# Magnetism. How?

1. Jul 9, 2007

### Joza

I have always wondered, what causes magnetism? I have only high school level physics at the moment, so I do not know the "whys".

WHY do two opposite magnetic poles attract, and likes repel?? And where does a magnetic get its energy to say, cling to a fridge?!!

Or is there something fundamental here, like asking why does the absence of light look black, and not green?

2. Jul 9, 2007

### CraigD

Magnatism is a fundamental force, that is, to say it just is.

CraigD, AMInstP
www.cymek.com

3. Jul 9, 2007

### belliott4488

That's an excellent question, Joza, and it's actually what motivated me to stick with my Physics education all the way through grad school. I always kept a magnet on my desk so that whenever I became discouraged and wondered why I was doing what I was doing, I would pick up the magnet and just remind myself that I simply wanted to understand as well as I could what was going on.

That said, CraigD is right - it's one of the "fundamental" forces, so in some sense it doesn't "come from" anything else more fundamental. On the other hand, the more precise statement is that it's the electromagnetic force that is fundamental -- so-called because electricity and magnetism were found to be different aspects of the same thing back in the 1800s. To understand magnetism, you must understand electricity, and vice versa.

In practical terms, however, you can explain how a given magnetic field comes about. One way that would be theoretically possible (according to Classical Physics), would be from a "magnetic monopole", which is a particle carrying a fundamental unit of magnetic charge, like a single North pole or a single South pole. As it happens, these have never been seen in nature. Instead we have particles that carry fundamental units of electrical charge, such as the electron.

You probably know that you can create a magnetic field by passing an electric current through a wire (i.e. an electromagnet). Well, even permanent magnets get their fields from the same source, namely the tiny electrical currents carried by the electrons in the substance. Some materials just happen to be such that the orientations of all the tiny fields generated this way all line up together, and you get a total field that you can detect.

I hope this was interesting to you.
-Bruce

4. Jul 9, 2007

### DeepGround

Photons are responsible for the magnetic field according to theoretical physics. Also Muons are responsible for the strong force that holds protons and neutrons together. Gravity well.. we don't know yet.

There is lots of information out there about how photons transmit the electromagnetic force.

I recommend reading "the elegant universe"

5. Jul 9, 2007

### Joza

Just as I thought, that it would be a fundamental force!

It does make you wonder sometimes doesn't it....WHY is that a fundamental force....maybe oneday, we will understand such fundamentals even deeper!

6. Jul 9, 2007

### rbj

sorry, but CraigD is not correct. the magnetic action is nothing other than the electostatic action (the fact that like-charged particles repel and unlike-charged particles attract) but with consequences of relativity taken into consideration.

there is a more rigorous treatment at this thread:

and in the Wikipedia article refered by it.

Last edited: Jul 9, 2007
7. Jul 9, 2007

### Brock

Has anyone ever tryed to combine astronomical physic's with quantum physic's, to figure out why the unbalanced electron in the magnets atoms causes N pole in one direction and S in the other? Also most of the magnetic force comes from the Electrons magnetic feild which is NOT in the same direction of the magnets feild direction, isn't that interesting, if it's true. Some of the magnetic force comes from the neucleous but not much.

8. Jul 10, 2007

### CraigD

Show me a monopole magnet and I will agree.

CraigD, AMInstP
www.cymek.com

9. Jul 10, 2007

### ranger

10. Jul 10, 2007

### rbj

the fact that the magnetic action is a consequence (as perceived by an observer) of the electrostatic action (with the effects of special relativity) has no dependence upon the existance of magnetic monopoles. in fact, i think that such indicates the utter lack of magnetic monopoles since it shows that what we perceive as magnetic forces only occur when there is electric charge moving relative to our frame of reference.

you don't need to agree (but you might want to get this in a physics class), but it's the other way around. the fact that

$$\nabla \cdot \mathbf{B} = 0$$
or
$$\oint_S \mathbf{B} \cdot \mathrm{d}\mathbf{A} = 0$$

is not only consistent with this relativistic fact, it is actually a consequence of it.

11. Jul 10, 2007

### PhillipKP

Damn hyperphysics rocks!

Yeah, in my mind I unify electromagnetism with the weak force too. Do you guys assume there is only three forces or do you still think of electromagnetism and weak nuclear as seperate forces?

12. Jul 10, 2007

### Ariste

I think (I may be wrong) that it's been shown that the electromagnetic, strong nuclear, and weak nuclear forces unify at extremely high temperatures. In other words, shortly after the Big Bang, the three forces were one.

13. Jul 11, 2007

### Neutralino

hyperphysics was really good, thanks for the link. :)
Why is it that magnetic monopoles do not exist?

14. Jul 11, 2007

### Neutralino

and i thought that gravity was a consequence of the curvature of spacetime? not because of gravitons as the hyperphysics states...

15. Jul 11, 2007

### cristo

Staff Emeritus
Classically, one of Maxwell's equations, namely $$\nabla\cdot\bold{B}=0$$ states that the magnetic flux through any closed surface is zero. Therefore there can be no magnetic "charges" in the same way that there exist electric charges.

General relativity states that gravity curves spacetime. The gravitons mentioned above are carriers of the gravitational force in quantum field theory. Since we have no (complete) theory of quantum gravity, both of these descriptions are acceptable.

16. Jul 11, 2007

### belliott4488

Well, that equation really states the observed state of affairs; it doesn't disallow monopoles, it simple says we don't have any. You could put a monopole charge density on the RHS of that equation and you'd have the analog of the equation for the static electric field. This equation asserts that the monopole charge density is zero, but it would be consistent with the theory for it to be non-zero.

To answer neutralino, there have been theories beyond the standard model that do incorporate monopoles, and there have been numerous experiments that attempted to find them. Unfortunately, even these theories predicted that they'd be so rare that you'd have to wait something like the age of the universe before one came wandering along. And then the chances of repeating the observation to confirm it were virtually zero!:grumpy:

I'm not sure what you mean by "acceptable". GR conflicts with the Standard Model, so a complete theory would have to modify one or both of those. But yes, any quantized form of the gravitational field includes spin-2 gravitons. It's the analog of the photon and the classical electromagnetic field, Mr. neutralino.

Last edited: Jul 11, 2007
17. Jul 11, 2007

### Neutralino

Oh i see... thanks. I hope there comes along a theory which will end the conflicts between GR and QFT.

18. Jul 11, 2007

### belliott4488

You and a whole lot of others, neutralino! That's the Holy Grail.

19. Jul 12, 2007

### rewebster

Yeah, me too, actually a 'matched' pair----a long time like the icon here: