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Using magnetic energy.

  1. Jun 29, 2005 #1
    Using magnetic energy........

    I am not sure if this is the right place for this type of discussion, but I am wondering if it is possible to suspend a magnet (permanent magnet) among other permanent magnets in such a way, that it will rotate by it self based on arrangement of mantic fields as such that there is no point of equilibrium.

    Free rotation for the life of the magnates.
  2. jcsd
  3. Jun 29, 2005 #2


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    Many have tried, all have failed. No, it isn't possible.
  4. Jun 29, 2005 #3


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    No. Conservation of energy prohibits this.
  5. Jun 29, 2005 #4
    Conservation only prohibits it if we include friction, in which case newtons second law is prohibited by conservation of energy, in your sense.

    The arrangement in the attached picture works if we ignore friction and air resistance (a necessary condition for perpetual motion).

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  6. Jun 29, 2005 #5


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    Wait, what are we talking about here ?

    Is the OP suggesting rotation sustained by a torque that's somehow provided by the magnets ? Or are we merely talking about rotation subsequent to an initial torque ?
  7. Jun 29, 2005 #6


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    I don't get this bit. What does this arrangement do ?
  8. Jun 29, 2005 #7
    The arrangement is a magnetic dipole in a constant uniform magnetic field.

    We can associate a potential energy U with this system:

    [tex] U = \vec{\mu} \cdot \vec{B}[/tex]

    Where B is the magnetic field strength (teslas) and mu is the magnetic moment (ampere-meter^2).

    The bottom line is, this is a conservative system (because it has potential energy). The magnetic field applies a (non-constant) torque on the dipole and rotates it forever in the absence of nonconservative forces (friction, air resistance). Think of it as a close analogy to a gravitational pendulum.
  9. Jun 29, 2005 #8


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    Okay...how is it any more of a PMM than a pendulum is ?
  10. Jun 30, 2005 #9
    It's not. But the original poster asked for a magnetic configuration that would spin indefinitely, and I answered the question as best I could using well established physics.
  11. Jun 30, 2005 #10

    I don't get it. I have never seen a magnetic arrangement that causes a torque beyond equilibrium.
  12. Jul 1, 2005 #11
    This helps clarify the confusion.

    Ask yourself, does a gravitational pendulum exert a "torque beyond equilibrium"? (I actually don't know exactly what you mean by this). Of course a gravitational pendulum has a stable equilibrium, and if the ball moves from this (the middle) state, it experiences a torque that restores it to equilibrium.

    Do you understand how my magnetic configuration does the same thing?

    What allows a gravitational pendulum to swing? Conservation of energy means that the pendulum swings through equilibrium, up to another point of maximum potential. This is the case for a magnetic dipole in a constant, uniform magnetic field.
  13. Jul 1, 2005 #12
    Assume there is an initial torque (or push) and that there are no dissipative forces

    Does it have to rotate?

    3-dimensional, 2-dimensional or 1-dimensional equillibrium?

    Can you create a continuous 3-dimensional magnetic field that allows a spinning or non-spinning magnet to move about never settling in a neutral position? If it were large enough it would have to curve to match the earths surface or else it would be flat and 2-dimensional thereby the object would find an "area" of equillibrium, due to gravitational attraction. This would settle down to a "point of equillibrium" over time. This assumes that this is at all possible. Can continuous flux be created over a large surface area?

    1-dimensional could not create "pointless equillibrium" for the same reason a flat 2-dimensional surface wouldn't. How about a 2-dimensional line, one that curves like the earths surface, sounds like magnetic levitation vehicles on a track. They are not spinning. I would hypothesize that spinning and non-spinning objects could be in 2-dimensional "non-equillibrium" along a curved line only.

    Taking away dissipative forces I would imagine a constantly accelerating object circling the earth along a track. Does this make sense?
  14. Jul 1, 2005 #13
    No this does not make sense and definitely violates conservation of energy.

    It is kind of Escher type thought, where you are constantly falling to the same place without ever climbing back up :smile:
  15. Jul 1, 2005 #14
    I read up and found Earnshaws Theorem which helped me understand that rotation must be present to have levitation with permanent magnets. This makes sense with my observation of the "levitron" magnetic top. The mag lev comparison I made violates the permament magnet assumption. This is interesting stuff ! I like the "Escher type thought" comment. Later !
  16. Jul 1, 2005 #15

    Actually there is another "toy" you can make that accomplishes levitation without anything spinning. Looks pretty simple to make too, I think I might try it.

  17. Jul 1, 2005 #16
    I had that idea in 6th grade. It is certainly very interesting idea. I never tried to make it happen (material reasons) but I think that it is a concept worth of research.
  18. Jul 1, 2005 #17
    I had that idea in 6th grade. It is certainly very interesting idea. I never tried to make it happen (material reasons) but I think that it is a concept worth of research.
  19. Jul 3, 2005 #18
    Ideally in the absence of non-conservative forces like air resistance, friction, eddy currents, it is possible to bring a magnet into infinite rotational motion regardless of whether it has initial thrust or not.
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