MadSci Network: Physics

Re: How does spinning aluminum push away a magnet?

Date: Mon Aug 23 15:24:00 1999
Posted By: Bob Novak, Other (pls. specify below), Sr Process Research Engineer, Carpenter Technology
Area of science: Physics
ID: 934838380.Ph

Hi Curtis,

Good question.  Aluminum normally is not magnetic.  Aluminum is an 
electrical conductor.  When the aluminum disk is placed near the magnet, 
the magnetic field exerts a force on the electrons in the aluminum.  The 
magnetic force will cause the electrons to separate from the positively 
charged atomic nuclei.  A portion of the disk will then become charged.  
Because the disk is spinning, the charges will move around the 
circumference of the disk.  The movement of electrical charge is an 
electrical current.  The electric current generated will have a magnetic 
field that is equal and opposite to the field of the magnet.  The induced 
magnetic field is what exerts the force on the magnet.

A good method to determine the induced force is by measuring the energy 
required to spin the disk.  More energy is needed to spin the aluminum disk 
to the same speed when the magnetic field is present.  A stronger magnetic 
field, or faster rotation of the aluminum disk, will require more energy to 
maintain the same rotational speed.

Bob Novak
Specialist - Process R&D
Carpenter Technology

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