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explain how magnets work

of the 4 main forces in the universe:

strong forces
weak forces
gravity
and magnetism

magnetism is said to be actually stronger than gravity.
 
wollery said:
Umm, no.

If you hold the fridge door so it's horizontal with the magnet hanging underneath it still stays stuck to the door. Please explain how friction is responsible for that.
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Sorry, but UserGoogle is correct.

In the usual scenario, magnetism explains why the magnet remains next to the fridge door. Friction is necessary to explain why it remains next to whatever spot you happen to place it on. In your alternate scenario magnetism explains why the magnet stays on the door. Friction explains why you'll feel resistance if you try to push the magnet along the bottom of the door.

In both cases you need to know about both magnetism and friction to understand the full phenomena that you're looking at.

Cheers,
Ben
 
wollery said:
Umm, no.

If you hold the fridge door so it's horizontal with the magnet hanging underneath it still stays stuck to the door. Please explain how friction is responsible for that.
Click to expand...

In that case it's magnetism doing all the work, but that's a completely different system from what the original poster asked. Topspy asked, although in a somewhat confused way, how magnets stick to refridgerator doors. I answered that question. You are referring to a very different question.

When a magnet is placed sufficiently close to a refrigerator door, the magnetic force pulls the magnet towards the door. (For reasons that have already been explained.) When the door is perpendicular to the ground, however, the force of gravity and the force of magnetism are pointing in completely different directions, with gravity pulling the magnet down and electromagnetism pulling it sideways. Thus, it cannot be the magnetism which is countering the force of gravity, but rather friction, in the way I said. When the door is parallell to the ground and the magnet is below it, then yes, it is magnetism doing all the work. And if the door is paralell to the ground and the magnet is above the door, (such as if you knocked the fridge on its back) then magnetism and gravity are pulling in the same direction, and it is the "contact force" of the door which is holding the magnet up. That is, the force that resists objects from passing through each other as if they were ghosts.

(Ultimately, contact forces and friction are just special kinds of electromagnetism, with the electrons on the outside of the outside of the objects interacting with each other, although that's getting a bit off the original topic.)
 
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It seems that we don't know. We know that magnets exert a field that reacts with other magnets and iron and steel but we don't know what that field is exactly. We can harness it to create electricity and do many important and benificial things. We can also measure the field, in fact we can measure it very precisely. We have some theories but beyond that I don't think we know. I've never met anyone who claimed to know. Some people want to play cute games like zep but those are not really helpful beyond making one feel stupid. Hell, who hasn't played games with magnets as children and understand intuitively how they work so that is not at all helpful. It makes him feel superior I supose so that's worth something.

Somethings we just don't know. Unless, I'm wrong and someone wants to let you and I in on the secret. I wouldn't hold my breath.
 
As for cool things to do with magnets...

Gaussian gun

(Apologies if this is off topic but I only recently came across it and just had to share.)
 
RandFan said:
It seems that we don't know. We know that magnets exert a field that reacts with other magnets and iron and steel but we don't know what that field is exactly. We can harness it to create electricity and do many important and benificial things. We can also measure the field, in fact we can measure it very precisely. We have some theories but beyond that I don't think we know. I've never met anyone who claimed to know. Some people want to play cute games like zep but those are not really helpful beyond making one feel stupid. Hell, who hasn't played games with magnets as children and understand intuitively how they work so that is not at all helpful. It makes him feel superior I supose so that's worth something.

Somethings we just don't know. Unless, I'm wrong and someone wants to let you and I in on the secret. I wouldn't hold my breath.
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My experience is quite different.

If you read The Feynman Lectures on Physics you'll find a very nice explanation of magnetism. Magnetism is the necessary relativistic correction to electric attraction between moving objects. That is if Coulomb's Law is to be true between stationary charged objects, then we can calculate the attraction between two charged objects that are moving at the same speed. This attraction is not (due to relativistic effects) the same as what Coulomb's Law predicts, there is another term. And that other term we call magnetism.

Of course this is historically backwards. Historically we discovered electricity and magnetism, then discovered how to relate them. (The synthesis was done by James Clerk Maxwell in the 1800s.) It was found that electromagnetic disturbances travelled at a fixed speed (barring various kinds of interference), and that speed was the speed of light. For a long time people tried to shoehorn this into classical relativity, but in the face of experiments suggesting that light always moved at the same speed, Einstein took Maxwell's prediction at face value and tried to figure out what must happen in a world where all observers see light going at the same speed. The result was the special theory of relativity.

So while we can understand magnetism as a relativistic correction to electricity, historically relativity was discovered as a consequence of studying the interplay between electricity and magnetism.

Cheers,
Ben
 
RandFan said:
It seems that we don't know. We know that magnets exert a field that reacts with other magnets and iron and steel but we don't know what that field is exactly. We can harness it to create electricity and do many important and benificial things. We can also measure the field, in fact we can measure it very precisely. We have some theories but beyond that I don't think we know. I've never met anyone who claimed to know. Some people want to play cute games like zep but those are not really helpful beyond making one feel stupid. Hell, who hasn't played games with magnets as children and understand intuitively how they work so that is not at all helpful. It makes him feel superior I supose so that's worth something.

Somethings we just don't know. Unless, I'm wrong and someone wants to let you and I in on the secret. I wouldn't hold my breath.
Click to expand...

I think the basic idea behind quantum electrodynamics is that electromagnetism works by particles swapping photons. A particle will emit an photon with the probability of the photon being emitted being proportional to its charge, and that photon then is absorbed by some other (or the same) particle, causing an exchange in the respective energies of the particles. And of course, all this is happening in with much "quantum weirdness" regarding the motions of the particles and the photons. When this whole thing is extended to very large numbers of particles coming together to form things like "magnets" and "refrigerators," you get the classical forces which we have been talking about so far.

(And again, Richard Feynman is awesome, and QED: The Strange Theory of Light and Matter is a pretty understandable layman's explanation to Quantum Electrodynamics, I think.)
 
Probably worth describing the relativistic correction to the electric force that results in magnetism in more detail, if we're going to "explain how magnets work."

Suppose there are two electrons just sitting there. They will act on each other by their electric charges of one unit each, with a force that is inversely proportional to the square of their separation; double the separation, the force is quartered; halve the separation, the force is quadrupled. Now force is mass times acceleration; so we see that it is doubly dependent upon time.

Now suppose that the electrons are both moving by at a substantial fraction of the speed of light. Note that from our point of view, they will experience time dilation; that is, we will see time as passing more slowly for them than for us. I have described elsewhere the spacetime rotation that results in this. If the force is doubly dependent upon time, you can see that this will result in a perception on our part that the force between the electrons is reduced. One way to explain this reduction in force is to posit another force that is acting in opposition to the original force; but since the electrons must be moving for us to see this force, its vector is not directly opposed to the vector of the original force. In fact, it acts at an angle to the original force, because of the movement of the electrons. We call this force, "magnetism." It acts always on moving charges, and always at an angle to the electric force; since the electric force moves at the speed of light, we see the angle as a right angle, both to the direction of movement, and to the direction of the electric force.

OK, so what's a magnet? The simple answer is, magnets form when materials have a net unbalanced charge in their electron shells. The spin of the electrons results in the relativistic effect we call magnetism, and the lack of balance results in residual force that is not canceled by the existence of equal and opposite force. A more detailed explanation is available at the linked article above.
 
Ben Tilly said:
Sorry, but UserGoogle is correct.

In the usual scenario, magnetism explains why the magnet remains next to the fridge door. Friction is necessary to explain why it remains next to whatever spot you happen to place it on. In your alternate scenario magnetism explains why the magnet stays on the door. Friction explains why you'll feel resistance if you try to push the magnet along the bottom of the door.

In both cases you need to know about both magnetism and friction to understand the full phenomena that you're looking at.

Cheers,
Ben
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UserGoogol said:
In that case it's magnetism doing all the work, but that's a completely different system from what the original poster asked. Topspy asked, although in a somewhat confused way, how magnets stick to refridgerator doors. I answered that question. You are referring to a very different question.

When a magnet is placed sufficiently close to a refrigerator door, the magnetic force pulls the magnet towards the door. (For reasons that have already been explained.) When the door is perpendicular to the ground, however, the force of gravity and the force of magnetism are pointing in completely different directions, with gravity pulling the magnet down and electromagnetism pulling it sideways. Thus, it cannot be the magnetism which is countering the force of gravity, but rather friction, in the way I said. When the door is parallell to the ground and the magnet is below it, then yes, it is magnetism doing all the work. And if the door is paralell to the ground and the magnet is above the door, (such as if you knocked the fridge on its back) then magnetism and gravity are pulling in the same direction, and it is the "contact force" of the door which is holding the magnet up. That is, the force that resists objects from passing through each other as if they were ghosts.

(Ultimately, contact forces and friction are just special kinds of electromagnetism, with the electrons on the outside of the outside of the objects interacting with each other, although that's getting a bit off the original topic.)
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Ah crap. You are indeed both correct. :boxedin:

That'll teach me to post without thinking and before I've had my morning coffee. :rolleyes:
 
Magnets on refrigerator doors -- good. They hold up photos, drawings, notes and good test papers.

Magnets on color CRT TV screens while on -- bad. They magnetize the grid and can actually make Whoopi Goldberg purple.
 
shuyun said:
of the 4 main forces in the universe:

strong forces
weak forces
gravity
and magnetism

magnetism is said to be actually stronger than gravity.
Click to expand...

Two points, shuyun: current theory holds that there are only 4 kinds of forces in the universe, in decreasing order of strength:

Strong nuclear force: holds atomic nuclei together despite the electronmagnetic repulsion of the protons. The strongest of the four forces, but short range.
Electromagnetism: light , magnetism, radio, heat, X-rays, not falling through the chair you're sitting on. Second strongest force, with long range.
Weak nuclear force: involved with beta decay and other nuclear transmutations. Weedy and very short range.
Gravity. The very puniest force of all, but it's very long range and can be built up and up and up until a black hole just falls out of the universe.

Nothing is weaker than gravity!

However, with the example of the fridge magnet parallel to the ground, the electromagnetic force is opposed by gravity, if only by the tiniest amount. On the surface of a neutron star, the star's gravity would whip it off the fridge as is nothing was holding it up.
 
Big Al said:
... However, with the example of the fridge magnet parallel to the ground, the electromagnetic force is opposed by gravity, if only by the tiniest amount. On the surface of a neutron star, the star's gravity would whip it off the fridge as is nothing was holding it up.
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It would probably strip the enamel paint off the door as well.
 
current theory holds that there are only 4 kinds of forces in the universe, in decreasing order of strength:
Click to expand...

It's often said that electromagnetism is much weaker than the nuclear force, but it's not exactly true.

Looking at a radioactive atom like uranium, you'll see that the nuclear forces are only barely overpowering the electromagnetic forces. The hold is so tenuous that a stray bump from a neutron is enough to shatter the nucleus, allowing the stored electromagnetic energy to launch pieces of the nucleus apart at great speed.

One might say that nuclear bombs are actually electromagnetic bombs, as it's actually the stored electromagnetic energy that's being released. The nuclear force is just the "catch", like a compressed spring bound with a piece of string. When the string is cut, the energy stored in the spring is released.

Electromagnetism is generally thought to be weak because charge is typically so well balanced, but within a nucleus you have positive charges held in very close proximity to each other.

- Dr. Trintignant
 
Dr. Trintignant said:
Looking at a radioactive atom like uranium, you'll see that the nuclear forces are only barely overpowering the electromagnetic forces. The hold is so tenuous that a stray bump from a neutron is enough to shatter the nucleus, allowing the stored electromagnetic energy to launch pieces of the nucleus apart at great speed.
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Hmm, I don't think I agree with that. Why do we get energy from fusing hydrogen nuclei into helium nuclei, then?

I'll have to get out my QM and nuclear physics textbooks tomorrow morning...
 
Hmm, I don't think I agree with that. Why do we get energy from fusing hydrogen nuclei into helium nuclei, then?
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Well, the forces do not always balance out so equally. For large nuclei like, oh, Ununoctium, electromagnetism wins out. For small nuclei like hydrogen, the nuclear force wins.

I'm not a physicist, but I know that the nuclear force does not follow a 1/r^2 curve the way electromagnetism does (and even electromagnetism doesn't follow that at very close range). So the "winner" is going to depend greatly on the exact distances we're talking about, which itself is going to depend greatly on how big the nucleus is and how it's arranged.

I'd posit that it's why, after a certain point, extra neutrons will make a nucleus less stable. The nuclear force falls off faster than the electromagnetic, and therefore a bigger nucleus is not held together so strongly as a small one. That's just a guess (and I realize there are many complicating factors) but I think the basic gist of it is true.

- Dr. Trintignant
 
Dr. Trintignant said:
The nuclear force falls off faster than the electromagnetic, and therefore a bigger nucleus is not held together so strongly as a small one. That's just a guess (and I realize there are many complicating factors) but I think the basic gist of it is true.
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After thinking about it further, this latter speculation is clearly untrue, since the nuclear force applies to all nucleons, whereas electromagnetism applies just to protons. So if anything, it is the opposite--more neutrons make the nucleus more stable, since the internucleon force is constant while the Coulomb force is reduced. Of course many other factors are at play.

Anyhow, I believe my first statement is still correct. I believe it appeared in the Feynman Lectures on Physics; I'll try to find a page number.

- Dr. Trintignant
 
Let's do a little math here. Wikipedia claims of the nuclear force:
At typical nucleon separation (1.3 fm) it is a very strong attractive force (104 newtons).

That is quite impressive; a very macroscopic force for just two subatomic particles. But let's see what the Coulomb force comes to.

The Coulomb force is:
F = k*q1*q2/r^2

Where k = 8.988e9 N m^2 / C^2

q1 = q2 = 1.6e-19 C
r = 1.3 fM = 1.3e-15 m

So:
F = 136 N

Well, that's pretty impressive, eh? It's greater than the nuclear force. It explains why Helium is not stable with just two protons--the Coulomb force is just too great, and to hold together you need some neutrons for spacing.

- Dr. Trintignant
 
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