Experiments with magnetic levitation
How can you magnetically levitate objects?
Magnetism is fascinating, especially when it is used to cause objects to levitate or float or be suspended in the air, defying the gravity which keeps us on the ground. How can this be done? There are 10 ways to magnetically levitate objects:
1. Repulsion between like poles of permanent magnets or electromagnets. However, there needs to be a way to constrain the magnets so they don't flip over and become attracted to each other. For example, floating donut magnets have the dowel rod in the center to keep them from flipping over.
2. Repulsion between a magnet and a metallic conductor induced by relative motion. However, the magnet needs to be restrained from moving in the same direction as the conductor, otherwise it will travel with the conductor.
3. Repulsion between a metallic conductor and an AC electromagnet. It is possible to shape the magnetic field to keep the conductor constrained in its motions, otherwise, a mechanical means is needed to keep the conductor in place. See also magpie.htm.
4. Repulsion between a magnetic field and a diamagnetic substance. This is the case of the floating frog, and the floating magnet between two diamagnetic disks.
5. Repulsion between a magnet and a superconductor. No mechanical constraints are needed for this.
6. Attraction between unlike poles of permanent magnets or electromagnets. This will work as long as there is a mechanical method to constrain the magnets so they don't touch.
7. Attraction between the open core of an electromagnetic solenoid and a piece of iron or a magnet. The iron or magnet will touch the inside surface of the solenoid.
8. Attraction between a permanent magnet or electromagnet and a piece of iron. Again, the iron needs to be constrained.
9. Attraction between an electromagnet and a piece of iron or a magnet, with sensors and active control of the current to the electromagnet used to maintain some distance between them.
10. Repulsion between and electromagnet and a magnet, with sensors and active control of the current to the electromagnet used to maintain some distance between them.
A couple of interesting links:
http://www.its.caltech.edu/~atomic/display/displaycase.htm good review of some levitation devices
Nature 400, 323 - 324 (1999) © Macmillan Publishers Ltd.
Nature magazine, July 22,1999, p323
Magnet levitation at your fingertips
A. K. GEIM, M. D. SIMON, M. I. BOAMFA & L. O. HEFLINGER
The stable levitation of magnets is forbidden by Earnshaw's theorem, which states
that no stationary object made of magnets in a fixed configuration can be held in
stable equilibrium by any combination of static magnetic or gravitational forces,.
Earnshaw's theorem can be viewed as a consequence of the Maxwell equations, which
do not allow the magnitude of a magnetic field in a free space to possess a maximum,
as required for stable equilibrium. Diamagnets (which respond to magnetic fields with
mild repulsion) are known to flout the theorem, as their negative susceptibility results
in the requirement of a minimum rather than a maximum in the field's magnitude,.
Nevertheless, levitation of a magnet without using superconductors is widely thought
to be impossible. We find that the stable levitation of a magnet can be achieved using
the feeble diamagnetism of materials that are normally perceived as being
non-magnetic, so that even human fingers can keep a magnet hovering in mid-air
without touching it.