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Magnetic levitation and suspension

  1. Jun 4, 2007 #1
    I was watching a video on youtube which showed how superconductors worked in magnetic levitation, and something struck me, the effect of magnetic levitation and suspension, where the magnet is both attracting and repelling at the same time thus keeping the superconductor at the same distance, made me think that would it not be possible to achieve the very same effect with permanent magnets at room temperature?, i may be wrong, and i have a feeling that may be an all too familiar case, but surely if you took a neo magnet and fixed it securely to a surface so that its repelling pole was facing outward and then surrounded it with other neo magnets, again fixed securely the surface but with their attracting poles facing outwards, would you not create an area that both attracted and repelled an object at the same time, thus causing anything that was placed with in this field to not only levitate but also overcome unstable equilibrium.? seems logical to me.
    Michael
     
  2. jcsd
  3. Jun 4, 2007 #2

    ZapperZ

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    Magnetic levitation for ordinary magnet is nothing new. The reason why such phenomenon are usually done with superconductors is that the magnetic field generated is exceedingly strong for the particular SIZE of the material. A comparable ordinary magnet of the same size typically cannot generate enough magnetic field to hold it up against its weight.

    Furthermore, to generate stability, a Type II superconductor having flux lines penetrating the its bulk can actually help with this issue. So such lines are actually beneficial to cause it to resist being pushed off.

    Zz.
     
  4. Jun 4, 2007 #3
    That's the problem of course (Earnshaw's theorem). One way you can get around it is with a spinning piece of iron (until friction takes its toll), and I suspect the lack of internal friction is similarly key to levitation with a supermagnet. Or you can confine the parts somehow (mechanically preventing the magnets from flipping around in the air) so that stability becomes irrelevant. There's also active control, and diamagnetic materials.

    ZapperZ, did you just make all that up? :grumpy:
     
    Last edited: Jun 4, 2007
  5. Jun 4, 2007 #4

    ZapperZ

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    Nope... Read what the OP was actually trying to do - magnetic "repulsion" by alligning like-poles together. This is nothing new and people have tried to confined suspended magnetic objects like that. So why would I make that up?

    Zz.
     
  6. Jun 4, 2007 #5

    siddharth

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    Last edited: Jun 4, 2007
  7. Jun 4, 2007 #6
    I assume you are referring to unstable magnetic levitation (which requires additional non-magnetic confinement, active control, diamagnetic material or rotating parts, in order to sustain itself). However, the OP seems to be referring to stable magnetic levitation, as is exhibited by any magnet above a type I (and slightly concave ) superconductor, and which is theoretically forbidden for ordinary magnets.

    Can you give any reference for that please? (I say it's obviously wrong.)

    Yes, that resists horizontal motion, but is unnecessary as levitation is also stable above a concave Type I.

    I'm not sure that you've interpreted this properly.

    No. To ever experience magnetic repulsion from below, surely your levitating "object" will be a magnet, right? Now, what's to stop your magnet from flipping over suddenly (it isn't so interesting if you mechanically confine it)? Now, the overwhelming repulsion from below has become an overwhelming attractive force, so what can stop the magnet falling? Even if the magnet didn't flip, the surrounding attractive magnets will only further encourage it to drift to one side, upsetting the equilibrium again. The details aren't always obvious, hence the mathematical theorem.
     
  8. Jun 4, 2007 #7

    ZapperZ

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    I don't think you've interpreted what I wrote correctly.

    A superconductor placed on a magnet levitates because of the induced current that generates an external magnetic field the repulses the field from that magnet. I assume that this is not the point of contention.

    Now, do the same for a "metal" that isn't in a superconducting state. Even when you move the metal to induce current in it to generate a magnetic field, such a field is too weak to be able to hold up the metal, especially when the force generate is trying to stop the metal from moving and reduces the induced current and field. At any given time, while the metal is trying to fall, the change in magnetic flux on the metal's surface doesn't generate enough of field to sustain a levitation.

    I addressed the OP question via pointing out that even when you don't do the complicated stuff, simply via using a simple metal plus magnet set up, you already face with no possibility of doing this, even when one assumes that one can align the metal "perfect" with the magnet that maintains it in its unstable equilibrium. Now, if one thinks that one can levitate a frog in a different geometry than this is a completely different matter.

    Zz.
     
  9. Jun 4, 2007 #8
    I'm disagreeing with your statement that superconductors are required (rather than ordinary magnets) because of their size to magnetic field strength ratio.

    It is true that a piece of metal will not levitate above a magnet, whilst a piece of superconductor will. This is irrelevant since nobody expects an ordinary piece of metal to levitate in the first place (It does not justify your statement which compared superconductive magnets with ordinary magnets, rather than referring to ordinary metals); the OP is only suggesting replacing superconductive material with permanent magnetic material. An ordinary permanent magnet will, like a superconductor, levitate above another magnet (but both are normally unstable in that configuration).

    I presumed the OP was comparing to the (inverted) arrangement where the magnet is levitated above the superconductor. This can easily be made stable (by creating a slight "well" shape in the superconductor, or by restricting the directions along which it is superconductive). Again, if the superconductor is replaced with ordinary magnets they will typically be strong enough to levitate the first magnet, but it can not be made stable simply with changes to the geometry of the supporting magnets.
     
  10. Jun 5, 2007 #9

    ZapperZ

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    This is what I wrote originally:

    I can see how you might have interpreted it that way. When I said

    "Magnetic levitation for ordinary magnet is nothing new."

    I meant IN GENERAL, as in using the diamagnetic effects to levitate objects in an external magnetic field. They claimed to have done this with frogs! That is why I said this phenomenon is not new. Now, I assume we have no issues with that.

    However, in the next line, I went specific to using an ordinary metal instead of a superconductor in a magnetic field. This may not answer the OP question directly, but it is rather an explanation on why one can't have a large enough magnetic field to overcome the object's weight. There is just never a large enough induced current to create an opposing field. Compare to the coverage of the the external field to the surface of the object, you cannot generate enough of such opposing force, unlike the supercurrent in a superconductor.

    Zz.
     
  11. Jun 16, 2007 #10
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