NIKAM N.D. M.Sc.NET DEPARTMENT OF CHEMISTRY The Meissner Effect NIKAM N.D. M.Sc.NET DEPARTMENT OF CHEMISTRY

Outline What is the Meissner Effect? Superconductors as diamagnets. Meissner effect in perfect conductors? Meissner effect in superconductors.

The Meissner Effect A diamagnetic property exhibited by superconductors. End result is the exclusion of magnetic field from the interior of a superconductor. What is diamagnetism?

Diamagnetism? A superconductor is not only a perfect conductor (R=0), but a perfect diamagnet. It will tend to repel a magnet.

So, Superconductors are Perfect Diamagnets? If a superconductor was only a perfect conductor, would there be a Meissner Effect? Recall Faraday’s Law of Induction.

Faraday’s Law of Induction

Faraday’s Law of Induction A change in magnetic flux will induce an emf in a conductor. There will be no induced emf if the magnetic flux is constant with respect to time.

The Minus Sign What does the minus sign imply physically? The direction of the induced emf will be such that the magnetic field produced by the induced emf resists the change in magnetic flux. The presence of the minus sign is referred to as Lenz’s Law

Lenz’s Law If the magnetic flux is decreasing out of the page, which way will the induced emf be directed? (Note: the induced emf has the same direction as the induced current.)

The direction of the induced emf (or current), will be counterclockwise. This will generate an induced magnetic field out of the page, counteracting the decrease in flux. (Found from the right-hand rule for current carrying wires.)

Perfect Conductor Move this perfect conductor into a magnetic field. By Faraday’s Law of Induction, a current is induced. The magnetic field generated by this current would oppose the change of the applied field.

How long will the induced current flow? Recall P= I2R. The induced current would flow indefinitely. There is no I2R power loss. The induced magnetic field will continue to oppose the change in the applied field. Conversely, if the conductor is in a magnetic field which is then removed, an induced current and corresponding magnetic field would tend to oppose the removal of the applied field.

What Do We See? Would a magnet levitate over the surface of a perfect conductor? No, if a magnet is placed on top of a material which becomes a perfect conductor, there would be no effect on the magnet. There would only be an opposing force if the magnet was removed.

In Superconductors Faraday’s Law does not explain magnetic repulsion by superconductors. Below its critical temperature (Tc) a superconductor does not allow any magnetic field to enter it.                                   

Circulating currents on the surface of the superconductor induce microscopic magnetic dipoles that oppose the applied field. The induced field repels the applied field, and the magnet associated with it. If a magnet is on top of a superconductor as it is cooled below its Tc, it would exclude the magnetic field of the magnet.

The Result

References Image 1: http://www.physics.ubc.ca/~outreach/p420_97/bruce/ybco.html Image 2: http://www.sci.kun.nl/hfml/froglev.html Image 3: http://physicsweb.org/article/world/11/12/6 Image 4: http://www.phys.warwick.ac.uk/supermag/Research/Superconductors/body_superconductors.html