What is the Nature of Magnetism?

[bozo the clown]

What exactly is that tuging / repelling force that you feel when you hold a pair of magnets close together ?
Also what is the attracting / repelling force formula with regard to distance between them ?

 

[Moe]

It is magnetism. The EM force is one of the four basic forces, I am not sure how to interpret your question. Surely you have seen the pictures with all the field lines around a magnet. Forces pull along those lines.
The force drops with 1/r^2.

 

[Integral]

A magnetic field surrounds all magnets, the field is visualized as closed lines which emanate from one pole of the magnet and enter the other. This field wants to be in a minimum energy configuration, this occurs when the field lines are as short as possible.

All materials have have a property called permeability (or was that permativity? , I am writing this without any references!) This property is an indication of how "easy" it is for magnetic fields to "pass through" the material. If a magnetic field encounters a material (like iron) which it can pass through easily, the minimum energy configuration will be with lots of field lines passing through that material, further the minimum energy configuration will occur when that material is oriented with the field and is as close as possible to the magnet. Thus Magnets attract Iron.

When 2 magnets are brought close together something similar happens. Now the minimum energy configuration occurs when the poles of the magnet are lined up and the magnets are as close together as possible. The pull you feel is the magnetic field seeking a minimum energy.

 

[bozo the clown]

It is magnetism. The EM force is one of the four basic forces, I am not sure how to interpret your question. Surely you have seen the pictures with all the field lines around a magnet. Forces pull along those lines.
The force drops with 1/r^2.

ok what is going on with the atoms between the magnets ?

 

[Moe]

What atoms between them? Those that make up the air? Or do you mean the ones inside the magnets?

Integral, permeability is correct. Permittivity is for electric fields.

 

[bozo the clown]

I mean what I said the atoms between the magnets air ,dust etc

 

[JohnDubYa]

"Forces pull along those lines."

No, the direction of the force depends on the velocity orientation of the charged particle placed within the field.

Essentially, we have discovered that when two charged particles are moving in relation to each other, they exert a force on each other. We invent the concept of the magnetic field to explain the origin of the force.

 

[kuenmao]

The idea of magnetivity is an abstract one. Some scientist(forgot who) introduced the concept of a "magnetic field" to explain it. That means that the first magnet produced a sort of a field which has no matter, and does not really have any "physical form", and this "field" exerts a force on the other. It's just another thing that exists; don't ask why. It doesn't even need a medium such as air, and it works in vacuum. Same goes for electric fields.

The microscopic view is that a magnet contains of many very small parts called "domains" which are all mini-magnets. When they get aligned in a certain direction, they act as a magnet and produce a field. I don't really know about the details, since I'm still only a student. Can anybody elaborate on this?

 

[Moe]

John: I was talking about ferromagnets, not about charged particles.

bozo: That depends. If they are already magnetic dipoles (we have to talk about molecules here, not individual atoms), they will turn into the magnetic field. If they are not, the internal structure will shift just a tad to create dipoles, which are oriented along the field lines as well.

 

[krab]

ok what is going on with the atoms between the magnets ?

Nothing. At least, they play no role in the force you feel.

The force law is 1/r^2 for poles close together, but 1/r^3 for distances larger than approx size of magnet, since all magnets are dipoles and one pole of a magnet sees both poles of the other magnet at once.

It is a fundamental force. Like gravity, or electric. It just is.

 

[bozo the clown]

ok take 2 repelling magnets now when you push on them you feel that cushion type force as i push the repelling magents together using my strength what am I pushing against what is acting between the magnets against my strength something is going on between the magnets something is there existent that is acting.

 

[krab]

Yes. There is a magnetic field between the magnets. It's like gravity. There is nothing pulling you down to Earth but a gravitational field.

 

[Adrian Baker]

I think Bozo is concerned (as Newton was with Gravity) of the 'action at a distance' problem i.e HOW does one magnet 'know' the other is there. What actually 'carries the force' across the gap.

For this you need to look into the fundamental force carrying particles - Fermions - in this case 'virtual photons', being exchanged between the two magnets. It is these interactions that you feel as a force.

 

[bozo the clown]

Is there any releation between the mathematics in regard to the force and distances of electro magnetism fields and gravitational fields ?

 

[krab]

Is there any releation between the mathematics in regard to the force and distances of electro magnetism fields and gravitational fields ?

Yes. They are essentially the same: they are of the form
F\propto {q_1q_2\over d^2}
where the q's are relevant quantities (mass or charge), and d is the separation. Choose the units of q appropriately and the proportionality constant is 1. But for magnets, as I said, you cannot have a positive q without a nearby negative one. This reduces the force between magnets because for example if you have north to north, each north also sees the south at the other end, partially cancelling the repulsion.

 

[bozo the clown]

So magnetic force and gravtitational forces are two diff forces that just happen to share the same math ?

 

[kuenmao]

Yes, they are different forces, and they share similar math, although not the same. Actually, the equation mentioned by Krab was Coulomb's Law, which involves electric charges only. I'm not sure about the one about magnetism, though.
The cushion-like force you feel is actually as follow:
Force acting on the second magnet, by the magnetic field which was created by the first magnet; Force acting on the first magnet, by the magnetic field which was created by the second magnet.
To visualize how the force can act over a distance, scientists think that a "field" is created by a first object, and acts on the second one.

 

[zoobyshoe]

what am I pushing against what is acting between the magnets against my strength

I love this question and I frequently ask myself the same thing.

A magnetic field is a version of the electric field. If you don't first have an electric field there will be no magnetic field.

The way you get from an electric field to a magnetic field seems to be intimately tied to motion of the particles from which the electric field emanates. On the other hand, a stationary electric field will be seen as a magnetic field by a moving charged particle (what John was describing).

A magnetic field will arise around a current carrying wire. I believe it is accurate to say this is because there are electrons in the wire actually changing their locations and carrying their fields along with them.

In a permanent magnet there is a certain small number of electrons in each iron atom whose electric field is not canceled out by the electric fields of other electrons. Everything would be a magnet except that in most elements all the electric fields are canceled out.

This is actually called "uncompensated spin". It's more complex, really, than "cancelling out". Only iron and a few other elements have electrons whose spin isn't completely compensated.

So, it is the electric fields of these "uncompensated" electrons that are free to form the magnetic field in a permanent magnet.

Which begs the question "What's an electric field?"

Both the electric and magnetic field have certain specific properties and dynamics which have been studied and quantified since that anonymous Greek person became fascinated with the static electric spark he generated when he touched something after polishing some amber. ("Electricity" comes from the greek word for "amber", I have read.)

It isn't really possible to answer the question "What is an electric field?" directly, because as people have pointed out, it is a thing unto itself. The only really informative answers involve descriptions of it's properties and qualities.

 

[turin]

All materials (that contain electrons, at least) will respond to a magentic field if the gradient is steep enough. I wish I could remember the article, it was a solid state physics article, they got a (live) frog to levitate inside a solenoid with a field gradient of some T over some cm.

Oh yeah, and something a little less exotic is the popular class room demo of dropping a bar magnet through a conductive pipe (such as copper). The magnet quickly reaches terminal velocity, not due to air resistance, but due to Lenz' Law. It can take several seconds for the magnet to fall out the other end.

 

[eJavier]

All materials (that contain electrons, at least) will respond to a magentic field if the gradient is steep enough. I wish I could remember the article, it was a solid state physics article, they got a (live) frog to levitate inside a solenoid with a field gradient of some T over some cm.

Oh yeah, and something a little less exotic is the popular class room demo of dropping a bar magnet through a conductive pipe (such as copper). The magnet quickly reaches terminal velocity, not due to air resistance, but due to Lenz' Law. It can take several seconds for the magnet to fall out the other end.

The Froglev

http://www.hfml.kun.nl/froglev.html
http://www.sci.kun.nl/hfml/frog-ejp.pdf

 

[turin]

eJavier,
Thanks for the leg-work. That's exactly what I was talking about. Also, your links reminded me that I should have included that single-word explanation that encompasses the entirety of the phenomena: diamagnetism.

 

[zoobyshoe]

Oh yeah, and something a little less exotic is the popular class room demo of dropping a bar magnet through a conductive pipe (such as copper). The magnet quickly reaches terminal velocity, not due to air resistance, but due to Lenz' Law. It can take several seconds for the magnet to fall out the other end.

I have heard of this but never seen it done. How long does the pipe have to be for it to take several seconds? Is some especially strong, rare Earth magnet required?

 

[turin]

I actually tried to arrange this demo in my class, but I couldn't get it to work significantly. When I saw the demo for the first time when I took physics at a CC (back in the dinosaur days), the pipe that the professor used was a 1" aluminum pipe about 4' long. He had a little bar magnet about 2" long and about 1/4" in diameter encased in plastic. I don't know what kind of magnet it was. It was quite dramatic, though. He dropped a plastic rod through the pipe that popped out the other end rather quickly. Then, he let the magnet fall through the pipe. We all thought it had gotten stuck, until it plopped out the other ender a few seconds later. Then, he let us all watch down the pipe and see the magent slowly descending at a constant rate. It was pretty neat. I think that the stronger the magnet, the better the demo. I don't know how the material of the pipe would come into the demo, though. I imagine that the pipe just needs to be a "good" conductor (to allow the ring currents), but, like I said, I couldn't manage to get the demo to work very well.

 

[zoobyshoe]

That's pretty interesting. I don't know what you tried, of course, but the purpose of the plastic casing in your professor's case may have been to keep the magnet equidistant from the sides of the pipe. Being closer to one side of the pipe than the other may not generate currents that are the right strength in the right places.

Yeah, I'm sure aluminim or copper are both fine. Steel would not be good, even though it conducts, for obvious reasons.

 

[turin]

I actually tried both aluminum (I think it was aluminum: dull silver, light weight, etc.) and copper. Neither one gave a dramatic effect.

When I saw the demo done (correctly), the outer diameter of the plastic surrounding the magnet was much less than the inner diameter of the tube; so much less, in fact, that the magnet acquired a quite noticeable diagonal orientation as it fell, with the bottom end sliding along one side of the tube and the top end sliding along the oposite side.

Actually, it looks like I'm going to have some spare time this afternoon to play, so I plan on going into the demo room and trying to get this to work. I'll let you know if it does, and what seems to make it work the best.

 

[pallidin]

Perhaps I am mistaken, but I thought that the "force carrier" of the magnetic phenomenon has never been actually identified in any verifiable experiment.

 

[zoobyshoe]

When I saw the demo done (correctly), the outer diameter of the plastic surrounding the magnet was much less than the inner diameter of the tube; so much less, in fact, that the magnet acquired a quite noticeable diagonal orientation as it fell, with the bottom end sliding along one side of the tube and the top end sliding along the oposite side.

OK, that theory's obviously shot. I can't think of any more suggestions, though.

Actually, it looks like I'm going to have some spare time this afternoon to play, so I plan on going into the demo room and trying to get this to work. I'll let you know if it does, and what seems to make it work the best.

OK. I would like to see this in action, sometime.

 

[bozo the clown]

http://my.execpc.com/~rhoadley/magwhy.htm
all about magnetism

 

[turin]

The following stuff in italics is old, and has been improved upon:

I'm sorry. I still couldn't get it to work dramatically.

my rough setup:
I got a copper tube and a PVC tube of approximately the same inner diameter (~3/4") and length (~4'6"). Then, I got ahold of two "identical" (designed to be identical) alnico cylindrical bar magnets of approximate dimension 2" x 1/3". I put a cushion on the floor to catch the magnets (sharp jolts to a magnet can demagnetize it, like if they just fall out the bottom end of a tube and hit the hard floor). I made sure the cushion was just hard enough so that I could hear the dull thud of the magnets hitting the bottom. I stood on a chair and held the tubes up vertically. As you may have noticed, this was all thrown together from scrap materials (other than the nice alnico magnets, fortunately, as they are the key ingredient in this demo).

my rough results:
In a nutshell, I got a difference of about 1/2 sec in fall time (more in the copper tube). I tried each tube with each magnet and both N and S poles facing down. It was always the same 1/2 sec longer in the copper tube. This is not dramatic enough for a class demo.

refinements:
I do believe that a stronger bar magnet is the key to improving this demo. I did not measure anything precisely, since the point is not to evaluate the physical process, but to evaluate the feasability of a class room demo (or personal intrigue). I did have one interesting experience that had not occurred to me previously. When I let the magnets fall, I held the tubes sometimes a little up off of the cushion, and I felt an unmistakable downwards tug in the copper tube. I will chaulk this up to Newton's third law.

Sorry for the disappointment, zooby. Maybe you can be a bit more serious than I was about it and try to get ahold of a really strong magnet.

All right! I did it, finally. I got a whoppin' 4.9 sec fall time through a 1.5 m aluminum tube! This magnet is ridiculously strong. I have no idea what it is - the mystery magnet. I'm too lazy to construct a test setup to evaluate the strength of this mystery magnet, but here are some examples:
- It can pick up a compass just by pulling on the needle inside.
- I picked up a metal stool with it by cradling it in a piece of paper and pulling up on the paper.
The magnet has one peculiarity. It is a rectangle of dimension:
3.25 x 1.70 x 1.05 cm, but the poles are on oposite sides of the 1.05 cm, very much unlike the "usual" bar magnet. The tube had inner diameter of 2.10 cm, so the magnet had to fall through with its poles facing sideways. Of course, Lenz' Law doesn't care, because, it's not the direction of the field, it's the direction of the gradient that matters.

For anyone who is curious and too lazy to do the calculation, I've done it for you: freefall time through the 1.5 m tube should be 0.56 s. I had to do the timing by hand with a stop watch, so I estimate a possible systematic error on the order of 100 ms (I only took 5 readings). I also dropped nonmagnetic weights through the tube, and they took between 0.6 and 0.8 s to fall the 1.5 m.

Here is the actual data:

TUBE
material: unkown, but most likely aluminum
length: 152.4 cm (measured with a 2-meter stick)
inner diameter: 2.10 cm (measured with an inner caliper)
outer diameter: 2.65 cm (measured with an outer caliper)

20 g WEIGHT
material: unkown, but most likely brass
length: 2.20 cm (measured with an outer caliper)
diameter: 1.40 cm (measured with an outer caliper)
fall times: 0.71 s, 0.71 s, 0.69 s, 0.68 s, 0.72 s

50 g WEIGHT
material: unkown, but most likely brass
length: 2.50 cm (measured with an outer caliper)
diameter: 1.90 cm (measured with an outer caliper)
fall times: 0.73 s, 0.72 s, 0.65 s, 0.76 s, 0.63 s

MYSTERY MAGNET
material: unkown
strength: uncharacterized
length: 3.25 cm (measured with an outer caliper)
width: 1.70 cm (measured with an outer caliper)
height: 1.05 cm (measured with an outer caliper)
fall times: 4.95 s, 4.92 s, 4.93 s, 4.86 s, 4.91 s

 

[bozo the clown]

so physicists still do not know what the magnetic field is actually made up of right ?

 

[zoobyshoe]

All right! I did it, finally. I got a whoppin' 4.9 sec fall time through a 1.5 m aluminum tube!

Congratulations!

So now we know that the visibility of the effect is highly dependent on the strength of the magnet.

-Zooby

 

[zoobyshoe]

so physicists still do not know what the magnetic field is actually made up of right ?

I think this is more or less true. It's true for almost everything, though. At a certain level they no longer know what anything is actually made up of. All they can tell you is what it does, not what it is.

 

[kuenmao]

Isn't the magnetic field just invented(YES, invented) by scientists? There is actually no such thing as a magnetic field, if you define things to be made up of some mass. It is just a construct of the human mind so as to easily see the effects of a magnetic field on the system we are talking about. If you ask what it is made of, then the best answer would be: nothing, save the imagination of the human brain.

 

[krab]

This magnet is ridiculously strong. I have no idea what it is - the mystery magnet.

My guess is that it is Samarium-Cobalt.

 

[krab]

so physicists still do not know what the magnetic field is actually made up of right ?

These sorts of questions are interesting. If you look back on these forums, you will find many similar: What is the real mechanism of gravity, of mass, of electric charge, etc. I cannot help feeling that it is a result of our conditioning whereby we have a real feel for mechanical effects, but none for other kinds of effects. I also cannot help feeling that those who ask these questions need something like microscopic mechanical devices like springs and linkages. If that's the case, they are out of luck. There are no mechanical devices operating behind the scenes. In fact, even the mechanical things we know about are fields (mostly electric) at the microscopic level. Yes, when you push your hand against something, the reason it does not go through is that there are electric fields between the atoms that keeps them together. So, in a sense, everything is fields. It's one of the things you get used to when you learn physics. Fields just are, and they have certain properties physicists know and have measured.

 

[zoobyshoe]

krab: "I cannot help feeling that it is a result of our conditioning whereby we have a real feel for mechanical effects, but none for other kinds of effects."

I think you've hit the nail on the head. That "real feel" you mention is a neurological one: our nerves and brain are able to make an extremely useful kind of sense out of mechanical forces. Not so with fields.

krab: "I also cannot help feeling that those who ask these questions need something like microscopic mechanical devices like springs and linkages."

Again: nail on the head. That is exactly what people are looking for when they ask these questions. When a person starts wondering about these things they have no other reference frame at their disposal from which to consider them. They are not at liberty, so to speak, to ask from any other standpoint: you push two repelling magnets together, your sences demand you to understand that this is a completely mechanical phenomenon. Your mind tells you: "there must be micro-springs too fine to be seen with the naked eye." or something similar.

Krab: "So, in a sense, everything is fields. It's one of the things you get used to when you learn physics. Fields just are, and they have certain properties physicists know and have measured."

Getting used to it is, I believe, dependent on a given individual acquiring enough information about fields to see that the micro-spring (or whatever) explanation just won't satisfy everything that a field can do. You have to learn a certain number of their mysterious properties before you can face the fact that fields are a thing unto themselves, that "they just are". In other words, you have to know a fair amount about how they behave in a fair amount of circumstances before you realize that "What fields are" isn't a secret that hasn't been unlocked yet in terms of micro-springs.

What fields do, is consistant and measurable, but what they are ends up not being expressible by mechanical analogy. Fields are fields.