Re: point particles

[Dan Crowe <dcrowe@SOTC.ORG>]

The following analogy might help clarify the issue:

Torques are calculated relative to a coordinate system.
The magnitude of the torque due to a single force
depends on the lever arm (perpendicular distance
from the line of action of the force to the origin
of the coordinate system).  The magnitude of the torque
due to a force couple (two equal and opposite forces
with different lines of action) is independent of
the coordinate system.

Similarly, the magnitude of the electric dipole moment
of a point charge depends on it distance from the
origin, but the magnitude of the electric dipole
moment of two equal and opposite charges is
independent of the coordinate system.

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


-----Original Message-----
From: Ludwik Kowalski [mailto:kowalskil@MAIL.MONTCLAIR.EDU]
Sent: Wednesday, January 21, 2004 4:37 PM
To: PHYS-L@lists.nau.edu
Subject: Re: point particles


On Wednesday, Jan 21, 2004, at 17:00 America/New_York, Dan Crowe wrote:

> 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.

Are you saying that the magnitude of the dipole
moment of a "system" can have any value,
depending on where the origin of our coordinate
system is? I was under the impression that the
dipole moment is a system property, like its total
mass or charge. Is this wrong?
Ludwik Kowalski