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peter eldergill
2010-May-08, 10:26 PM
I understand that charged particles in motion create a magnetic field and if you put a beam of charged particles through a magnetic field you'll get deflection of the beam.

My question is why does there have to be motion? What is it about motion that would induce the field or vice versa?

Pete

loglo
2010-May-08, 11:52 PM
Very good question!

I have no answer but note that electric and magnetic fields can be related by the Lorentz transform from special relativity. :-

Lorentz transformations can also be used to prove that magnetic and electric fields are simply different aspects of the same force — the electromagnetic force. If we have one charge or a collection of charges which are all stationary with respect to each other, we can observe the system in a frame in which there is no motion of the charges. In this frame, there is only an "electric field". If we switch to a moving frame, the Lorentz transformation will predict that a "magnetic field" is present. This field was initially unified in Maxwell's concept of the "electromagnetic field".

Somehow, motion breaks the symmetry of electromagnetism.

Strange
2010-May-10, 12:35 PM
I'm not sure that "why" has an answer. Or at least, not in physics; metaphysics maybe.

I would add that it is symmetrical: a moving (or changing) magnetic field can generate an electric current.

Geo Kaplan
2010-May-18, 02:48 PM
I understand that charged particles in motion create a magnetic field and if you put a beam of charged particles through a magnetic field you'll get deflection of the beam.

Indeed -- and this is exactly how a CRT works. An electron beam is shot toward a phosphor-coated screen, and is magnetically deflected from left to right, and from top to bottom to scan the whole screen dozens of times a second.

My question is why does there have to be motion? What is it about motion that would induce the field or vice versa?

Pete

That's a really fine question. As it happens, these forces are frame-dependent. That is, depending on the relative motion of the entities involved, you may describe what happens in terms of electric, or magnetic, or a combination of the two forces. That's why motion matters. Ed Purcell showed (see, eg, his wonderful textbook on E&M, written as part of the old Berkeley Physics Series of books) that magnetism may be viewed in a relativistic context. In a current-carrying wire, oppositely-charged carriers move in opposite directions. In the frame of one of those carriers (let's call that the reference frame), the other is moving, and thus is Lorentz-contracted. The charge density therefore changes! So an observer in the reference frame will feel a force from this charge imbalance, but will interpret it as magnetism. It is a testament to the strength of electrostatic attraction/repulsion that a tiny charge density imbalance can produce sensible effects.

There's much more that could be said (and has been written), but perhaps this is enough to whet your appetite for now.

DrRocket
2010-May-18, 05:54 PM
I understand that charged particles in motion create a magnetic field and if you put a beam of charged particles through a magnetic field you'll get deflection of the beam.

My question is why does there have to be motion? What is it about motion that would induce the field or vice versa?

Pete

There are two answers to that question. The first is simply the relationships that come with Maxwell's equations.

The second is that the electric and magnetic fields are really part of a single thing, the electromagnetic field, and the resolution into electric and magnetic components is dependent on the reference frame. One thing that comes with special relativity is that what one perceives as the electric and magnetic fields depends on ones relative state of motion.

You might to want to take a lesson from your signature and go read "that book by that wheelchair guy." A good book on electrodynamics might also be appropriate. Jackson's Classical Electrodynamics is a standard graduate text, though Marion's Classical Electromagnetic Radiation is a bit easier.

peter eldergill
2010-May-18, 09:34 PM
You might to want to take a lesson from your signature and go read "that book by that wheelchair guy."

Heh..that's Homer Simpson for you. I've read his book but I don't recall anything about electromagnetism.

Problem is, I've never taken any undergrad courses in electromagnetism and the last course I took was in 1992 (it was relativity and mechanics for non-physics students)

and the resolution into electric and magnetic components...

I seem to recall that the electric field and magnetic fields are perpendicular to each other in an EM wave...is that true?

And thanks for the answers :)

Pete

DrRocket
2010-May-18, 10:30 PM
I seem to recall that the electric field and magnetic fields are perpendicular to each other in an EM wave...is that true?

And thanks for the answers :)

Pete

They are perpendicular in free space (vacuum). In other media the angle can change. In air they are essentially perpendicular.