Repelling force of magnetism

1. Jun 25, 2004

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 ?

2. Jun 25, 2004

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 $$1/r^2$$.

3. Jun 25, 2004

Integral

Staff Emeritus
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.

4. Jun 25, 2004

bozo the clown

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

5. Jun 25, 2004

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.

6. Jun 25, 2004

bozo the clown

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

7. Jun 25, 2004

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.

8. Jun 25, 2004

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?

9. Jun 25, 2004

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.

10. Jun 25, 2004

krab

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.

11. Jun 25, 2004

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 existant that is acting.

Last edited: Jun 25, 2004
12. Jun 26, 2004

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.

13. Jun 26, 2004

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.

14. Jun 26, 2004

bozo the clown

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

15. Jun 26, 2004

krab

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.

16. Jun 26, 2004

bozo the clown

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

17. Jun 27, 2004

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.

18. Jun 27, 2004

zoobyshoe

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 cancelled out by the electric fields of other electrons. Everything would be a magnet except that in most elements all the electric fields are cancelled 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.

19. Jul 5, 2004

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.

Last edited: Jul 5, 2004
20. Jul 5, 2004

eJavier

The Froglev

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

Last edited by a moderator: May 1, 2017
21. Jul 6, 2004

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.

22. Jul 6, 2004

zoobyshoe

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?

23. Jul 7, 2004

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.

24. Jul 7, 2004

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.

25. Jul 8, 2004

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 aquired 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.