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Misc. Explain this trending science DIY video

  1. Dec 2, 2014 #1

    My take is:
    The magnets are conductive and complete an electric circuit composed of the battery and a section of the coil (toroid) thereby forming a solenoid. The battery-magnet assembly is in the center of the solenoid. The magnetic field of the solenoid is maximum at the center and decreases towards the end. Move further away from the ends and the field decreases even faster.
    The north and south poles of the magnet is oriented so that the magnetic field of solenoid and of the magnets are oppositely oriented, and the solenoid field will tend to push the magnets away from the center. And hence the assembly moves away. Repeat.

    If the assembly is flipped over, it will still work, because the battery polarity will also be reversed and the solenoid will produce magnetic field in the other direction. However, if only the magnets are flipped over the battery, then I think the assembly will tend to be stuck at the center. Also, instead of flipping the magnet, if a different toroid, where the wire is wound the other way is used, it will fail.

    Am I right?
  2. jcsd
  3. Dec 2, 2014 #2
    Well, the magnetic field strength of the solenoid (B) depends on the current of the circuit (constant) the number of turns. The number of turns is the number of turns between the plus and minus, so that is pretty much constant through the whole, so I don't think that the field strength will change very much.

    For your second question, yes it would work the same way if the whole assembly was reversed, the solenoid N will become S, and the Magnets N will become S as well.

    But if just the magnets are flipped around, then the solenoid will remain the same (battery isn't flipped), but the magnets will be the same, so it wouldn't just be stuck in the middle, the solenoid would repel it outward as soon as both magnets touch the coil.

    The polarity isn't affected by wether the wire is wound clockwise or counterclockwise, only which end is connected to positive and which to negative, so if just the solenoid is flipped around, the train would work the same.

    Please somebody correct me if I am wrong.
  4. Dec 2, 2014 #3
    I agree on that.
    But, when I talked about field strength decreasing, I was actually talking about the magnetic field distribution of a solenoid, at any given instant, formed around the battery-magnet assembly. I was trying to make the point that, since the magnetic field is non-uniform the magnet-assembly CAN have a net force. If whole of the toroid was powered by external power supply, thereby creating a near-uniform magnetic filed throughout, I think there will be no net force on the magnets, and hence it won't move.

    In that case, the magnetic fields of the instantaneous solenoid and the magnets would be facing the same way, and hence will tend to pull each-other not push.

    The direction of magnetic field produced by a coil depends upon the winding direction of the coils.
  5. Dec 2, 2014 #4
    Oh yes, the rotational direction of the current you are correct. Solenoids ARE different from electromagnets in that respect, oops, my bad.
    I'll also need to remind myself that a coil simply turned around is still wound the same way as well, which isn't the same as reversing the winding.

    I initially thought that the magnets were oriented to the solenoid, however I was wrong about that too, if that was the case they would be attracted to the center, so you are right they would be stuck in the middle if the magnets were flipped.

    I better check myself before I wreck myself.
  6. Dec 8, 2014 #5


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    Nobody yet mentioned how cool that demo is! Why aren't they in the shops right now?
  7. Dec 8, 2014 #6
    Finally someone to appreciate its coolness. :). I had played a lot with wires, and magnets, but it never occurred to me that you could make such a cool thing with just those basic ingredients. I think somebody just doesn't first imagine they are going to make this and then make, rather I believe, these things are accidentally discovered.
    BTW, what do we call this thing? Its certainly not a coil gun, not a rail gun (although quite near), not a magnetic train, not a motor. Maybe, its some form of permanent magnet linear motor?
    The fact that the power source is travelling makes it distinct from other elementary electromagnetic motion system, I think.
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