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



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

Dr. Donald 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: Friday, June 10, 2011 8:19 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] solenoidal and cylindrical EM sourced magnetic fields.


On 2011, Jun 10, , at 13:31, Moses Fayngold wrote:


No, it does not. We can as well recall that a bar-magnet is a system of
aligned
electron spins, whose individual magnetic fields can be modeled by coaxial

microscopic current loops. This does not change the asymptotic form of
magnetic
field far away from the magnet, we still have B ~ 1/r^3.

Moses Fayngold,
NJIT
______
Of course, thank you, and re: the "solid coil", is it different from the
solenoid very near field..



I thought very near (R (axial) <<D) the field is constant.

bc


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