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Homework Help: Quadrupole moment

  1. Apr 20, 2008 #1

    Looking for quadrupole moment of spherical shell with surface charge (sigma)=(sigma_o)*cos(theta) and (sigma_o) => constant
    Sphere is at the origin with radius=a.


    Well, I think I am using the right equation, by integration of the quadrapole moment taken from the quadrupole term, but I am questioning my [(r')^2] factor.


    I use vector product I get a zero. I guess a vector crossed with itself is always zero because of no span. I have already found the monopole and dipole (monopole =0), and I think I take the vector product of r', and not the dot product, because the dot product is a scalar... and I found by another theorem that whether it is shell or sphere, the quadrapole and higher terms are zero. So is this where the quadrapole zeroes out?? from the [(r')^2] factor?
  2. jcsd
  3. Apr 21, 2008 #2


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    Homework Helper

    i'm pretty sure only the dipole moment should be non-zero. you can write all the moments in terms of the charge density integrated against spherical harmonics. E.g., Y_{0,0} for monopole term, Y_{1,m} for dipole term, Y_{2,m} for quadrupole term, etc. But since the charge density itself is proportional to Y_{1,0} only the l=1 multipole moments should be non-zero due to orthogonality of spherical harmonics.
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