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Curious question about Magnets!

  1. Dec 1, 2004 #1
    When playing around with magnets, one can notice that they attract objects from a distance. No only that, but they can also attract quite heavy objects many, many times in a row. Where does the energy that moves the objects come from/stored? Does it ever "run out"? My guess is that magnets have electricity (in the form of magnetism) stored in them, kind of like a capacitor?

    Can anyone answer this? I'm very curious
     
    Last edited: Dec 1, 2004
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  3. Dec 1, 2004 #2

    krab

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    Yes, they attract many many times and each time you pull them apart again. Pulling apart means doing work. That's where the energy comes from.
     
  4. Dec 2, 2004 #3
    Suppose the magnet starts out from rest at a far distance away. Then there will be a difference in potential energy (like that of a particle which is at rest far from the Earth). The magnet will slowly start to move toward the original one which, say, is nailed to the floor. As the magnet picks up speed then its kinetic energy increases while its potential energy decreases - all the time the total energy remains constant. If, instead, you do work on it then you've supplied energy such that the kinetic energy goes to zero. The potential energy will be reduced from what it started out to be but the total energy (energy in yoru muscles doing work + initial potential energy) remains constant.

    It is then reasonable to ask what is doing the work since magntic fields can't do work - This is pretty complicated stuff so I don't have an easy answer at hand. There are articles written on this topic in the American Journal of Physics which I obtained but have not gotten around to reading yet. Its in the stack of the other 1,000 papers I want to read. :eek:

    Pete
     
  5. Dec 2, 2004 #4
    This property of magnetism is comparable to gravity as far as your question is concerned, you can think of it that way.
     
  6. Dec 2, 2004 #5
    It's amazing how strong magnetism is, compared to gravity. When you pick something up with a magnet, the magnetic force from your small magnet is overcoming the gravitational force of the whole Earth pulling in the opposite direction.

    The mutual gravitational force exerted between everyday human scale objects is so tiny that it is incredibly difficult to measure. In contrast you can measure the force between two small magnets using an ordinary spring balance.
     
  7. Dec 2, 2004 #6
    Ok, the gravity example was helpful, but what I'm trying to get at is for example :

    You have one magnet with its + pole facing UP sitting on the bottom of a plastic container that completely envelops the magnet on its bottom and sides. You then drop another magnet with its - pole facing UP into the container. We know that that magnet will "magically" float above the bottom magnet. What I'm trying to figure out is if the top magnet will hover there infinitely, or if it will gradually "lose its repelling energy" and drop.

    Krab said: Yes, they attract many many times and each time you pull them apart again. Pulling apart means doing work. That's where the energy comes from

    However, in this experiment, Im not pulling the magnet away or anything, it just floats there. In addition, the whole earth is pulling on the magnet

    I understand that if one were to do this experiment with electromagnets, the magnet will keep hovering as long as there is a current

    My guess is that the magnets gradually "lose their magnetism".....but does that loss proceed more quickly if the magnet is constantly repelling another magnet? Doesen't it take tons of enery to keep a mass floating in the air for such a long time? There must be some "output" of energy.

    Thanks
    Anton
     
    Last edited: Dec 2, 2004
  8. Dec 2, 2004 #7

    ZapperZ

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    Your question then is now no longer about the nature of the magnetic FIELDS but the nature and origin of MAGNETISM of the material. While they are related, they are not of the same field of study.

    If you are simply interested in the FIELD part of the problem, then no. Under the ideal condition given within this scenario, there is nothing to indicate that the magnetic field will leak out and lose strength over time.

    However, if you are looking at it from the MAGNETISM point of view, then there can be, because the nature of ferromagnetism involves statistical distribution at a finite temperature of the individual dipole moments in the magnetic material. I do not wish (and I don't think you want to read it) to go any deeper into quantum magnetism to explain all this.

    Zz.
     
  9. Dec 2, 2004 #8
    It doesn't take 'tons of energy' (your phrase) to just keep the magnet in the same place. You could just stand the magnet on a table. It stays put, supported by the table, and you (correctly) wouldn't say the table is using energy to keep it there.

    The magnets could stay repelling each other for years - thousands of years. Magnets may eventually lose their magnetism for a number of reasons, But magnetically levitating another stationary magnet isn't one of them.

    By the way - in your example, the supported magnet would have to be prevented from flipping over by the plastic tube it's sliding in. You can't make a stable arrangement of permanent magnets that will levitate. There is however a fascinating toy called 'Levitron' that is a levitating permanent magnet, but it is stabilised from flipping over by spinning like a top. Google on Levitron if you want details.
     
  10. Dec 2, 2004 #9

    DaveC426913

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    "It doesn't take 'tons of energy' (your phrase) to just keep the magnet in the same place. You could just stand the magnet on a table. It stays put, supported by the table, and you (correctly) wouldn't say the table is using energy to keep it there."

    The key here, is that no "work" is being done (i.e. causing a mass to move). Work is what requires energy. F=ma: if a is zero, then F is zero.


    "There is however a fascinating toy called 'Levitron' that is a levitating permanent magnet..."

    I have a Levitron and it is way cool!

    Attracts crowds to my desk every time I demo it. My favourite trick is setting it going and then working a coffee mug under the levitating top (which floats about 2 inches high ) so the top is completely inside the mug, all while never touching the top. Also, cupping it in my hand is cool.
     
  11. Dec 2, 2004 #10

    krab

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    This myth is amazingly widespread. The "magnetic fields can't do work" statement applies to the effect of magnetic fields on charged particles; not to the effect of one magnet on another.
     
  12. Dec 2, 2004 #11
    Magnets

    Ceptimus said: You can't make a stable arrangement of permanent magnets that will levitate.

    Couldn't you just have a plastic box that exactly fits the dimensions of the magnets, so that the magnets would just be able to slide up and down the box , but not flip over? That would make one magnet levitate above the other

    I think I get it now. A levitating magnet isn't doing any work, and magnets pulling other things is potential energy being transformed into kinetic energy
     
  13. Dec 3, 2004 #12
    Yes you are right. I should have made it clearer. Permanent magnets can levitate, but you need something like your plastic box, or some strings or something to stop them flipping over. The box or strings don't hold the magnet up - they just steady it.

    You might think you could make a frame with an arrangement of magnets - some to hold it up in the air, and some sideways facing ones to stabilise it etc., but no, it's been shown that any arrangement of magnets can not form a stable levitating arrangement - you always need something non magnetic to act as a stabiliser.

    The stabiliser can be strings or gyroscopic spin, like in the 'Levitron'. or maybe something else like air jets.

    By using electromagnets, energised by AC, you can produce stable levitation. You can also do it with superconductors.
     
  14. Dec 3, 2004 #13

    HallsofIvy

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    You seem to be confusing "energy" with "force".

    When a magnet holds something at a constant level, it is exerting a force equal to that object's weight but is doing no work and so is using no energy.

    There is a "conservation of energy" law. There is no "conservation of force" law.
     
  15. Dec 5, 2004 #14
    Force

    Oh, so is your example equivalent to a person leaning against a light and slow-moving go-cart travelling towards them, and thus preventing the go-cart from going forwards?

    The person is not expending enrgy to lean, but he is stopping the go-cart from moving.

    Is that a good analogy for the magnets?
     
  16. Dec 5, 2004 #15

    krab

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    Sure. You only understand this for go-karts? How about a concrete pillar holding back a go-kart? There is the same amount of energy required whether it is a concrete pillar or a person; namely zero energy.

    There are simpler examples. A hoist can lift my 3000 pound car, and in so doing does a lot of work. But holding it in place requires no energy at all. Otherwise you would have to believe my garage floor is doing work right now. It's not.
     
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