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Re: point particles



A multipole expansion can be calculated for any charge
distribution, including a single point charge. The
multipole expansion is calculated relative to a particular
coordinate system. If a point charge is not at the origin,
the dipole moment, relative to the coordinate system, is
nonzero. See, for example, Eyges or Jackson.

Daniel Crowe
Oklahoma School of Science and Mathematics
Ardmore Regional Center
dcrowe@sotc.org

References:

Eyges, Leonard. The Classical Electromagnetic Field.
New York: Dover, 1972, pp. 20-23.

Jackson, John David. Classical Electrodynamics, 2nd ed.
New York: Wiley, 1975, pp. 136-141.


-----Original Message-----
From: Ludwik Kowalski [mailto:kowalskil@MAIL.MONTCLAIR.EDU]
Sent: Wednesday, January 21, 2004 3:03 PM
To: PHYS-L@lists.nau.edu
Subject: Re: point particles


On Wednesday, Jan 21, 2004, at 15:44 America/New_York, Richard A.
Lindgren wrote:

If the electron had a slight displacement of its center of
mass to its center of charge, it would have what is called
an electric dipole moment.

This is not clear to me. In my mind a dipole
consists of two equal and opposite point-like
charges. Please explain.