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Levitation with Permanent magnets

  1. Oct 24, 2008 #1
    Hi everyone, I've been trying to find info about getting an object to levitate and how strong the magnets need to be but haven't had much luck. I'm trying to build a levitating picure frame using permanent magnets rather electromagnets. The plan was to take apart a lamp and put in two ring magnets at the base and at the end of it. I wanted to put two magnets at the top and bottom of the picture frame as well. I included a picture of what i want it to resemble as well.

    [​IMG]

    First of all... will this work the way i have it?

    Secondly, assuming that the picture frame and photo will be a few pounds in weight, and I want it to be about .5-1 inches from each magnet, how strong do the magnets need to or what kinds of magnets should I be trying to find?
     
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  3. Oct 24, 2008 #2

    Nabeshin

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    How in the world do you plan on making this stable? Sure, if you set this up, the thing will kinda undergo SHM at any little perturbation along the axis aligned with the magnets, but any other slight change in position will cause the magnets to torque the picture, and there goes grandma :(
     
  4. Oct 24, 2008 #3
    Well, shouldn't the ring magnets keep the magnets on the frame in the inner circle of the ring?

    [​IMG]

    This, except upsidedown. And having that on both the top and bottom of the frame makes it stable. I could be wrong?
     
  5. Oct 24, 2008 #4

    mgb_phys

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    It can be done - you can get small map globes that do this.
    Balancing the fields is a little tricky, I don't know if you can do it with a flat picture frame - I think you might need a ring of magnets in the horizontal plane on the floating part.
     
  6. Oct 24, 2008 #5
    I definitely know it's possible, simply because the idea comes from this: [​IMG]

    The main problem is that they use electromagnets, and from my understanding they are stronger than permanent magnets? Either way, my main problem is figuring out how strong the magnets need to be? Is there an equation that relates the amount of lift or push a magnet gives at a point away from it's surface?
     
  7. Oct 24, 2008 #6
    The problem is not with the strength of the magnets its balancing the force created by the fields of the magnets. Because of this, you have two choices, use an electromagnet on the top or bottom of the cradle with a control circuit, or use some type of di-magnetic materials. If you look at all the floating picture frames and globes out there they all require power to operate their control circuit and electromagnets.

    [​IMG]

    I strongly suggest you just build something with an electromagnet and control circuit like the globes use because its much more practical, simpler, and cheaper. Or you can do it the hard way.

    http://www.fieldlines.com/other/diamag1.html
     
  8. Oct 24, 2008 #7
    I would use an electromagnet, but batteries die out and plugs are limited to where they can reach an outlet. The whole point of using the permanent magnets is that they will not die out. And though I understand that the batteries will last a while and this and that, I wanted to make this so that you can put it somewhere and no longer worry about it and also, on the chance that the batteries do die out, there goes the picture frame.

    Having said that, what kind of dimagnetic materials are we talking about here. I've been looking at neodymium magnets, but I'm not exactly savy in magnetics if you haven't noticed yet. Do they work the same as regular magnets, just with a particularly high strength?
     
  9. Oct 24, 2008 #8

    DaveC426913

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    I've always wondered about this. Every device I've seen uses electromagnets.

    I'm not convinced it's possible in principle to build one with permanent magnets.
     
  10. Oct 24, 2008 #9
    Why wouldn't it be possible?
     
  11. Oct 25, 2008 #10

    f95toli

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    It is quite literally impossible to create a stable configuration using a static field unless you use a diamagnetic material (and AFAIK the only materials that are "diamagnetic" enough to be practical are superconductors), this is known as Earnshaw's theorem.
    Look at the wiki if you want to know more.

    Hence, in order to "balance" something you either need a control circuit which continuously adjusts the field or a rotating field, the latter is what is normally used for e.g. magnetic traps.
     
  12. Oct 25, 2008 #11

    Danger

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    Excel, there is a very simple solution to this, but you probably won't like it.
    Magicians use something called 'invisible thread' for levitating cards, cigarettes, etc., and it can be obtained at any magic supply house. You can't see the stuff from more than a few inches away. It's strong enough that you could suspend your picture from a couple of strands, and nobody could tell that it wasn't defying gravity.
    I know that it isn't what you want, but it might serve your purpose.
     
  13. Oct 25, 2008 #12
    Indeed. The second link I posted shows how to perform magnetic levitation without super conductors but for an application such as this it is impractical.
     
  14. Oct 27, 2008 #13
    Hi, excel110. I only have a couple of minutes here, but would a setup something like I've sketched below work for you? That is, if it must be the case that the picture frame "hangs" on magnetic threads (as it were) from above rather than being supported by a magnetic cushion from below. Both would work to achieve the levitation effect, although I think the "hanging" method would be more stable.

    [​IMG]

    What you could try is to place a powerful permanent magnet (in the diagram: the large magnet at the top) which is more than strong enough to support your picture frame at a 1-inch distance (or whatever distance you choose). It is important that it be more than strong enough.

    Below this magnet I have placed a much weaker magnet, with poles oppositely aligned with the larger magnet.

    The picture frame has a magnet embedded in it as shown. The picture frame magnet is therefore attracted to the strong magnet and repelled by the weak magnet. It seems to me that the picture frame would hang stably at a certain distance. If the frame were to vibrate or wobble such that it drifted slightly downwards, it would quickly fall out of the range of the weak, repelling magnet above it, but only very slowly fall out of the influence of the stronger, attracting magnet. This is because the two control magnets are at different distances from the picture frame's embedded magnet, and their magnetic fields diverge in a roughly inverse-square manner. Well, I'm out of time now, but am I hopelessly wrong here or would this work?

    - m.e.t.a.
     
  15. Oct 27, 2008 #14

    RonL

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    Look at this link, and see if the principle will apply to what you are trying to do.


    http://www.centralhobbies.com/props/balancer.htm
     
  16. Oct 27, 2008 #15

    f95toli

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    You are wrong:wink:

    As I pointed out above: it is IMPOSSIBLE to find a stable configuration using static fields, i.e. unless you have stumbled across a flaw in Maxwell's equations you are wrong...
     
  17. Oct 27, 2008 #16
  18. Oct 27, 2008 #17

    f95toli

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    I don't have to, that's the nice things with theorems: they apply to ALL designs.

    However, your design is so simple that if you think about it a little you will realize that it is critically unstable, ANY disturbance will cause it to collapse. The point of Earnshaw's theorem (which follows directly from Gauss law) is that this is true for all designs that use static fields;in mathematical terms: there are no local maxima or minima; only saddle points.

    Or, to put it another way, if you were to analyze your system using control theory you can be sure that you would find that you will always end up with poles in the right hand plane...
     
  19. Oct 27, 2008 #18
    m.e.t.a., unfortunately it is impossible with permanent magnets. Anything balancing in the middle would be too unstable -- if it moved even a little in either direction (assuming you could get it to that perfect place in the center, it would fly off or flip. I was fortunately able to get my hands on a former physicist and some big ring magnets, and he was able to show me why it wouldn't work. Thanks everyone for your help though.
     
  20. Oct 28, 2008 #19
    Shame -- it would have made things so much easier! I still haven't managed to spot the flaw, so perhaps understanding will dawn if I prove it to myself with real magnets. Good luck, Excel.

    - m.et.a.
     
  21. Oct 28, 2008 #20

    DaveC426913

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    I have a Levitron. It is awesome. It suspends a weight in air using only permanent magnets, i.e. seemingly defying everything we've just established.

    The catch of course, is that it needs to spin to keep balanced. As soon as the spin slows the weight flips over and crashes down.
     
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