What causes magnetism?

1. Sep 20, 2004

chound

What causes magnetism? :surprised

2. Sep 20, 2004

krab

There are 2 sources: Moving electric fields, and elementary particles.

3. Sep 20, 2004

Gonzolo

Moving electric fields (charges) explains pratically all that we see in our daily lives.

Which elementary particles and how?

4. Sep 20, 2004

ZapperZ

Staff Emeritus
If I may hazzard a guess, Krab is pointing out to the spins and angular momentum of quantum particles, which is the origin of magnetism in matter. Spin,for example, isn't "moving charges", and yet, they exert a magnetic moment.

Zz.

5. Sep 20, 2004

krab

Right, Zz. Besides "electromagnets", there are materials that are intrinsically magnetic. These are quite common in our daily lives, and do not owe their magnetism to moving charges. In these, there are unpaired electrons so the magnetic moments from all electrons in the bulk material do not cancel. All permanent magnets are of this kind.

6. Sep 20, 2004

Gonzolo

I see. The explanation I was aware of for permanent magnets was that they were due to the electron revolution around nuclei. It's only as good as the Bohr model and perhaps not accurate numerically. At least it allows not having to introduce spin (which, I agree, is more accurate).

7. Sep 20, 2004

ZapperZ

Staff Emeritus
Well, if that is true, then EVERYTHING would be a permanent magnet, since every material has an "electron revolution around nuclei". But the fact that we don't, and only certain types are paramagnet, ferromagnet, etc., implies that it is a lot more complicated than that. Quantum magnetism is one of the most complex and complicated many-body problem.

Zz.

8. Sep 20, 2004

Gokul43201

Staff Emeritus
And few people know that this is one of Heisenberg's most important works.

Yoohooo, Heisenberg was a Cond. Mat. physicist !

9. Sep 20, 2004

Gonzolo

Well, it could be argued that in magnets, the normals to each orbit are aligned (same z), while in all other materials, they are not (random z). Again, I don't claim this to be a sufficient explanation to a scientist, but it is one I that have seen in a (perhaps an elementary) physics textbook. I am not sure where exactly the theory fails, although it surely does at some point.

10. Sep 20, 2004

reena

Magnetism of the brain and heart:

Both brain and heart exhibit magnetism as the magnetic field in both cases is huge. Can someone venture to guess the cause and effect of this magnetic field?

11. Sep 20, 2004

ZapperZ

Staff Emeritus
I'm sorry, but the magnetic field of the brain and heart are HUGE? Define "huge"!

If they are THAT huge, then it doesn't explain why detecting neuron signals from the brain require some of the most sensitive, superconducting curcuit we can build today.

Zz.

12. Sep 20, 2004

Gokul43201

Staff Emeritus
Ummm...that's nearly true, but not as much an explanation as it is hand waviness.

Clearly there's a lot more involved in understanding why certain materials have the spins (normals, in your textbook) aligned in the same direction, while others want the spins to line up in opposite directions, while a third class doesn't really care which way they line up. Your textbook has no explanation for what causes these differences, and surely is not expected to. It is sufficient for the high school student to understand that there can be interactions (or exchange mechanisms) between spins that make them want to line up some certain way or another.

13. Sep 20, 2004

ZapperZ

Staff Emeritus
It fails because for an s-orbital, there are no prefered direction of symmetry. So even if they are all "alligned", you still get no net magnetic moment due to all of those "revolution" around the nuclei.

Zz.

14. Sep 20, 2004

reena

I am really sorry that I am not able to find the article I read the other day which referred to the effect of microwaves on the brain. I would like to quote the numbers and I just dont have them now!

15. Sep 20, 2004

ZapperZ

Staff Emeritus
The effect of microwave on the brain say nothing about the magnetic field of the brain. Microwave has a HUGE effect on water. But does water have a "huge" magnetic field? Try passing water through a coil and see if it can induce any measureable current.

Zz.

16. Sep 20, 2004

Gokul43201

Staff Emeritus
This reminds me of a paper I read, in Nature (or Science, not sure which) some ten years ago. It was mostly about magnet design. A group in Tokyo (I think), built these giant conventional (not-superconducting, if I remember right) electromagnets fed by 4 storey tall capacitors, which make a huge transient current.

I don't remember the field strength of the magnet, but I remember a picture where a magnet held above the center of a long trough of water caused it (the water) to mound up below the magnet. Subsequently, a little copper sulphate was added to the water making it (blue, and more importantly,) diamagnetic. Now, bringing the magnet close made the liquid part under it. They called this the "Moses Effect" !!

17. Sep 20, 2004

CharlesP

Microwaves can cook your brain. Stay out of the oven. The brain magnetic field is very tiny.

18. Sep 21, 2004

Gonzolo

A group in Europe also used very powerful magnets to levitate a live frog.

19. Sep 21, 2004

Gonzolo

ZapperZ and Gokul43201,

I just checked again in the book in question (pre-calculus and pre-modern physics level). There are about 17 pages based on magnetism in matter based on the Bohr model of the atom. The most impressive part is that it predicts $$M = \frac{e}{2m}L$$ for the magnetic moment of a hydrogen atom, apparently the same value as a quantum theory predicts. Paramagnetism, diamagnetism and ferromagnetism are also "covered". I consider it to be an excellent introduction, as it gives very good insight into the matter (I like puns).

20. Sep 21, 2004

Theelectricchild

But why is no one answering simply? : Current.