Cause of Magnetism: An Overview

[chound]

What causes magnetism?

 

[krab]

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

 

[Gonzolo]

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

Which elementary particles and how?

 

[ZapperZ]

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

Which elementary particles and how?

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.

 

[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.

 

[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).

 

[ZapperZ]

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).

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.

 

[Gokul43201]

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

Yoohooo, Heisenberg was a Cond. Mat. physicist !

 

[Gonzolo]

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.

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.

 

[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?

 

[ZapperZ]

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?

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.

 

[Gokul43201]

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.

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.

 

[ZapperZ]

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.

It fails because for an s-orbital, there are no preferred 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.

 

[reena]

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.

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 don't have them now!

 

[ZapperZ]

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 don't have them now!

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.

 

[Gokul43201]

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" !

 

[CharlesP]

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

 

[Gonzolo]

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" !

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

 

[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).

 

[Theelectricchild]

But why is no one answering simply? : Current.

 

[ZapperZ]

But why is no one answering simply? : Current.

You seemed to have completely ignored the mentioning (several times) of the fact that there are no moving charges in the "spin" of an elementary particles. You should have made the connection that "no moving charges" means "no current". That's why no one answered it "simply" as current.

Zz.

 

[ZapperZ]

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).

The magnetic moment of a hydrogen atom is based on how it responds in a strong external magnetic field. You can put almost anything in a strong enough field, and it will produce a magnetic moment (we wish we can have an infinite mu metal). However, leave an H atom by itself, or with a conglomerate of others, and you do NOT get a spontaneous magnetism nor do you see an allignment of magnetic moment.

Paramagnetism, ferromagnetism, antiferromagnetism, etc., requires not only the presence of individual magnetic moment of each particle in the system, but also the "correct" Heisenberg coupling, "J" between each magnetic moment to allow for such long range order in the material. This is why I mentioned that this is a many-body problem. Again, if this is that simple, we wouldn't have books solely on the quantum magnetism phenomenon.

Zz.

 

[Creator]

... 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" !

He, he; I like that Gokul; the 'Moses' effect.

I remember seeing in a (very) old issue of Scientific American some very interesting pictures of water (due to its dipole nature) 'mounting up' beneath a strong electric field.

Creator

 

[Theelectricchild]

I need to understand more physics, I haven't even taken QM yet.

 

[chound]

I would appreciate if someone could make it all sense for me. I can't understand a bit what you are talking about. And in my 10th grade textbook there is no mention of what causes magnetism. So please explain detailedly

 

[Theelectricchild]

Theres no such thing as magnetic charges.

 

[Gonzolo]

I would appreciate if someone could make it all sense for me. I can't understand a bit what you are talking about. And in my 10th grade textbook there is no mention of what causes magnetism. So please explain detailedly

Until you start learning quantum mechanics, you can consider that the cause of magnetism is moving electric charges, which are basically the same as an electric current.

An electric charge (+ or -) causes an electric field, whether it is moving or not. But if it is moving, say, in the x direction, it causes an extra field, called a magnetic field. This field is circular, so that if the charge is moving in a wire, such that it's creating a current, the magnetic field is around the wire (the needle of a compass while near a wire will indicate the direction of the magnetic field). A wire shaped in a loop will produce a nearly straight magnetic field in the center of the loop.

In matter, such as a magnet, the classical view is that it is the electrons around each atom that create tiny currents producing the magnetic field of the magnet, (like the loop wire I mentioned). As Zapperz and Gokul have said though, this view has flaws and cannot explain many (if not most) phenomena related to magnetism, but its useful until about halfway of a physics degree. For a complete and detailed explanation, you need to understand the notions of "spin", atomic angular momentum, magnetic moment etc.

If you understood what I just said, you may be ready for an introduction to this theory, but if you learn only one thing, may it be that it is moving charges that cause magnetism, and that there something important in physics called "spin" .

 

[chound]

Thanks a lot

 

[geethdesai]

consider any object say 'xyz' it contains atoms, and atoms contain electron which revolv earound the nucleus in an atom.As we know that moving electron is similar to moving charge and therefore it produces an electric current on moving,on moving it works as a current carrying circular loop therefore it behaves as a magnet.A magnet can also be called as a magnetic dipole with different direction which eventually cancel out each other in case of a non-magnetic object but,in case of a magnet of a magnetic substance the direction are such that the resultant is not 0 that is they don't cancel each other