Magnetism

humbert

I've been studying physics for a while and have had magnetism described to me in a number of different ways(also I've missed quite a lot of physics and probably missed many descriptions). Anywho the most comforting description of it is special relativity applied to current flowing in a wire, it's tractable and suggests that it's derived from an electric field which is in turn a result of charge(which I really don't get, what is charge). what I'm asking is magnetism a fundamental physical concept or is it more a symptom of more fundamental concepts. Oh and I was reading a thread in philosophy which mentioned A and φ which I've encountered as vector potential and am curious as to how they fit in

Delphi91

Try here: http://en.wikipedia.org/wiki/Magnetism

Mike

Son Goku

humbert wrote:

Anywho the most comforting description of it is special relativity applied to current flowing in a wire, it's tractable and suggests that it's derived from an electric field which is in turn a result of charge(which I really don't get, what is charge).

Charge is basically the strength of matter's coupling to the electromagnetic field. How "attached" it is to electromagnetism. It has a very fundamental and weird explanation that goes deeper than this, which I'll leave aside for the moment.

humbert wrote:

what I'm asking is magnetism a fundamental physical concept or is it more a symptom of more fundamental concepts.

It is derived from more fundamental physical principles. For instance, you might know this but I'll say it anyway, in special relativity you can be stationary next to a wire and feel a magnetic field. However if you accelerate to match the speed of the electrons in the wire, the magnetic field disappears. This obviously suggests magnetism isn't fundamental because it disappear for observers moving at certain speeds. If at rest with respect to the charge generating the magnetic field you won't see the field.

If you want to think of it one way, the magnetic field is a relativistic correction to the electric field coming from time dilation and length contraction.

To see what I mean, think of a really heavy, very negatively charged particle at the origin, and you are at rest with respect to it. (Heavy because I don't want it to move) It has a coulomb potential which will attract nearby low mass weak positive charges.
If you stay at rest you'll see a positive charge particle fall towards the negative one.
If you accelerate to let's say a tenth the speed of light, to exaggerate the effects. Your clock would slow down with respect to the two charges and so you would see the particle fall faster. From your point of view there will be no explanation for this. What is the extra force causing the particle to move faster toward the charge? This extra force is magnetism.
So as I said, it basically compensates for relativistic effects.
(In fact even if you look at the units you'll see this. If you call the units of the electric field "electrics", the units of the magnetic field is "electrics per velocity".)

humbert wrote:

Oh and I was reading a thread in philosophy which mentioned A and φ which I've encountered as vector potential and am curious as to how they fit in

The magnetic field is basically the curl of A and the electric field is the gradient of φ, which I'd say you know. So basically the magnetic field and the electric field come from these two objects and so these are more fundamental.

The electric and magnetic fields have 3 components each, which are basically degrees of freedom, so combined they have 6 degrees of freedom.
You'd then think that electromagnetism has 6 degrees of freedom.

However A has 3 components and φ has only 1. Which makes 4 degrees of freedom. Since you can use A and φ to get the electric and magnetic fields, you can use 4 degrees of freedom to get 6 degrees of freedom. Obviously from this 2 degrees of freedom in the electric and magnetic fields are fake, so we've seen that electromagnetism has actually 4 degrees of freedom.

In special relativity you can combine A and φ into one four dimensional spacetime vector, which also called A. The old A is the spatial components of the new A and φ is the temporal component. This new object is basically the spacetime electromagnetic potential.
Since it has four components (3 spatial ones coming from A and 1 temporal one coming from φ) it has four degrees of freedom, just the same as before.
However when you do the maths it turns out another two of degrees of freedom are fake. So we're down from 4 to 2.

This came as a surprise to most physicists, because it meant that all of electromagnetism comes from only two fundamental properties. Later it was found that electromagnetism is "made up" of photons/Photons are the electromagnetic particle.
It then turned out that photons can only have spin 1 or spin -1. These were the two degrees of freedom hinted at by classical electromagnetism. So all of electromagnetism comes from photons spinning up or down.

humbert

Thanks, that explanation has clarified my understanding quite a lot. I've heard people talking about magnetic monopoles, not so much saying they exist but not ruling out the possibility either. I don't understand how they could possibly exist if magnetism is a relativistic phenomenon, though it may well simply be a lack of understanding on my part.

Son Goku

humbert wrote:

I've heard people talking about magnetic monopoles, not so much saying they exist but not ruling out the possibility either. I don't understand how they could possibly exist if magnetism is a relativistic phenomenon, though it may well simply be a lack of understanding on my part.

Magnetic Monopoles are exotic effects coming from grand unified theories. These are theories were the electromagentic, weak nuclear and strong nuclear forces are combined into one force, so the monopoles are a side-effect of new physical effects coming from the unification of the forces which causes magnetism to act differently.

However none of these theories have been verified and no magnetic monopole has ever been found.

cunnins4

Hi guys, i read here from time to time as i find it quite interesting. What about the magnetic field in an ordinary bar magnet? what causes that?

Son Goku

cunnins4 wrote:

Hi guys, i read here from time to time as i find it quite interesting. What about the magnetic field in an ordinary bar magnet? what causes that?

Without going really far into Quantum Mechanics*, the explanation comes from the fact that the electrons in a bar magnet are all spinning around. If anything that spins has an electric charge, the combination of the spin and the electric charge creates a magnetic field. And electrons have charge, so there's your magnetic field.

(The reason you don't see it in most materials (stones, people, e.t.c.) is because the electrons' magnetic fields don't line up with each other, meaning they cancel each other out and so you don't see any magnetism.)

*It would make the explanation far longer and unclear.

Professor_Fink

cunnins4 wrote:

Hi guys, i read here from time to time as i find it quite interesting. What about the magnetic field in an ordinary bar magnet? what causes that?

It's been explained previously in this thread.

Hope it's useful.

humbert

Son Goku wrote:

Magnetic Monopoles are exotic effects coming from grand unified theories. These are theories were the electromagentic, weak nuclear and strong nuclear forces are combined into one force, so the monopoles are a side-effect of new physical effects coming from the unification of the forces which causes magnetism to act differently.

However none of these theories have been verified and no magnetic monopole has ever been found.

Ah, that's well beyond my current level of understanding. I just wanted to make sure I wasn't misunderstanding something about the relativistic explanation. Thanks again.