# Re: [SI-LIST] : Magnetic field of a solenoid

From: Kim Helliwell ([email protected])
Date: Fri Dec 08 2000 - 08:36:36 PST

The field outside equaling 0 is an approximation for a long, thin
(read: infinitely long) solenoid, as you conjecture.

And the reason that fields on the axis are all that are calculated is that
that is the only problem that is easily solved with handwritten methods.

To get the field everywhere, I assume you would have to write a computer
program to solve Ampere's Circuital Law for whatever geometry.

You can go slightly off-axis and get an approximation for the small radial
component of the B field inside a solenoid by using handwritten methods. See
_Classical Electrodynamics_, by J.D. Jackson, Problem 5.2 on page 165.

Kim Helliwell

Steve Rogers wrote:
>
> I wish to determine the magnetic field surrounding a solenoid (at any point around the structure). All literature I have consulted (Books re Electromagnetics) give only expressions for the on axis field inside the solenoid or just outside (on axis). All of these fields are derived from Biot and Savart law.
>
> I have seen stated in a number of texts that the external field of a solenoid is zero! I can see this intuitively for a toroid and it may be so for a solenoid that is infinitely long, but what about practical finite size solenoids? Clearly if you connect a coil to a DC power supply set it down on a white sheet of paper and cover it in iron filings you will see lines of force outside the coil!
> So there is field outside!!!
>
> The reason for my interest in all of the above is as follows.
>
> I wish to determine the magnetic field surrounding different solenoid structures (different in length to diameter, shape - round or square or what have you).
>
>
> Regards,
>
> Steve Rogers
>
>
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```--
Kim Helliwell
Senior CAE Engineer
Acuson Corporation
Phone: 650 694 5030  FAX: 650 943 7260
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