# Another spin question

1. Jan 24, 2005

### jet10

I have read somewhere that magnetism of material comes from a spinning charge. Protons and neutrons are supposed to have spins too. But neutrons are not charged and yet they produce a magnetic field. Why?

How do you count a spin of let's say an atom anyway? Is the total spin the sum of the spins of the neutrons, protons and electrons? Why is He-3 a fermion and He-4 a boson? Is it just because He-4 has an additional 1/2 spin from its additional proton?

2. Jan 24, 2005

### dextercioby

Spin is an entirely quantum concept.Magnetism,at classical level,is created by moving electric charges,it doesn't matter whether bosons or fermions.Read QM,SM and solid state physics to learn about the quantum explanation of magnetism.
Alpha particle is a boson and has one extra NEUTRON compared to He3.

Daniel.

3. Jan 24, 2005

### NateTG

In on a macroscopic scale, magnetism is associated with moving (not necessarily spinning) charges. There are, for example, magnetic fields associated with straight wires conducting electricity.

'Spin' as the property of an object corresponds to angular momentum, not magnetic moment, so it is possible for non-mangetic objects to have spin. As far as I am aware Neutrons do not interact with a magnetic field in any fashion. However, even in a classical setting, it's not difficult to imagine systems that have zero net angular momentum and charge, but non-zero magnetic moment such as a pair of concentric counterrotating charged rings.

4. Jan 24, 2005

### Staff: Mentor

Neutrons do have a magnetic dipole moment: -0.966 x 10^-26 joule/tesla (-1.91 nuclear magnetons). In fact, it's the same order of magnitude as for protons, which have a magnetic dipole moment of +1.41 x 10^-26 joule/tesla (+2.79 nuclear magnetons).

So they must interact with a magnetic field, about as strongly as protons do, as far as their spin is concerned.

5. Jan 24, 2005

Staff Emeritus
The magnetic moment of the neutron led to the conjecture that the neutron was not elementary but composite, long before the quark model came around. In modern terms, the neutron is composed of three quarks, and they are fermions, hence they have spin 1/2. They move around inside the neutron, and so they generate a magnetic moment.

6. Jan 25, 2005

### jet10

Thanks. So the quarks are responsible for the magnetic moment of the neutron.

An alpha particle is a boson, having one additional neutron. What happens when an atom is ionized? Let's say a fermion atom looses an electron or gains an electron. Does it make it a boson? As far as I know, bosons are force carrying particles e.g. photons or phonons and fermions are "matter" particles - correct me if I am wrong. But I would say that an ionized atom is still a "matter" particle.

7. Jan 25, 2005

### marlon

correct

yes, if an initial neutral atom loses and electron is becomes "charged" or more appropriately it becomes ionized. But it is still a matter-particle

regards
marlon

ps : not all bosons are force carries (eg : the Cooperpairs in superconductivity) but all force carriers are bosons

8. Jan 25, 2005

### dextercioby

YES.

You are wrong.Matter fields are either (pseudo)scalar fields or secind class constrained fields...

Obviously.

Daniel.

9. Jan 25, 2005

### jet10

thanks again.

what happens if I put particles together? e.g. if you put 2 fermion atoms together, does it make it a boson, because the total spin would be a natural number? Does it depend on the no. of particles in it, in order to tell if a solid-state body is a fermion or a boson?

10. Jan 25, 2005

### dextercioby

Let's leave the boson-fermionic character to the microscopic/quantum description.Okay?

Yes,for the first part of the question...

Daniel.

11. Jan 25, 2005

### marlon

This is very easy. You can take two fermions and make a boson if and only if the potential energy of the two fermions together is lower as the potential energy of each fermion apart. Given the fact that everything in nature wants to be in lowest possible potential energy-state, this means that the two fermions "prefer to sit together" and therefore form one composed particle that indeed is a boson. This is how two electrons make up a Cooperpair (this is a boson) in the case of superconductivity

marlon

12. Jan 25, 2005

### marlon

This is rubbish...i want to challenge you and ask the original poster if this "obvious" "explanation" brought any clarity...Besides dexter , do you even know what you are saying yourself ??? Matter-particles are indeed fermions...

this is basic QFT-stuff

marlon

13. Jan 25, 2005

### jet10

no, dexter, i don't understand what want to say. what do you mean by a second class constrained field? or how is the matter field connected with the scalar field?

14. Jan 25, 2005

### dextercioby

Matter field fermions??????????:surprised Think again,Marlon...

Daniel...

15. Jan 25, 2005

### dextercioby

Okay...Let's not go into technical details...I could speak an hor without pause about second class constrained fields.
Scalar field is a basic example of matter field.I is the simplest possible matter field,as it doesn't involve constraints..

Daniel.

16. Jan 25, 2005

### marlon

Again i restate my challenge. You see i was right: your first explanaition did not help and this won't do either. You don't know what you are talking about because you say nothing here, i am very sorry. Look at any QFT-book, you shall see that fermions are represented by the socalled matterfields. Forces are mediated by gauge-fields. Apparently you don't know this but a matter-particle is an excitation of the matter-field. That is the story, there is no point in trying to argue with this dexter. If you want proof just ask another advisor here or a mentor. Or (and this is my suggestion to you) go study QFT...

marlon

17. Jan 25, 2005

### marlon

not all matter-particles can be represented by scalar-fields. It really depends on what representation you chose. EG : the spinors

marlon

18. Jan 25, 2005

### dextercioby

Marlon,you're so wrong,that i cannot believe you find the strength inside u not to see it...Or u do and you won't accept it,which is even worse...Go study...

Are $\pi^{+}$ and $\pi^{-}$ particles electrically charged...??Are they described by matter fields...?????Have u heard of SED???

Daniel.

19. Jan 25, 2005

### marlon

this is irrelevant to this discussion. Point is that fermions are described by matterfields. And YES all matter-particles are described by matterfields.
Do you even know what a matter field is...

really, my friend, WHAT'S IN A NAME ??? :uhh:

marlon

besides, who the hell is talking about electrically charged particles here ???

Last edited: Jan 25, 2005
20. Jan 26, 2005

### jet10

my question is answered, marlon. I will probably deal with QFT later. Matter field and gauge field is something new fo me...