Physics Photo of the Week

February 20, 2009

Levitating magnet
Big image - please have patience ...Discussion by Jenifer Donovan

Generally, a material’s resistivity - its opposition to the flow of electric current - decreases with temperature. Most common materials are unable to reach “zero” resistance even when pure. However, a few materials may reach superconductivity when cooled below a critical temperature. These materials are called superconductors. Below the critical temperature, which is unique to each material, superconductors exhibit “zero electrical resistance” and exclude interior magnetic fields. The latter characteristic is known as the Meissner effect.

The video picture above shows the black disk consisting of the high temperature superconductor - a ceramic consisting of YBa2Cu3O7.  This black disk is chilled by liquid nitrogen that reaches the bottom surface of the disk.  Liquid Nitrogen is below the critical temperature for this material.  The small object levitating above the disk is a tiny magnet.  The superconductor levitates the small magnet as explained below.

In a weak applied magnetic field, below the critical temperature, superconductors produce spontaneous “surface currents” called persistent currents. These currents, which do not decay over time, form a magnetic field that opposes the magnet's magnetic field. Combined, the two opposing fields yield a “net zero” magnetic field. The interactions of the two fields can repel nearby magnetic fields. For example, if a magnet was place atop a superconductor that had been cooled below its critical temperature, the magnet would float in thin air. The levitation is caused by the repulsion of the magnet’s field by the magnetic field of the supercurrents in the superconductor. Repulsion from the superconductor pushes the magnet “up” while Earth’s gravity pulls the magnet down so that it does not fly out into space.  If the magnet is lowered onto a normal metal, the induced currents rapidly die out due to electrical resistance and the magnet sinks to rest on the metal, in stark contrast to the levitation above the superconductor.

Interestingly, the levitating magnet may “spin without friction” while maintaining its position above the superconductor. Flux pinning keeps the magnet from sliding out of position atop the superconductor. This effect is due to the magnetic forces of the material becoming entangled in microscopic voids in the superconducting ceramic.

Our thanks to Sara Bacon for help in image processing to obtain the video clip.



Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu. 

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