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

[Bernard Cleyet <bernardcleyet@redshift.com>]

On 2011, Jun 11, , at 12:44, Donald Polvani wrote:

> Jackson, "Classical Electrodynamics", 1962 works out (unfortunately, in cgs
> units) the exact magnetic field from a circular loop carrying a constant
> current I and of radius a.  Although the general solution involves the
> complete elliptic integrals K and E, the exact B field, on axis (B_r), is
> given simply by:
>
> B_r = (2I*pi*a^2/cr^3)*1/(1 + a^2/r^2)^3/2
>
> Where c = speed of light, pi = the usual math constant 3.14...
>
> > From this, we see that on axis:
>
> 1) For r >> a, B_r varies as 1/r^3
>
> 2) For r << a, B_r is constant
>
> 3) For r ~ a, B_r varies, approximately, as 1/r
>
> Don

so as:

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 ".  

then intermediately 1/r^2,       a < r < >> a     ?

bc thinks K&J should post the limits to the region of approximation.


p.s. I used the calculator, because I could read my table of elliptical integrals.