- #1
JulienB
- 408
- 12
Homework Statement
Hi everybody! I'm very stuck trying to solve this problem, hopefully some of you can give me a clue about in which direction I should go:
Determine the multipole expansion in two dimensions of the potential of a localized charge distribution ##\lambda(\vec{x})## until the quadrupole term.
Hint: a point charge situated at the origin in 2D has the potential ##\phi(\vec{x}) = - k q \ln\vec{x}^2##.
Homework Equations
The multipole expansion formula I have (up to the quadrupole term): ##\phi(\vec{x}) = k \big(\frac{q}{|\vec{x}|} + \frac{\vec{x} \cdot \vec{p}}{|\vec{x}|^3} + \frac{1}{2} \sum Q_{ij} \frac{x_i x_j}{|\vec{x}|^5} \big) ##
The Attempt at a Solution
I am afraid I don't even know where to start. If I go cylindrical coordinates I can rewrite the given potential as ##\phi(\vec{r}) = -2kq \ln(r)##. But I don't really know how to link this equation with the multipole expansion formula I quoted above. I tried expanding ##\ln(r)## but I am not sure if that brings anything.
Any hint?
Thank you in advance for your suggestions.Julien.