Re: is the magnetic field really moving?

On Fri, 24 Jul 2008, phil-news-nospam@xxxxxxxx wrote:

Is the magnetic field really moving if the magnets move but the field remains
the same?

Personally, I'd say "no" (and more below on this), but it doesn't matter one way or the other - the physical effects (as far as we know) only care if the field _changes_. If we just ask what effect will there be on charges (freely moving charged particles, charged particles in a conductor, whatever), "motion" of the field has no effect.

Beyond this, and I think that this is a strong argument for saying that fields don't move (even if they do change) is that the division of a magnetic field into "field of object A", "field of object B", "field of object C" is a mathematical convenience, at most. I would say that physically, we have one magnetic field (perhaps better to say EM field). If a source moves, the field might change, but how can that make it _move_?

For example, we are free to use the scalar and vector potential instead of E&H. Given an electrostatic potential, the potential physically tells us the potential energy of a test charge at any position. How can this "move", in any physically meaningful sense of "move"? Likewise, the E-field tells us the force that would act on a test charge. Ditto.

I like the analogy of the surface of a pond. Our field is the height of the surface. We can have waves in the pond, from many types of sources, but we only have one height of the surface at any time at any horizontal position. Is it meaningful to say that height(x,y) moves (horizontally)?

Now, electromagnetic field energy, momentum, and angular momentum can move, but these are transported by EM waves. Considering that EM waves can pass through the same point at the same time, travelling in different directions, in what direction would the field be "moving" at that point?

For even more fun, we can have an EM wave moving energy, momentum, and AM, with the magnitudes of E&H remaining constant at every point in space (a circularly polarised wave - the magnitudes remain the same, but the field vectors rotate).

--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html

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