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New idea for experiment

  1. Jul 30, 2012 #1
    If you could suspend a magnetic sphere in a electromagnetic field, how fast could you spin it?
    Last edited: Jul 31, 2012
  2. jcsd
  3. Jul 31, 2012 #2
    You might look up the spinning rotor gauge. It sounds similur.
  4. Jul 31, 2012 #3


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    Gold Member

    Nukedropout, Welcome to Physics Forums!

    Here is an experiment that spun the suspended sphere so fast that it disintegrated:
    J.W. Beams, “The Production of High Centrifugal Fields”, Journal of Applied Physics, 1946, pp. 886-890
    High centrifugal fields were produced by spinning small solid steel spherical rotors up to their bursting speeds. The rotors were supported magnetically in a vacuum by an improved method and spun by rotating magnetic fields. The peripheral velocities at which the rotors of various sizes, made of the same (flaw free) steel and having the same shapes, exploded, were roughly the same and of the order of 105 cm/sec. A centrifugal field of 2.4×108 times gravity was obtained with a .795‐mm spherical rotor which was the smallest diameter tried. Calculations indicate that in some cases plastic flow probably occurred in small regions near the centers of the spherical rotors somewhat below their bursting speed.

    Here are some cool toys that suspend spheres magnetically:

    Levitron is a brand of levitating toys and gifts in science and educational markets marketed by Creative Gifts Inc. and Fascination Toys & Gifts. The Levitron top device is a commercial toy that displays the phenomenon known as spin stabilized magnetic levitation. This method, with moving permanent magnets, is quite distinct from other versions which use changing electromagnetic fields, levitating various items such as a rotating world globe, model space shuttle or VW Beetle, and picture frame.
    http://www.fascinations.com/unique-toys-gifts/levitron-revolution-series [Broken]

    If you like, you can buy some cool High strength NEODYMIUM SPHERE MAGNETS to experiment (or play) with:

    Finally, this article seems to indicate there are limits to how fast the sphere may rotate. I do not pretend to understand this:
    Rev. Sci. Instrum. 43, 1413 (1972); http://dx.doi.org/10.1063/1.1685456 (5 pages)
    Residual Drag Torque on Magnetically Suspended Rotating Spheres
    J. K. Fremerey
    Institute of Physical Chemistry, University of Bonn, 53 Bonn, Germany

    An almost quantitative explanation of the residual drag torque on freely spinning steel spheres in magnetic suspension is given by comparing previously published experimental data with theoretical results of Keith (1963). Accordingly, the residual drag torque is mainly caused by the earth's rotation. The associated decay ratio −ω∕ω increases quite linearly with the rotor angular velocity ω. A distortion of the rotational symmetry of the supporting magnetic field enhances the decay ratio. This effect decreases with increasing rotor speed. The influences of rotor imperfections and of lateral rotor vibrations are also considered.

    © 1972 The American Institute of Physics
    http://rsi.aip.org/resource/1/rsinak/v43/i10/p1413_s1?isAuthorized=no [Broken]

    Last edited by a moderator: May 6, 2017
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