- #1
Destroxia
- 204
- 7
Homework Statement
Consider a physical dipole consisting of charge +q and −q separated by a distance a.
• At first consider the origin of coordinates to be located in the middle of the dipole and a positive charge to be located at a distance y = a/2 and negative charge at position y = −a/2. Find the first two nontrivial terms to the potential of this system at large distances.
• Second, place the charge −q at y = 0 and then place the other charge at y = a. Again, find the first two nontrivial terms to the potential of this system at large distances.
• Discuss the difference. In both cases use spherical coordinates.
Homework Equations
## V(r) = \frac 1 {4\pi\epsilon_0} \sum_{N=0}^{\infty} \frac 1 {r^{(n+1)}} \int (r')^n P_n(cos(\theta))\rho(r')d\tau' ##
## V(r)_{mono} = \frac 1 {4\pi\epsilon_0} \frac Q r ##
## V(r)_{dip} = \frac 1 {4\pi\epsilon_0} \frac {p \cdot \hat{r}} {r^2} ##
## Q = total charge ##
## p_dip = qd ##
The Attempt at a Solution
This is just my attempt for the first bullet point:
The V(r)_{mono} is 0, because the total charge is ## Q = +q - q = 0 ##.
The ##p_dip = \frac {qa} 2 \hat y - \frac {qa} 2 (-\hat y) = qa \hat y ##
The ##V(r)_{dip} = \frac 1 {4\pi\epsilon_0} \frac {qasin(\theta)sin(\phi)} {r^2}## because ## \hat{y} \cdot \hat{r} = sin(\theta)sin(\phi) ## within sphereical coordinates.
So, this gives me the first non-trivial term. My question from this is, how do I get the term for the quadrupole potential, or even the quadrupole moment, for that matter? I've tried getting it using the multipole expansion formula I gave above, but it doesn't simplify down as nice, and I'm not sure what to do with some of the components.
## V(r) = \frac 1 {4\pi\epsilon_0} \sum_{N=0}^{\infty} \frac 1 {r^{(n+1)}} \int (r')^n P_n(cos(\theta))\rho(r')d\tau' ##
## V(r) = \frac 1 {4\pi\epsilon_0} \frac 1 {r^3} \int (r')^2(\frac {3cos^2(\theta)-1} {2}) \rho(r')d\tau' ##
I'm not at all sure what to do with this integral, or even how to do it, or it's bounds. I've found some other derivations online, but I don't know anything about tensors, or kronecker delta functions. Is there anyway to just get into a simple enough form I can deal with like ## V(r)_{dip} ## ?