What IS magnetism?

Dg101

Same vein as the what is fire thread. I understand the effects of magnetism, and I understand how to magnetise something, but what exactly is causing the magnetic effect? In other words, how is the force along the field lines being generated? If someone could explain what exactly is happening I would be much obliged.

Hellm0

http://en.wikipedia.org/wiki/Magnetism#Physics_of_magnetism

Dg101

Thanks for the reply.

So, if as the wikipedia article suggests, the magnetic forces arise from the movement of electrical charge, how is it that they differ from regular electrical forces? And, does that suggest that there is always a flow of electrons in, for example, a piece of magnetised iron?

Son Goku

Dg101 wrote:

Thanks for the reply.

So, if as the wikipedia article suggests, the magnetic forces arise from the movement of electrical charge, how is it that they differ from regular electrical forces? And, does that suggest that there is always a flow of electrons in, for example, a piece of magnetised iron?

For one, this will actually be easier if I use special relativity.
Basically there is only Electromagnetism, the basic force.

To observers this split itself up into two different subforces (for reasons I can explain if you want) called, obviously, Electricity and Magnetism.

Depending on how you're moving relative to the source of the electromagnetic field more of the electromagnetic field will look more/less like magnetism and more/less will look like electricity.

Imagine a huge, very long wire with trillions of amps of electricity passing through it at 100 km/hr.
If you're moving at the same speed as the source, i.e. 100km/hr, then the whole electromagnetic force will look like it is just an electric field.

However if you slow down or speed up you will see a new force appear to pull on some of your equipment made of iron, this is magnetism.

So basically magnetism is a "fake" force that only appears because of a relativistic disagreement between you and the source.

This probably isn't detailed enough or too vague, so just ask away if there is anything unclear.

Professor_Fink

Dg101 wrote:

Thanks for the reply.

So, if as the wikipedia article suggests, the magnetic forces arise from the movement of electrical charge, how is it that they differ from regular electrical forces? And, does that suggest that there is always a flow of electrons in, for example, a piece of magnetised iron?

Hi,

Magnetism comes from the movement of electric charge, as you say, and as described by Son Goku. However magnetic materials generally don't have electric currents in them (well, obviously electro-magnets do). So what is going on?

Well the electrons have a property called spin. On a classical level, you can think of this like angular momentum (imagine a ball spining). So the electrons are 'spining' even when they're not going anywhere. This generates a magnetic field which depends on the direction of spin. This happens in all materials, not just magnets.

Electrons usually fill shells in pairs of opposite spin (cancelling each other out). If you have an unpaired electron, then the contribution to the total magnetic field from it's spin is not cancelled.

There are many classes of magnets, but the one I assume that you are talking about is called a ferromagnet (the kind you stick to fridges, etc). In ferromagnets there is an energy minimum when the free spins are all aligned. So at absolute zero, they would all be aligned. At finite temperatures some of the spins get flipped, and the chances of a spin being flipped depend on the temperature. One result of this is that the strength of the magnetic field produced by a magnet is temperature dependant. So if you heat a magnet enough, its magnetism goes away.

Protons and neutrons also have magnetic moments, but there is no need to worry about these for the moment.

CathalMc

Re: Son Goku's answer

Is the electrical/magnetic field distinction purely a relativistic effect or is can it be explained equally well with classical physics. Also, do you have any references for that application of special relativity to this matter - or if you're feeling especially generous, could you give a more thorough explanation of this, maybe including the nature of EM radiation. I'm more interested in the fields themselves rather than illustrations with currents in wires.

On a related subject, coming from a related field (EE), I don't have an intuitive grasp of the notion of a field apart from a behavioural or mathematical descriptive sense. For instance, I've a healthy understanding of Maxwell's equations, but not necessarily of what actual underlying physical process causes the repulsion of two electrons. Again, any reference/explanation would be appreciated.

Guest

Well, i think by using a magnet..Rub it against a pice of metal the same way, over and over this pulls all of the electrons the one way, It charges them., Well to be honest i dont really know, what i do know is that I have 2 extremely strong magnets, which stuck togehter pinching my skin and it was Sore:mad: They were neodium magnets, get hem on ebay!, Fantastic item worth buying!

sunnyjim

Hmmm... Maybe someone can confirm this fact that I came upon recently...

[fact?]Apparently the poles of the earth have switched ends over 400 times over the last 300 million years[/fact?]

Guest

hmm, Sounds good sunnyjim..
I think that might be true but im not sure better ask jeeves lol

Dg101

Thanks Professor Fink, great answer. So would that mean that elements with satisfied valencies would have less magnetic charge? I'm getting slightly worried that this is all going to go quantum but, here goes, would the magnetic effect of the proton and neutron be because of the fact that there are 3 quarks in each and they don't cancel out spins? Wouldn't the strong nuclear force mean the spin would be negligible though?

*massages head to make headache go away*

Professor_Fink

Dg101 wrote:

So would that mean that elements with satisfied valencies would have less magnetic charge?

Well magnets don't have charge as such, but they would have lesser magnetic moments (which I think is what you mean by charge in this context).

Not every element will be magnetic in every compound, it all depends on how it interacts with neighbouring elements. If the are strongly coupled, for example if they can exchange electrons easily, then this will lead to stronger magnetization. If they are completely uncoupled then you get a paramagnet, which basically means the spins will try to line up with an external magnetic field, adding to it, but in the absence of a magnetic field they do not have a strong magnetization.

I'm getting slightly worried that this is all going to go quantum but, here goes, would the magnetic effect of the proton and neutron be because of the fact that there are 3 quarks in each and they don't cancel out spins? Wouldn't the strong nuclear force mean the spin would be negligible though?

*massages head to make headache go away*

Well the proton is like an electron in that it has an electric charge, and is in effect 'spinning'. The magnetic moment of the neutron does come from the quarks, but not in the same manner you mention.

The neutron is made up of three quarks (2 downs and an up quark). These quarks each have an electric charge (-1/3 for each of the downs and +2/3 for the up), and all have spin-1/2 (i.e. the can spin one way or the other, but always with the same angular momentum). So while their total charge cancels, on a microscopic scale you have electric charge moving about, giving rise to a small magnetic moment. Each of these moments is much smaller than the electron magnetic moment, so they have a negledgible effect on the overall magnetization of a material.

Dg101

Ahhh, right. Thanks. I think I follow it now. That had been bothering me for a while. Hurray for the wisdom of Professor Fink!

Son Goku

CathalMc wrote:

Is the electrical/magnetic field distinction purely a relativistic effect or is can it be explained equally well with classical physics.

It can be modelled perfectly well with classical physics, but classical physics can't explain what magnetism is, unfortunatly. All classical physics says is that there is some field B that acts in such and such a way in relation to electric charge. Its origin/explanation is totally relativistic.

CathalMc wrote:

could you give a more thorough explanation of this, maybe including the nature of EM radiation. I'm more interested in the fields themselves rather than illustrations with currents in wires.

Thankfully this will be easy if you've already done Maxwell.

You notation might be different, but I'm sure you've seen the electric and magnetic vector fields written in this form:

E = (Ex, Ey, Ez)
B = (Bx, By, Bz)

The source of the electric field is the charge density, ρ.
The source of the magnetic field is the the current density, which is a vector whose components describe the electrical currents running in various directions.
j = (jx, jy, jz)
Although after doing Maxwell you've probably seen this a million times, but just in case.

In relativity the charge density and the current density get combined into one entity called the four current vector, which I'll write with a captial J.

Now,
J = (ρ ,jx, jy, jz)

This thing is the source of the combined Electromagnetic field.
Remember that jx, jy and jz describe the electrical currents running in each direction and are the source of the Magnetic Field.

The trick is in Relativity different reference frames will see different densities and currents in the same way that they see different times and spaces. Basically because what I call the x, y, z and t axis, mightn't be what another observer would call the x, y, z and t axis.

In fact if in one referecne frame the four current is:
J = (ρ , 0, 0, 0).
(In other words purely the source of an electric field)

From the point of view of another frame it will be:
J = (ρ ,jx, jy, jz)
(In other words the source of both an electric and magnetic field.)

This is the very basics of what's really going on in electromagnetism.
If there is any confusion or you want more depth (and there is way more depth) just say so.

The next post will be the real explanation.