# Question regarding how to interpret dipole moment for bound charges

• I
• Harikesh_33
In summary, a dipole moment is when two charges are separated by a small distance and are able to rotate under the influence of an Electric Field. It is also the second term of the Multipole expansion. This definition technically works for the dipole moment per unit volume, but it is unclear how it applies to bound charges. The positive and negative centers of an insulator shift under an E field, creating a dipole. It is uncertain how the rate at which it turns affects the volume density of bound charges. The concept of dipole moment is not well-defined, and it may be easier to discuss in terms of electric field strength rather than dipole moment vectors.

#### Harikesh_33

How do I interpret physically what dipole moment is ? The explanations that I received were "two charges seperated by a small distance " ,"it talks about ability of a dipole to rotate under the influence of an Electric Field " ,"Second term of the Multipole expansion" ,I get that these terms technically works for the definition of what a dipole moment is ,but I can't see how this definition works for bound charges .I mean P is the dipole moment per unit volume .I don't understand why should negative divergence of dipole moment per unit volume should give us the volume charge density .

What's stopping the dipole from attracting and collapsing in to each other .That begs the question ,do physical dipoles exist only in the case of insulators ?(In other cases (ie) in Multipole expansions we are just approximating the potential due to a charge distribution asthough a part of the potential is contributed by a dipole to make the problem simple) In the case of insulators the positive and the negative centres get shifted due to the E field ,and this creates a dipole ,now how does the rate at which it turns affect the volume density of bound charges(as the volume density is the negative divergence of dipole moment per unit volume ) ?

I am not sure if I will be addressing any of your concerns here but - as this is an unanswered thread - here is my ha-penny's worth!

"Dipole moment" seems to be a not-very-well defined concept. As I understand it, a dipole creates a far-field electric field (positive or negative) which then attracts the oppositely charged end of a separate molecule creating dipole-dipole forces.

Interpretation of 'bound' dipoles is difficult. For example the C-H bond is nominally polar (non-zero dipole moment). But in methane the 4 dipole moments cancel each other creating a non-polar molecule. Considering one of the 'polar' C-H bonds where electron charge density (of the shared pair) is presumed to be closer to Carbon on account of higher electronegativity, one would imagine the counter-active electron repulsion effect from the remaining 3 C-H bonds adjusts the average position of such charge density such that each C-H bond is effectively non polar in and of itself.

One study I looked at cited the C-H bond in methane as being polar with Carbon as the negative end and another with H as the negative end ?? So what gives actually ?

I understand the "physics" of what's happening better if we discuss in terms of electric field strength rather than the somewhat nebulous concept of dipole moment vectors cancelling each other. In particular the electric field strength along each bond line axis - and the extent to which it is affected by other bonds in the same molecule.