# Homework Help: Spherical Multipoles.

1. Nov 17, 2009

### MathematicalPhysicist

1. The problem statement, all variables and given/known data
2 q charges are placed at (0,0,a) and (0,0,-a) and a third charge -2q is placed at the origin.
1. find the charge density distribution in spherical coordinates.
2. Find all the spherical multipoles of the above distribution.

2. Relevant equations
The spherical multiple formula:
$$q_{l,m}=\int r^2 r^l sin(\theta) dr d\theta d\phi Y*_{l,m}(\theta,\phi) \rho$$ where rho is the charge density, Y_l,m are the spherical harmonics.

3. The attempt at a solution
1. Here's what I got by dirac delta function:
$$\rho =-2q\delta (r)\delta (\theta) \delta (\phi)+ q\delta(r-a) \delta(\theta-\pi/2) delta(\phi) +q\delta (r-a) \delta(\theta +\pi/2) \delta (\phi)$$.

2. q_{0,0}=1/sqrt(4pi)[qa^2-qa^2]=0
q_{1,0}=sqrt(3/4pi)[0+0]=0
q_{1,+-1}=-+sqrt(3/8pi)[qa^3+qa^3]=-+2sqrt(3/8pi) qa^3
q_{2,0}=0
and the rest spherical multipoles are vanishing cause Y_{l,m} is a multiplication of P_l(cos(theta)) (Legendre polyonimal) and exp(m\phi) and cosine(+-pi/2)=0

So we are left with only one multiple here, am I right or wrong?

2. Nov 17, 2009

### gabbagabbahey

That doesn't look right to me....$\theta=\pm\pi/2$ corresponds to a point in the xy-plane, not on the z-axis....

3. Nov 17, 2009

### MathematicalPhysicist

So theta should be pi and -pi?

If it's pi instead of pi/2 then I get that all the multipoles are zero because sin(+-pi)=0, and this doesn't look right to me either.

4. Nov 17, 2009

### gabbagabbahey

Yes.

Slow down there....you still have another problem with your charge density....if you integrate each term over all space, you should get the charge of each corresponding point charge...do you?

5. Nov 17, 2009

### MathematicalPhysicist

Yes you are quite right I need to divide by $$4\pi a^2$$ which is the surface area of a sphere with radius a.

6. Nov 17, 2009

### MathematicalPhysicist

No, obviously this isn't right either, I guess I need to divide this by r^2 sin(theta), I mean each term in the density I need to divide by this factor.

7. Nov 17, 2009

### gabbagabbahey

Okay, that should work...now what do you get for the multipoles?