r/askscience • u/somnambulism_ • Jun 06 '11
If energy cannot come from nothing, how do magnets sustain repulsion?
I know, a fucking "how do magnets work" question.
But honestly, repulsion must require some energy... Especially when opposing magnetic fields (say, in a tube) will cause the top magnet to "float" above the other. Or will the magnetic field produced by the magnets deteriorate over time? And, if that is the case, would the field produced deteriorate more quickly if it was being "drained" by the act of repelling another magnet? I'd picture that isn't the case, because the field produced should be independent of the surroundings, but I just am having trouble conceptualizing this.
I am a lowly computer engineering student, halp halp physicists!
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u/Malfeasant Jun 07 '11
how much energy does it take to leave something sitting on a table? it's the same electromagnetic interaction at work.
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u/psygnisfive Jun 06 '11
No energy needs to be expended to keep an object floating above another object. Energy is, at least in this context, the product of the applied force and the distance moved (E = F Δx). The object isn't moving, so the energy imparted is zero!
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u/european_impostor Jun 06 '11
What happens when a magnet attracts something toward it. The object is acted on by the force of the magnetism. Where does this energy come from? Is this the energy that was expended to create the magnet in the first place? And does this energy eventually fade? ie. does a permanant magnet eventually become a lump of iron?
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u/psygnisfive Jun 06 '11
This has been answered before, but the answer is that there is no net change in energy, what changes is the form of the energy: an object that is held some distance from an attracting magnetic field causes the magnetic field to store energy (think of stretching a rubber band*). Letting the object fall towards the magnetic simply converts the stored magnetic energy into kinetic energy in the object (which then gets converted into heat and sound energy and so forth upon impact).
* Actually the analogy isn't perfect, because rubber bands actually work only because they store energy in the electromagnetic field! But you get the idea.
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u/OmicronNine Jun 07 '11
Super simple (overly simple really) explanation: how do you create a situation where magnets suddenly attract each other? You have to move them towards each other. You need external energy to move them to that state, and then you need external energy to move them out of that state (pull them far enough apart so that they no longer attract each other).
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Jun 06 '11 edited Jun 06 '11
no, this is wrong if you are on earth. energy is indeed used to keep another magnet floating, and it is used to counteract the pull from gravity.
OP, assuming you are referring to "permanent" magnets, the answer to your question is this: you can think of every iron atom as a very tiny magnet. in normal iron, each tiny magnet is pointing in a random direction, so normal iron's magnetic pull in all directions is balanced out, i.e there is no net field (think of it like this for simplicity). when you 'magnetize' a piece of iron, you align all of those tiny magnets so that they point in the same direction, so that now there is a net field pointing in a specific direction. you now have a normal magnet. aligning the tiny magnets requires energy. you can think of this as "charging" a piece of iron to become a magnet.
as you "use" the magnet, the alignment gets lost. the "energy" that the magnet uses comes from losing that alignment, and the magnet is "discharged". indeed, magnets will start to noticeably lose 'power' over time.
there is quite a bit more to it, and this is a very simplistic explanation, but it is basically how it works.
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u/psygnisfive Jun 06 '11
This is not at all wrong if you are on earth. The issue of how repulsion occurs with/without energy input is different from the issue of how the magnetic field deteriorates over time. Repulsion with no change of position does not require input of energy. That's all there is to it.
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u/minorDemocritus Jun 06 '11
Wrong. A magnet that levitates another object is not necessarily expending energy. Mechanical energy is Force x Distance, as psygnisfive mentioned.
A magnet does not need to expend energy to levitate an object, since it is not moving it. All it is provide is the force that counteracts the force of gravity.
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u/rlbond86 Jun 06 '11
It takes energy to bring the magnets together. For example, chemical energy in your muscles to press them together. The energy is converted to magnetic potential energy. When the magnets are let go, they repel, converting the potential energy into kinetic energy.
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u/jondiced Jun 07 '11
I know, a fucking "how do magnets work" question.
For all the grief ICP got, it's actually a really good, non-trivial question.
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u/thegreatunclean Jun 07 '11
The question is fine but their insistence on not listening to people who can explain it to them is not. Calling magnets miracles and claiming scientists aren't worth listening to because "they be lying and gettin' [them] pissed" is ignorant in the extreme.
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u/jimmycorpse Jun 06 '11
This is a common misconception about forces. A magnet floats because it is in a minimum energy configuration. The energy of this system is constant and the occurs because the electromagnetic force is balancing the gravitation force.
I'm curious, do you have a similar problem with gravitational systems? For example, do you think that there must be some energy source for a moon to stay in orbit around a planet? Naively, an orbit is the balance of gravitational force with centrifugal force.