Physics Photo of the Week

Warren Wilson College

Feb. 11, 2005

High temperature superconductor levitates a small magnet.


Photo by physics student Jesenia Majias

The Meissner effect by physics student: Hannah Barks     

The Meissner effect was discovered in 1933 by Walter Meissner and Robert Ochsenfeld. The Meissner effect occurs when a metal (i.e. Pb, Al, Sb, Sn, Hg, YBa2Cu3O7) are cooled to a low enough temperature. The Meissner effect is the expulsion of all magnetic fields from the superconductor. Resistance in metals is caused by the lattice of the metals. At high temperature the metal lattice is vibrating. These lattice vibrations collide with he electrons and increase the resistance of the metal. When cooled to a low enough temperature these vibrations are almost no vibrations and there is a small resistance.    See the article below written by physics student George Keel for a more thorough explanation of superconductivity.  

The Meissner effect is the exclusion of all magnetic fields from a superconductor. (http://en.wikipedia.org/wiki/Meissner_effect) In this experiment a superconductor was placed on top of a cork in a styrofoam cup. A magnet was placed on the superconductor (YBa2Cu3O7). Liquid nitrogen was poured into the cup to cool the superconductor. When the ceramic YBa2Cu3O7 became a superconductor, because it was lowered to its superconductivity temperature, the magnet started to levitate on its side and and was free to spin. This was possible because the superconductor repels all magnetic fieldds. But YBa2Cu3O7 is a ceramic, which implies that it is porous.  This allows the magnetic fields to sometimes go through the voids in between the ceramic and become trapped.  The magnetic fields trapped within the voids – called “flux pinning” – is responsible for the stability of the levitating magnet. 



A brief explanation of Superconductivity by physics student: George Keel

 In order to understand a super conductor, one must first understand conductivity. Certain materials are conductive due to electrons that are free to move through out the element. These electrons are free because the atoms in the material share electrons that provide a sufficient negative charge to allow each atom to free an electron(s). A conductive material also has a resistance.  As these electrons are traveling freely through the material they collide with the nuclei of the atoms creating a resistance to current.

The atoms in a material oscillate at different rates depending upon the temperature of the substance. As the temperature gets cooler, the oscillation of the atoms slows down. As this oscillation slows, the electrons can take a more direct path with fewer collisions through the substance. This causes the resistance of the substance to decrease. If the temperature of a certain substance (a super conducting substance) is low enough, an interesting phenomenon called superconductivity occurs. The vibrations of the atoms slow down to a certain rate. According to theory, electrons begin to move in pairs. One electron is the pair electrically distorts the molecular structure of a super conducting material as it travels. This distortion creates a near-by positive charge, and the other electron is attracted to this. This coordination between the charges prevents them from colliding with the molecules of the material and eliminates electrical resistance. With no resistance, the electrons can follow an undisturbed path with conductivity at a maximum.



Physics Photo of the Week
is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an 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.

Click here to see all Physics Photo of the Week for 2005

Click here to see all Physics Photo of the Week for 2004.