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Name: Unknown
Status: N/A
Age: N/A
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Date: Around 1993

Why can a moving charge produce a magnetic field?

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 g-factor, 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!


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