function of the variable r). That's not something obvious to me. Is that true?