Is magnetism magic or what?
August 25, 2006 1:07 PM   Subscribe

Is magnetism free energy? How exactly does it work?

I have always been mystified by magnetism. Where does it come from? Does a magnet that's holding a metallic object ever "run out" of magnetism, and have to drop it? If not, isn't magnetism therefore free energy? It would seem to defy thermodynamics if a magnet can't exhaust its magnetism.

The rekindling of my juvenile fascination with magnetism is spurred by that Steorn story, as you might have guessed. It probably is a scam of some sort, I know. But I'm still curious how science explains magnetism, and I'm not ashamed to admit that I'm utterly clueless about magnetism.
posted by evariste to Science & Nature (16 answers total) 4 users marked this as a favorite
 
It's not free energy any more then gravity is free energy. Remember, energy is the amount of force times the distance traveled under that force. So if something doesn't move, then no energy is expended.

Think about a rock on the ground, is any energy used holding the rock on the ground? No. It's the same with magnets. Any energy gained by bringing the magnet closer to another one is required to separate them again.
posted by delmoi at 1:18 PM on August 25, 2006


Actually I think the reason there is so much hullaballoo over this story in particular is that magnetism is one of those forces (like gravity) that could possibly be misunderstood due to the mismatch between special relativity and quantum mechanics.

I can't really explain this eloquently (thanks, preview button!) but I'd like to reccomend Richard Feynman's Lectures on Physics. Volume 3 specifically deals with magnetism I believe. Six Not So Easy Pieces is probably my favorite book by him, and includes magnetism to a much smaller degree, but does wedge it in with other tricky bits, which is a nice frame.
posted by shownomercy at 1:18 PM on August 25, 2006


Does a magnet that's holding a metallic object ever "run out" of magnetism, and have to drop it? If not, isn't magnetism therefore free energy?

Would a bold holding up the same thing need energy? The answer is no. Magnetism does not require energy if nothing is moving, and when it does, it's simply potential energy being converted to kinetic energy, which is reversed when the object is removed.
posted by cillit bang at 1:20 PM on August 25, 2006


It takes energy to make a magnet. (To properly arrange the atoms.)

You cannot (according to scientific laws) get free energy out of a magnet. Perpetual motion devices have been designed that work through permanent magic, but in reality, any extraction of energy from the permanent magnet weakens the field and eventually it does "run out," and your machine has not produced more energy than was used to make the magnet.

I think a magnet just holding something shouldn't "run out," in this way, though, just like a screw holding something to a wall doesn't run out. There's probably some other effects at work that would eventually weaken the magnetic field, but just holding something up isn't extracting energy from it, to my knowledge.
posted by TheOnlyCoolTim at 1:25 PM on August 25, 2006


permanent magic -> permanent magnets.

If you had permanent magic, you probably could build working perpetual motion.
posted by TheOnlyCoolTim at 1:27 PM on August 25, 2006


I don't think magnetism is something that's misunderstood that way.
1) classically magnetism is very well explain by Maxwell's Equations
2) quantum mechanically it's nigh-on perfectly by quantum electrodynamics
3) special relativity gives you magnetism as soon as you have an electrostatic force - the whole of magnetism is fundamentally relativistic
4) there is no mismatch between special relativity and quantum mechanics, only general relativity and quantum mechanics.

Actually, point 3 is pretty significant in understanding how magnetism comes about actually. It's basically implied as soon as you have charges which produce electric fields and when you derive special relativity by assuming that the speed of light is a constant no matter how you move. Seeing intuitively how this comes about is pretty hard though, since relativity is pretty hard.

Anyway, lots of things like static electric fields, gravity, and static magnetic fields are what physicists call 'conservative fields'. These have the wonderful property that the energy of something in that field is only a property of where it is, not how it got there. This means if you go in a loop and get back to where you started you've not gained or lost energy. Steorn quite explicity state that they believe they have produced a non-conservative magnetic field from static magnets. I believe this is a mathematical impossibility unless magnets are not doing what we think on a fundamental level, and them having found this out by sticking a few together seems more than a tad unlikely.
posted by edd at 1:33 PM on August 25, 2006 [3 favorites]


the whole of magnetism is fundamentally relativistic

there's a very good explanation of this in one of the feynman lectures btw.
posted by sergeant sandwich at 1:37 PM on August 25, 2006


Magnetism, like gravity, is what is called a conservative force. If I travel any path in a magnetic field and come back to the original point, I will have the exact same energy I started with (minus friction). Magnetism is no more free energy than gravity is.

Two factors make magnetism attractive to people who want free energy: First, it's handheld and at a scale that can be felt. It's tough to make your own gravity or your own strong nuclear force for attraction, and working with electricity is dangerous, but magnetism is right there in the palm of your hand.

The second part that people don't get is that magnets don't do any "work" on free particles, which move perpendicular to the magnetic field. This peculiar sideways operation just seems counterintuitive to people, and then they get lost.
posted by adipocere at 1:40 PM on August 25, 2006


Perpetual motion devices have been designed that work through permanent magic, but in reality, any extraction of energy from the permanent magnet weakens the field and eventually it does "run out," and your machine has not produced more energy than was used to make the magnet.

I wasn't aware of this? Has anyone been able to extract the energy invested from the creation of a magnet mechanically? Not that it would be terribly practical, but it would be cool.

One great demo I've seen is of two poles drilled into a wooden board. On pole has a ferrous (metal that attracts magnets) tip and the other has a chain with a magnet on it. The chain is just shorter than the two poles, so the magnet is held in mid-air. It is used to demonstrate conservation of energy in a case like this.
posted by phrontist at 2:23 PM on August 25, 2006


Magnetism, like gravity, is what is called a conservative force.

Not quite. The magnitude of the force a charged particle feels as it travels through a magnetic field is proportional to its velocity. No velocity-dependent force is conservative. You can get an intuitive feeling for this just by thinking about wind resistance. It slows you down, and you can't regain the energy you lost.

However, what enforces energy conservation is the fact that magnetic fields can do no work (add or remove energy). The direction of the force is always perpendicular to the velocity of the charged particle--so it just ends up going in a circle, but it doesn't speed up or slow down.

You can do work by introducing a changing magnetic field, say by passing a magnet through a loop of wire. This will induce an electric field in the wire, which will do work against the electrons and move them along the wire (note that it's the induced electric field that does the work, not the magnetic field). In order to do this, you have to move the magnet (or the wire), and that's where the energy comes from. If you take a handheld crank generator and short its outputs, it'll be pretty easy to turn. Now hook the thing up to a lightbulb and you'll notice that it becomes considerably harder to turn... you're adding the energy, not the magnet in the generator.
posted by dsword at 5:49 PM on August 25, 2006 [1 favorite]


dsword: " If you take a handheld crank generator and short its outputs, it'll be pretty easy to turn. Now hook the thing up to a lightbulb and you'll notice that it becomes considerably harder to turn..."


I think you meant 'open' the outputs, not 'short' them. A short is a larger load than a light bulb.... an open is no load. Hence, no counter EMF.
posted by FauxScot at 6:56 PM on August 25, 2006




The thing that's always confused me is that it's possible to rig something up where two magnets repel each other, and so you can get mag-lev. In theory both magnets could be non electromagnetic, so something could levitate in the air permanently if positioned correctly, right?
posted by wackybrit at 7:58 PM on August 25, 2006


wackybrit: Yep, that's a simple permanent magnet maglev train. But it would just sit there until you applied a pushing or pulling force to get it moving.
posted by junesix at 8:08 PM on August 25, 2006


In theory both magnets could be non electromagnetic, so something could levitate in the air permanently if positioned correctly, right?

Yes.
posted by Wet Spot at 8:15 PM on August 25, 2006


dsword is right. It's not strictly conservative - but if you fix things to have constant fields, and you're not talking about moving charges then I think it acts like one in the case of static magnets interacting. But I could be wrong, I'm having trouble showing that magnets attract at the moment, my EM is that rusty :-)
posted by edd at 3:02 AM on August 26, 2006


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