MadSci Network: Physics

Subject: Does the following situation violate conservation of energy?

Date: Mon Nov 29 19:58:44 2004
Posted by Ryan
Grade level: undergrad School: No school entered.
City: No city entered. State/Province: No state entered. Country: Canada
Area of science: Physics
ID: 1101779924.Ph

If I have a permanent bar magnet, and I place a static charge in the middle, 
(Imagine a box -the magnet- and a point charge q embedded inside).
Then the magnet will produce a static dipole magnetic field, and the static 
charge will produce a static electric field.

If you calculate the direction of the Poynting vector S (ExB), it will seem to 
be circumferential, ie. it looks like it is circulating around the bar magnet.

Now EM fields have a momentum associated with them, in the direction of this 
Poynting vector S.  If you attach a solar sail that is attached to the bar 
magnet frictionlessly, placed so that it can rotate about the magnet in the 
same direction as S.

Won't the sail be pushed?  I understand that it takes work to place the charge 
inside the magnet, etc.  But this is a finite amount of work, and since 
Magnetic field and Electric field are static, nothing is changing so none of 
the energy that is stored in their fields, (~Integral[E^2 +B^2]all space) is 
lost.  So where does the energy come from?  And does it seem to be infinite?

Re: Does the following situation violate conservation of energy?

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