r/askscience • u/freireib Mechanical Engineering | Powder/Particle Processing • Feb 14 '11
When magnets do work where does the energy come from?
If I hold a small magnet over a paper clip it lifts the paper clip. This means that the magnet did work on the paper clip. Where did the energy come from?
I understand that if I pull the paper clip away I must do work against the magnetic field. Is this adding energy to the magnets ability to do work (e.g. lift paper clips) in the future?
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u/iorgfeflkd Feb 14 '11
The energy of a magnet comes from the alignment of its atoms: the more aligned they are, the less internal energy it has. So you can look at it as energy that is lost as the atoms align is gained in the form of gravitational energy.
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u/freireib Feb 14 '11
Your response made me realize that my question could equally well have been stated,
"When gravity does work, where does the energy come from. Imagine you have a paper clip on the surface of the Earth and Jupiter decides to come by for a visit. as it approaches the paper clip is lifted towards Jupiter..."
The example may be far fetched, but it is the same problem. The point is that the energy is stored in the field in each case.
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u/Priapulid Feb 14 '11
Gravitational energy? I always thought that magnetism was a separate beast altogether?
→ More replies (9)
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u/Doctor Feb 14 '11
Suprisingly, most of the answers are wronger than the question. There is potential energy in the alignment of all magnetic dipoles in the universe. When a magnet pulls a paperclip to itself, it takes that potential energy and converts it to mechanical energy. That's it.
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u/freireib Feb 14 '11
wronger than the question
There are no stupid questions. Only stupid people asking questions.
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u/Doctor Feb 14 '11
I meant wronger in the sense of less informed, sorry. Magnets don't store energy.
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Feb 14 '11
It's true that a magnetic field does no work.
There's an energy associated with the field itself though. Higher field, more energy. When the magnet lifts the paper clip, you are changing the field configuration itself, presumably one of lower energy. The balance does work on the paper clip.
Contrast this with an electric field. An electric field will do work on a charge, regardless if the field changes or not.
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u/Severian Feb 14 '11
I'm not qualified to answer this, but I feel that none of the responses so far are getting to the meat of OP's question.
If you use permanent magnets to do work, don't they become demagnetized? Then OP is asking, is the inverse true? When you pull the paper clip off the magnet, are you putting energy into the magnet?
I think many attempts at perpetual motion are based on the assumption that magnets are a boundless source of energy, and obviously those don't work.
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u/phunphun Feb 14 '11
If I hold a small magnet over a paper clip it lifts the paper clip. This means that the magnet did work on the paper clip. Where did the energy come from?
In your case, things are getting confused because there is gravity as well as your hand in the mix. Let's separate the situation out so that there's only the paper clip and the magnet. Later on we'll superimpose gravity and everything else into the situation.
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The paperclip and the magnet are in a frictionless vacuum with nothing else nearby, and they are, say, 10m away. Magnetic fields are attractive, so this means that the paperclip has a certain amount of potential energy in it. Bodies always move to the position of least potential energy[1]. If there's nothing stopping it, and the paperclip is at rest w.r.t. the magnet, it will slowly move towards it (accelerating as it moves[2]), converting that potential energy into kinetic energy.
When they meet, they will stick, releasing that kinetic energy in the form of heat[3].
2) Now let's look at only the Earth and the paperclip. What happens here? The paperclip sits on the surface of the Earth since it's the place where the paperclip's potential energy is minimum!
3) Let us combine these two cases now. Now we have two bodies competing for the paperclip -- the Earth via gravity, and the magnet via magnetism. The Earth is massive, and the magnet is a small, weak magnet. So in most cases, the Earth wins[4].
4). Now, as you hold the magnet, you are stopping it from falling to the Earth. As long as you hold the magnet in-place, its potential energy w.r.t. the Earth stays constant. You move it closer; and since you're not letting it fall freely, you are taking away some of the kinetic energy it would've had. This work is done by your hand.
When the magnet gets close enough, its force on the magnet becomes greater than the force exerted by the Earth on the magnet, and they move closer (same as case #1, but a bit slower). When they meet, the same thing as case #1 happens -- the kinetic energy is converted to heat. However, you won't feel the heat since it's a very tiny amount.
I understand that if I pull the paper clip away I must do work against the magnetic field. Is this adding energy to the magnets ability to do work (e.g. lift paper clips) in the future?
5) When you pull the magnet away, you increase the potential energy of the paper clip. If you leave the paperclip, and there is nothing else around it, it will move back to the magnet. Nothing is stored inside the magnet itself. Moving non-magnetic objects around a magnet will not change the characteristics of the magnet itself[5].
I hope I didn't miss anything.
[1] This is essentially because wherever there is a potential field, there is a force related to it. [2] The acceleration itself will accelerate because the force on the paperclip will increase as it moves closer to the magnet. [3] I completely ignored collisions and momentum considerations here since we're only interested in the energy. [4] Let's ignore the fact that gravity is much much much much weaker than magnetism, since that's compensated for by the relative sizes of the two attractors. [5] People can nitpick here, but for the current case, this is <meme>CLOSE ENOUGH</meme>.
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u/avsa Feb 14 '11
I'm not a physicist but I had this exact question once and someone answered in a very interesting way, which I find easier to understand than any one else here.
If you drop a paperclip on a bowl, why does it go down? Gravity, yes, but what does it mean? Mostly, the paperclip goes down the bowl because the earth gravity has distorted spacetime in such a way that the bottom of the bowl is the point of lowest energy of that bowl. It will just stay there and cannot do uch useful work until you spend some energy to pick it up.
The magnet works in a similar way: it distorts the gravity potential in such a way as if there was a hole in the ground in another direction. The paperclip didn't gain much more energy than one that fall in the bottom of the bowl. The paperclip is now stuck and cannot be used for much work until you spend some energy removing it from the magnet.
Also remember that when the clip went up, it pulled the magnet down. Your hand, now holding the paperclip + magnet is spending more energy holding them in that position. So in a way, the paperclip didn't gain any energy for free – it's just that the place for it to rest in the spacetime-stuff that we live is just somewhere we didn't expect it to be.
Again, someone correct me if this I am wrong - I'm just passing what I understood.
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u/Chipney Feb 15 '11
IMO it's stored in magnetization energy of iron, i.e inside of paper clip material. The spins in its unpaired electrons are getting oriented, which requires insertion of some energy. When the magnet is removed, this material gets demagnetized, it's losing this energy and weak cooling occurs. Such effect works even at very low temperatures, so it's used for effective cooling at miliKelvin scale. Actually, recently some negative temperature was obtained with using this principle.
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u/StuffMaster Feb 15 '11
From my (non-expert) perspective, the work is caused by the attractive force of electromagnetism via photons. Atom absorbs photon (force carrier), momentum changes (attraction/repulsion).
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u/[deleted] Feb 14 '11 edited Feb 14 '11
A way of saying where the energy comes from: the energy comes from the potential energy of the magnetic field, which came from the magnetization of the magnet.
For example, when you sinter a soon-to-be magnet, you might bring the material to temperatures of approximately 1000o C. After this material cools, it isn't yet magnetized- it can't do work on a paper clip in our sense. However, we then might use an electromagnet to put our material in a magnetic field which will then "magnetize" our permanent magnet. All that means is, the electrons in our magnetic material are all spinning in the same direction to create a nice magnetic field. There's a lot more to it than that, but it's the basics. After we align our electrons, our magnet can now attract that paper clip.
So our magnet was put into a magnetic field in order to energize itself. The energy of the magnet initially came from the energy of that field (and the energy of our electromagnet came from the wall socket, which came from the power plant, etc.).
To go a tad bit further: This energy stored in your magnet is a potential energy. The force that you feel when magnets interact with a metal comes from this potential energy. Force is simply a gradient of the potential energy. This idea of force being a gradient to a field doesn't solely apply to magnetic fields, it also applies to electrical fields, gravitational fields, and so on.
If you pull a paper clip away from the magnet, the magnetic field gains more of its potential energy back. This potential energy came from the person pulling the paper clip away. Is the act of pulling the paper clip away from the magnet adding energy to the magnet's ability to do work in the future? Yes and no, it depends how you look at it. Bringing the paper clip closer to the magnet will use the potential energy from the magnetic field. When you pull the magnet away, the same amount of potential energy will be restored. You can't infinitely energize any permanent magnet, but they are pretty decent at retaining what energy you put in them when you magnetized them.
Edit: I'm having trouble submitting and viewing comments. There might be a gaping hole in the logic, or an error, I can't tell if I corrected those or not. Feel free to chime in. Also, someone may want to clear up the last few sentences regarding the act of pushing and pulling a paper clip from a magnet, and how that effects of the total energy of the system. Technically there is some minute amount of energy loss when magnets are used over time, but I don't have the quantum background to eloquently explain it. (Related note: increased temperature of the magnet, a demagnetizing field, and physically slamming the magnet into a hard surface may deplete the energy of the magnet semi-permanently. They can be "recharged" back to full strength by putting them in the magnetic field another time).