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Can you use magnets to push other magnets unlimited amount of times?

  1. Aug 30, 2012 #1
    As I was bored at the lesson, I did some thinking of my own and magnets always fascinated me, still kinda like black magic.

    Anyway, i was thinking abou creating this:

    http://img201.imageshack.us/img201/7476/prepetummobile.jpg [Broken]
    I created this in paint as fast as possible so it's not super clear, but the light blue lines in the vacuum tube hae to represent the direction in which the magnets point their energy or whatever they deliver, the magnetic ball is, at the surface, of the same type as the other magnets, so instead pulling together they should be pushing each other, from each other, right? Place the tube horizontal on a surface, create vacuum in the tube to reduce friction, put the ball in it and give it a little move... prepetum mobile?

    My teachers said it Could be possible but you have to research much more than I did yet..

    In short: can the ball move without stopping if the magnets are pointing in the right direction?
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Aug 30, 2012 #2


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    It is very unfortunate that your teacher would indicate that such a device could be possible. What you have here is a variation on a fairly common perpetual motion design. You can read Wikipedia article on perpetual motion to get some idea of why these sort of devices don't work in general.

    Specifically why this particular design doesn't work requires a bit of a better understanding of magnets. For starters, a "magnetic ball", like the one you show, is impossible. It has to do with the fact that magnetic field lines cannot terminate. If you start at a field line passing through one of the poles, it will go out, wrap around to come back to the other pole, and then return to the point it started from through interior of a magnet. So an outer surface of a magnet cannot be all one pole.

    This isn't crucial to this design, however, and it can be modified to fix that particular issue. However, same kind of rules are followed by all of the outer magnets, and if you actually connect all the field lines properly, you would see that the pattern does not provide you with a constant pushing force in one direction. Instead, you'll find places where the "ball" tends to get stuck.

    There are some specific theorems that deal with why you can't create a cycle where magnet is being pushed always in the same direction, but they involve vector calculus, so they are probably a bit above your level.
  4. Aug 30, 2012 #3
    Actually the biggest problem is making the ball.
    If I understand your drawing, it should have only one magnetic pole since its other pole is inside.
    However this design is impossible in the real world because of the magnetic fields properties.
  5. Aug 30, 2012 #4
    Not quite you could stick lots of small magnets into the surface ball like object.Like cocktail sticks in an orange.The north end of the magnet just sticking out.
  6. Aug 30, 2012 #5


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    No, you cannot, because divergence of magnetic field is zero.If you do place magnets like that, you'll either end up with field escaping through gaps, meaning you'll have surface consisting of north and south poles, or if you manage to close gaps perfectly, you'll kill the magnetic field.
  7. Aug 31, 2012 #6
    If wern't so perfect could you get the desired effect.
  8. Aug 31, 2012 #7


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    No, because the gaps would act as the opposite pole. You can sort of think of magnet's poles as magnetic charges. Well, the amount of positive and negative magnetic charge on magnet's surface must be equal. So you can either have both North and South poles, or you can have neither.
  9. Aug 31, 2012 #8
    I guess this is not the only design flaw
    Even if making the sphere was possible, you would have as many magnets pushing before and behind the ball, which would anihilate the thrust.
  10. Sep 1, 2012 #9
    As far as I remember such a ball would have a magnetic field of zero. There is some integral law that guarantees this if I remember correctly.
  11. Sep 1, 2012 #10


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    There's no perpetual motion. There's no magnetic monopole, which is what you are trying to create with your sphere. But the negative and positive poles of the sphere cancel out and give no overall magnetic charge. The integral law is known as Gauss's law which is a special case of Stokes' Theorem.
  12. Sep 1, 2012 #11

    Vanadium 50

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    Sorry - we don;t discuss perpetual motion machines here.
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