

Moving Charges and Magnetic Fields
Name: Unknown
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993
Question:
Why can a moving charge produce a magnetic field?
Replies:
This is a really great question! It is hard to answer. The
question is really what kind of field area is created in the space around a
charged particle. When a charged particle is not moving the electric field
lines emanate from the charge outward in all directions and until we move the
charge we might think this is the only kind of field that the charge can
create. When the charge moves, we find that there is the possibility for a
new kind of field that can have lines which do not start or end on charges,
but which can form closed loops. These are the magnetic field lines. As soon
as there is movement of the charges, there is time changes of the electric
fields, and that can produce magnetic fields. There is really no profound
answer other than the fact that the electromagnetic field has 6 different
degrees of freedom and that it requires the electric and magnetic field to
represent these. Also when objects are moving relative to each other, the two
fields can mix together. I do not think I have done a good job. Your
question is very profound. Maybe we do not yet know the answer.
Sam Bowen
Maybe it would help you to know that we humans actually know of
only two ways of creating a magnetic field. One is based on moving charges
around, and is how an electromagnet works, for example. The other uses the
fact that all elementary particles have a tiny magnetic "moment" (like a
little bar magnet) associated with a fundamental property called "spin" (again
making it sound like the motion of charges, although this time that does not
seem to be the explanation), and under certain circumstances you can get those
moments to all line up on their own  this would form a permanent magnet.
However, if you know anything about electrons in atoms, you will realize that
they whiz around the atomic nucleus, and since electrons are of course
charged, they must produce another magnetic field, giving them an "orbital"
moment, which adds in complicated ways to their "spin" moment. In real
permanent magnets, such as those based on iron, it is these combined moments
that all line up, and so even in most permanent magnets the magnetism comes at
least in part (and perhaps mostly) from the motion of charges  this time
electrons moving around the atomic nuclei. Actually, the "spin" property of
elementary particles seems to me even more mysterious than the factthat moving
charges create magnetic fields. Maybe some particle theorists out there can
explain it a little better than I can. No particle theories have risen to the
challenge? Well, I actually looked up "spin" in the Encyclopedia of physics,
and was reminded that it is not all that mysterious, or at least that, since
we know all the interactions between the electron (as a point particle) and
the electromagnetic field. In terms of the theory of quantum electrodynamics,
we can actually predict the ratio of the electron's true magnetic moment to
what would be expected for an electron with an angular momentum of 1/2 hbar in
a semiclassical treatment. This ratio, called the gfactor, turns out to be
pretty close to 2, and can be calculated to 11 digits which all agree with
experiment. This is one of the big achievements of QED, in fact.
Unfortunately, the electron is the only particle we know how to do this with
yet. The magnetic moments of the proton and neutron are really known only
from experiment, since the theory of the structure of the neutron and proton
(quantum chromo dynamics) is not yet (with current computers) able to
calculate it.
Arthur Smith
I just wanted to add that Dirac's famous equation, in which he
presented a "relativistic" equation of motion for a charged particle,
predicted that: (1) electron's had an intrinsic property called "spin," and
(2) that a particle existed with equal mass and spin but with opposite charge,
which was dubbed the "positron" when it was eventually observed
experimentally. So, "spin" falls out of a relativistic treatment of quantum
mechanics  or at least this is what they told me in school. I also would
like to hear from a particle theorist on this question!
Topper
Click here to return to the Physics Archives
 
Update: June 2012

