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Re: point particles

Dear Ludwik,

No. Tt is a fundamental property. But I don't have time
to explain now. I sent a quantum mechanical scattering
resonse last night.

Here is a classical one. If the center of mass of the
object and the center of charge are not at the same place,
the object has a dipole moment. Suppose you have a disk
with some massless charge on its perimeter and you put
the disk in a uniform electric field so it is free to
rotate about the center of mass, then it would turn until
the charge lies along an axis through the center of the
disk aligning with one of the field lines. A torque is
created about the center of mass. I believe this would
behave like an electric dipole but I must admit I have
not thought much about the classical situation.

regards,
richard lindgren

On Thu, 22 Jan 2004 11:43:42 -0500
Ludwik Kowalski <kowalskil@MAIL.MONTCLAIR.EDU> wrote:
In other words, the dipole moment of an object
containing a single point charge is not an
intrinsic property of that object, (as it would be
the object contained two equal and opposite
charges separated by some distance). Right?
Ludwik Kowalski

On Thursday, January 22, 2004, at 09:46 AM, Dan Crowe
wrote:

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
=66rom 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-----
=46rom: 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 wrot=
e:

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