# Significance of dipole moment

1. Jan 24, 2016

### kiwibird4

Hi, I was wondering why dipoles are singled out as a separate section in my physics textbook. For instance, after discussing electric fields the textbook specifies what a dipole is, dipole moment, and the electric field for a dipole. Finding the e-field for a dipole would be the same as finding an e field at a point due to two separate opposite charges though. Why is the concept of a dipole singled out? Is a dipole supposed to be treated as a singular system that has this separation of equal charge or can one think of is as two random charges which are separated that happen to be the same magnitude of charge.

Also (sorry for so much) what is the dipole moment telling us? I understand it is the charge and the length of the separation multiplied to give us this vector quantity, but why would that even be important or some helpful thing to calculate (other than a stepping stone for equations that use the moment like torque with E cross moment and potential energy )

2. Jan 25, 2016

### blue_leaf77

It's not that easy if the charge distribution is continuous.
Given an arbitrary charge distribution, the fields and potential associated to this charge can be expanded into multipole terms, with the dipole term in most cases being the second largest contribution after the monopole term. Therefore, truncating the expansion up to the dipole term will usually yield a good approximation to the actual fields and potential. Beside this, the field due to an oscillating dipole will serve as a benchmark of understanding of antenna radiation. The importance of dipole moment also crosses over the boundary between classical and quantum physics In particular, dipole moment of an atom or molecules determines the behavior of the atom/molecule when interacting with elecromagnetic field.

Last edited: Jan 25, 2016
3. Jan 25, 2016

### gleem

. It is used extensively in discussing the affects on the local electric field of a dielectric and in Maxwell's equations in substances.