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Magnetism and circles

  1. Nov 16, 2011 #1
    I was watching some FX yesterday and Robert Downey's best, IronMan was on. I got to thinking, if possible, how does his little energy producing circle actually works. The idea is this:

    Picture a circle lined with magnets all facing negatively inward. In the center is a blade with parallel edges, much like a propellor of a lawn mower. At those edges, you place magnets facing negatively outward.

    Question: if this circle and blade are aligned on the same plane would the blade spin dependent on the angle of the outward pushing magnets, and if so for how long?

    I understand perpetual motion isnt possible but I can foresee many uses of a long lasting rotation.

    Thanks
     
  2. jcsd
  3. Nov 16, 2011 #2

    DaveC426913

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    By movie FX.

    It had to be said. :biggrin:

    But we'll look at your design aside from the movie FX source that inspired it.

    The device would not make a single rotation. It would rotate a few degrees to place the negative poles as far as possible from each other (depending on how many magnets in your rings, but no more than 90 degrees) and then it would stop.
     
  4. Nov 16, 2011 #3
    What about with three blades. The whole idea is if the the force pushing out on an angled surface would push against the outer ring disproportionately, sending the inside structure into motion
     
  5. Nov 16, 2011 #4

    DaveC426913

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    And indeed it will move - as much as 60 degrees before stopping.

    Once the inner magnets reach a point of maximum distance from the outer magnets, they are being repelled, and will not have enough momentum to reach them, let alone move past them.
     
  6. Nov 16, 2011 #5

    Drakkith

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    Ryanuga, take away all the magnets and just imagine a blade spinning on a shaft. What happens if you try to produce energy from it? It slows down. So the maximum amount of energy you can get out of it depends on the mass of the blade and the speed of rotation. So long lasting rotation is ONLY possible if we DON'T try to get power and work out of it. Make sense?
     
  7. Nov 16, 2011 #6

    DaveC426913

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    The problem is, it's even worse than that. Even with zero load and perfect frictionless system, the rotor will not rotate even once.
     
  8. Nov 16, 2011 #7

    Drakkith

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    Of course. I was pointing out the fact that even if you stuck your finger inside and started it spinning it only has as much power as you just gave it. The OP seems to think that long lasting rotation equals lots of power, which is not the case.
     
  9. Nov 16, 2011 #8
    How will the magnets reach a maximum distance between each other when the blade is centered and confined within a circle. The distance will be equal all the way around wouldn't it
     
  10. Nov 16, 2011 #9

    DaveC426913

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    Yes.
     
  11. Nov 16, 2011 #10

    Drakkith

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    I think he means the distance between the poles. When one of your blades is in the middle of two poles of the magnets in the circle they will be at maximum distance from them.
    (Imagine the minute hand on a clock as your blade and the poles of the magnets being the numbers. It would stop in the middle of two of the numbers.)
     
  12. Nov 17, 2011 #11
    Is there a way to post a document or design on here, I feel like it would be easier to explain my idea better
     
  13. Nov 17, 2011 #12

    Drakkith

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    When you hit the "New Reply" button, there are options above the text box for attaching things. The paperclip button will do attachments of documents.
     
  14. Nov 17, 2011 #13

    DaveC426913

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    You certainly can, but I feel it is important to point out that we get your idea. There's no confusion.

    You are not the first person to suggest the use of permanent magnets as a source of work. Regardless of the details of your particular setup, it is flawed in principle. What you are trying build - though you do not realize it yet - is a perpetual motion machine.

    And discussion of PPMs is explicitly forbidden here on PF.

    We'd like to help you learn but this thread will not last long on its current course.
     
  15. Nov 18, 2011 #14
    It's easy to suppose that if the outward pushing magnets, the ones attached to the blade, are only tilted the right amount you will get a force tangential to the circle at any point you choose, a force that should, you think, keep the blade moving.

    In fact, though, if the outer magnets form any kind of uniform field, there is no way to orient the inner magnets attached to the rotating blade so that they feel any net force one way or the other.

    Suppose all the outer magnets are oriented with their north poles facing inward. And suppose the magnets on the blades are oriented with their north poles facing outward. The blade magnets have no incentive to move from one repulsion into an equal repulsion. You could repel them into a weaker north pole, for whatever that gets you; you could repel them into an area of no north repulsion (i.e. a place where there's no magnet), and you could most easily repel them into any place into which it's also attracted by a south pole. But they won't move by themselves into an area equal in repulsion to the one they're already in.

    Now suppose the blade magnets are all south poles. You have the same problem in so far as they are not attracted to any north pole down the line any more than they are attracted to the one they're already facing. No motion.

    Now, and this is what I suspect you think will work, suppose you put the magnets on the blade sideways so that just as much north pole is exposed to the outer ring as south pole. Shouldn't that create repulsion at the north pole and attraction at the south pole such that the blade will start to rotate and will rotate so long as the magnets last? Unfortunately the repulsion and attraction don't act in the direction you want them to. The north poles will repel straight inward, toward the center, and the south poles will attract straight outward, from the center. You'll have radial forces instead of the tangential forces that would make the blade move. There's no way to arrange the inner magnets to get the disproportional (unbalanced) force you want in the direction you want it.
     
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