MadSci Network: Physics

Re: What can you use to cancel out Magnetism

Date: Thu May 3 18:02:15 2007
Posted By: Prater Brian D., Staff, Magnetics, Cavetronics R&D labs
Area of science: Physics
ID: 1178115689.Ph

Hello, Kevin

Diamagnetism is a form of magnetism that is only exhibited by a substance 
in the presence of an externally applied magnetic field. It is the result 
of changes in the orbital motion of electrons. Applying a magnetic field 
creates a magnetic force on a moving electron in the form of F = Qv  B. 
This force changes the centripetal force on the electron, causing it to 
either speed up or slow down in its orbital motion. This changed electron 
speed modifies the magnetic moment of the orbital in a direction opposing 
the external field.

Consider two electron orbitals; one rotating clockwise and the other 
counterclockwise. An external magnetic field into the page will make the 
centripetal force on an electron rotating clockwise increase, which 
increases its moment out of the page. That field would make the 
centripetal force on an electron rotating counterclockwise decrease, 
decreasing its moment into the page. Both changes oppose the external 
magnetic field into the page. However, the induced magnetic moment is very 
small in most everyday materials.

Fritz London and Heinz London developed the theory that the exclusion of 
magnetic flux is brought about by electrical "screening currents" that 
flow at the surface of the superconducting metal and which generate a 
magnetic field that exactly cancels the externally applied field inside 
the superconductor. These screening currents are generated whenever a 
superconducting metal is brought inside a magnetic field. This can be 
understood by the fact that a superconductor has zero electrical 
resistance, so that "eddy currents", induced by the motion of the metal 
inside a magnetic field, will not decay. Fritz, at the Royal Society in 
1935, stated that the thermodynamic state would be described by a single 
wave function.


Brian D. Prater
Cavetronics Labs

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