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Re: [Phys-l] solenoidal and cylindrical EM sourced magnetic fields.



Bernard Cleyet wrote on Thu, June 9, 2011 11:30:27 AM:


>The K&J again -- I plotted the far field result and it fitted the 1/r^2 ver
well.

As Donald Polvani noted in his message,
"...the text said that the field will be calculated NEAR a disc or a
cylindrical magnet. In the near field for such geometries, the field should fall
off as 1/r^2 ".
Although I disagree that near field in such geometries should generally fall
off as 1/r^2 either, this is not an issue right now since the original question
was about the FAR field. Whatever the NEAR field of such objects is, it is
certainly NOT 1/r^3. This resolves the question.


So a solid coil, i.e. not a solenoid has the same field?
As far as we are interested in the FAR field (its dependence on distance), it
is the same for all three objects - a circular coil, a solenoid, or a
cylindrical magnet. A solenoid can be considered as a set of identical coils put
one onto another along the stretch a of the axial symmetry axis. For a distant
observer, this detail is immaterial as long as a << r .
Moses Fayngold,
NJIT