Re: point particles

[John Denker <jsd@AV8N.COM>]

Quoting Bob Sciamanda <trebor@VELOCITY.NET>:

> J Denker's specification of the CM as the assumed origin for the
> "physics" dipole moment is news to me.  In my experience no mention of the
> CM occurs in any textbook definition of the physics electrical dipole
> moment.  John, would you give a reference for this distinction between a
> "mathematical" vs "physics" dipole moment?

It was news to me, too!  I know of no reference that openly
discusses the subject.  I made it up this morning, the
distinction and the terminology.  But I think it's right.
Ludwik kept asking the question, and kept getting conflicting
answers.  There was only one way I could see to resolve the
conflict.

If I had it to do over again, I might call one of them just
the "first moment" of the charge distribution, and call the
other one the "electro/inertial" dipole moment.

Bob then partially answered his own question:

> Look at:
> http://physicsweb.org/article/news/6/2/13
>
> These experiments are seeking to measure the dipole moment of the electron
> relative to its CM.

So at least *some* people are calling the electro/inertial
dipole moment "the" electric dipole moment.

> This is an instance where people in a specific specialty usurp established
> terminology and customize it as if it applies to only their specialized
> application.
> Well established textbook definitions should be respected by researchers.
> Let us not compound the students' sources of confusion.

I agree in principle ... but the electro/inertial version is
perhaps more widely established and more respectable than you
might suspect.  The fact that CODATA reports a numerical value
for "the" electric dipole moment of the proton lends a fair bit
of respectability;  see pages 951/952 of
  http://www.motionmountain.net/CC-PARTS.pdf

Also note that in the usual introductory cases (i.e. objects
with zero net charge) the fancy definition is equivalent to
the prosaic definition, so I see no urgent need to deprecate
the fancy definition.