# Magnetic Field outside a solenoid

## Main Question or Discussion Point

I am a physicist, a young physicist. I can't find information that agrees on how to determine the magnetic field at a distance from a solenoid.

We use a ferrite rod of 7.5" long wound with wire over 6.5" of it. The frequency it oscillates at is 73 kHz. If I know the current how can I determine the magnetic field at any point in space. I have looked everywhere and I get different information everywhere. I need some help.

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mjsd
Homework Helper
I suspect that you need some info on the properties/material physics of ferrite and the magnetic fields that it produces first.

the ferrite we are using has a permeability of approximately 8000 and the fields we are measuring are very colse to uniform, (possible human error). What I need is an equation that will give me an accurate magnetic field strength at any point away from the solenoid.

Meir Achuz
Homework Helper
Gold Member
You can treat the 7.5" ferrite rod as a uniformly magnetized magnet of magnetization
M=\mu nI/c, where n =turns/length, all in Gaussian units.
If the ends of the rod are circular, the B field outside will be the same as the E field due to two uniformly charged disks with surface charge \sigma =+ M and -M.
This field can be found using Legendre polynomials for off axis.
Reasonable approximations can be made if you are close to or far from the rod.
This all depends on the hysteresis relaxation time being small compared with the period of the oscillation. You need a magnetically very soft ferrite.

hey woje
did you ever find a way to calculate the field? I'm having the same problems you were with finding information on how to calculate the fields, so if you found anything I would really appreciate your help. Thanks.