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

["Donald Polvani" <dgpolvani@verizon.net>]

I just looked at the link Bernard Cleyet offered.  I did not try an actual
calculation, but 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.

Don
Dr. Donald G. Polvani
Adjunct Faculty, Physics
Anne Arundel Community College

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Bernard
Cleyet
Sent: Thursday, June 09, 2011 11:30 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] solenoidal and cylindrical EM sourced magnetic fields.


On 2011, Jun 09, , at 07:11, Moses Fayngold wrote:

> Bernard Cleyet wrote on Mon, June 6, 2011 2:22:13 PM:
>
> > Am I correct?  The field from a solenoid is the same as that of a dipole
and on 
> > axis ~ 1/r^3.   (1)
> > Using a field calculator (2) I found (far field) the on axis field of a 
> > cylindrical magnet ~ 1/r^2.  Is this correct, and if so, would a
cylindrical 
> > coil be a correct model?
>
> > (1) the last equation of Magnetic Fields due to a Solenoid
> > (2) K&J Magnetics - Magnetic Field Calculator
>
>
> The far field from of a cylindrical magnet must be the same as that of a 
> solenoid or just of a current loop, B ~1/r^3. In none of these cases can
it fall 
> off with distance as ~1/r^2. The latter would be characteristic of a 
> hypothetical isolated magnetic charge (monopole), but not of the known
sources 
> of a time-independent magnetic field.  I do not know what could have
caused the 
> result that you report. It might come from a computational error or some 
> erroneous initial expression in the input. 
>
>  Moses Fayngold,
> NJIT
> ________________

Mmmmm -- 

did my links work?


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



http://www.kjmagnetics.com/fieldcalculator.asp

bc puzzled

p.s. So a solid coil, i.e. not a solenoid has the same field?



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