# Homework Help: Approximation of linear quadrupole as a dipole in radiation zone

1. May 29, 2008

### dorist84

Hello,

This question is more conceptional - I think I can do the algebra (mostly approximations) in this problem ok.

1. The problem statement, all variables and given/known data

I am wondering why it is for a radiation zone, that a linear electric quadrupole can be approximated as a dipole. I am wondering if this is just a coincidence or if this is a specific case due to an intrinsic geometry or a characteristic about radiation zones that allows us to do this.

I doubt it is coincidental - if it were, I would guess that higher order dipoles (meaning bigger than quadrupoles) can be approximated as such then - that doesn't make sense to me.

There must be something "special" about radiation fields that allows us to say this: does this have to do with the energy density then? Maybe the magnitude of the Poynting vector? Perhaps the amplitude of the wave oscillation in the B-field in the radiation zone...(But what would dipoles/quadrupoles have to do with this then?)

Anything clarification would be greatly appreciated. Thanks so much! Take care.

--Doris

2. Relevant equations

3. The attempt at a solution