# Why is electric dipole moment represented by “p”?

"I recently came across a text where the electric dipole moment was represented as "d" instead of the more commonly used "p". The reason behind using "d" here is probably that "dipole moment" begins with a "d". However, it also made me wonder why the most commonly used symbol of electric dipole moment is "p" and not "d". What is the reason behind using "p" to represent electric dipole moment?

Also, is there any historical reason for this?"

I originally asked the above question at the Physics SE (https://physics.stackexchange.com/q/458157). There, a user (@Avantgarde) suggested that I should ask this question here too.

Also, another user (@frapadingue) commented that this "p" is from "p" in polarization. This led me to wonder about the following questions:

Historically, what was studied first - electric polarization in materials or the concept of electric dipole moment? If the concept of electric dipole moment was studied first, then, was the representation of this vector quantity changed later when polarization was discovered?

• May be we should look for the origins of momentum operator p in QM. Perhaps one can then rationalize this choice. – M. Farooq Feb 1 at 22:38
• The "p" used in polarization has a very different meaning. In chemistry, the Greek letter mu is also used. – M. Farooq Feb 2 at 0:51
• Because in physics Momentum is usually named $\text p$. – Mauro ALLEGRANZA Feb 2 at 9:11
• @MauroALLEGRANZA Can you please explain how momentum is related to dipole moment? Dipole moment is 1st moment for two opposing charges but momentum itself is not a moment (see: en.wikipedia.org/wiki/Moment_(physics)). So, what is the analogy between them? – A B Feb 2 at 21:46
• Please check early quantum mechanics paper (mostly German) and see what the used for elektrischen Dipolment or elektrischen Dipolen vom Moment. One can see that the Greek letter "mu" was commonly used. link.springer.com/chapter/10.1007%2FBFb0111913 – M. Farooq Feb 5 at 0:58

First the dipole moment is defined in terms of a distance $$\textbf{d}$$, so that letter is "right out" .
Next, as the dipole moment in aggregation indicates the degree of polarization of a macro object, so using $$\textbf{p}$$ is not unreasonable.