MadSci Network: Physics

Subject: How do you resolve the Faraday Disc paradox?

Date: Thu Sep 9 13:08:36 1999
Posted by No name entered.
Grade level: nonaligned School: No school entered.
City: No city entered. State/Province: Wisconsin Country: No country entered.
Area of science: Physics
ID: 936896916.Ph

The Faraday disc will generate an output with either the magnet stationary 
or rotating with the conductive disc.  It is said that with the magnet 
stationary, application of DC power to the disc will cause a motor action 
to rotate the disc, but with the magnet attached to the disc, it will not 
act as a motor.  It is also said that holding the disc still and rotating 
the magnet will not cause voltage to be generated in the disc.
Question: Does the current in the disc (acting either as generator or 
motor) react against the mechanical magnet or against the magnetic field? 
If against the magnetic field, how is this reaction force "absorbed"?
Does the answer to the above explain how attaching the magnet to the 
rotating disc prevent motor action but not generator action?  To generate 
equal amounts of output power with and without the magnet attached to the 
disc, is equal mechanical input power required?
An aside, but relevant: Why do physics books always discuss the force on a 
charged particle moving in a magnetic field, but never, that I have seen, 
the force of a charged moving particle on the field?  Is there a force on 
the field (invoking Newton's laws dosen't "explain" a YES answer)


Re: How do you resolve the Faraday Disc paradox?

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