Mathematical Methods of Physics/The multipole expansion
Tensors are useful in all physical situations that involve complicated dependence on directions. Here, we consider one such example, the multipole expansion of the potential of a charge distribution.
Consider an arbitrary charge distribution . We wish to find the electrostatic potential due to this charge distribution at a given point . We assume that this point is at a large distance from the charge distribution, that is if varies over the charge distribution, then
Now, the coulomb potential for a charge distribution is given by
Here, , where
Thus, using the fact that is much larger than , we can write , and using the binomial expansion,
(we neglect the third and higher order terms).
The multipole expansion
Thus, the potential can be written as
We write this as , where,
and so on.
Observe that is a scalar, (actually the total charge in the distribution) and is called the electric monopole
We can write
The vector is called the electric dipole. And its magnitude is called the dipole moment of the charge distribution.
Let and be expressed in Cartesian coordinates as and . Then,
We define a dyad to be the tensor given by
Define the Quadrupole tensor as
Then, we can write as the tensor contraction