Magnetic shielding, or field line redirection - why does it work ?

[taylog1]

Could someone please explain how magnetic shielding works ? (not superconductors - the mu metal type)

Most of the explanations on the site talk about a material with high permeability providing an 'easier path' for magnetic field lines to pass through than the surrounding space, so reducing the field in those areas.

http://www.lightandmatter.com/html_books/0sn/ch11/figs/eg-magnetic-shielding-sphere.png

This all seems fine, but what is actually happening ?

Is it that the original field induces a magnetic field in the high permeability material, and the field lines outside of the shielding material then 'cancels out' some of the original field, so the net effect is a stronger field inside the shielding, and less outside ?

Gareth

 

[Born2bwire]

Yep, that's it. Here's one way to look at it in the classical sense. In a ferromagnetic material, there are microscopic loop currents that are randomly aligned. A loop current will produce a magnetic dipole, so these are the equivalent of microscopic magnetic sources. Since they are randomly aligned, in the macroscopic picture there is no net magnetic field. However, if we were to align these currents then the microscopic fields would support each other and get a noticeable magnetic field. This is how we describe ferromagnetic materials. In iron say, we induce these dipoles to line up permanently (or at least for a long time) to create permanent magnets.

Now if we have a normal ferromagnetic material that hasn't been magnetized and apply a magnetic field, this will cause all these microscopic loop currents to line up with the field (you can think of this as minimizing the energy since it is more energetic to have a moment antiparallel to the field than parallel). These loop currents add up to make a macroscopic loop current. This is because adjacent current elements between loops are opposite, except at the boundary of the material where there are no adjacent loop currents to cause cancellation, see here: https://www.physicsforums.com/attachment.php?attachmentid=22270&d=1259928311 . Apparently I cannot reuse attachments.

So now we have two fields, the original applied field and the field produced by the magnetization of the the material. The superposition of these two fields is one where the field outside the material is diminished and enhanced inside the material, effectively redirecting the field lines into the material.

 

[Bob S]

If a vertical magnetic field B is perpendicular to the horizontal axis of a hollow magnetic tube (soft iron, mumetal, etc.), the magnetic field lines are attracted to, and flow through the shield. The maximum field in the shield is midway between the top and bottom, because the field lines are concentrated in the steel tube. At the boundary between the steel tube and the inside shielded volume, the parallel component of the magnetization current H is continuous across the boundary. So if the magnetic field B in the mumetal shield is 1000 Gauss, and the permeability is 40,000, then the magnetic field in the shielded volume inside the shield is only 1/40 Gauss (for an annealed mumetal shield).

Bob S

 

[taylog1]

Thanks very much - I've searched high and low for this - everywhere offers up explanations based on field lines being attracted/diverted, but I'm much more comfortable thinking about magnetism from the relativistic viewpoint, and then using field lines as a tool to aid comprehension. [I do find it amazing how well two apparently different models can both give the same results though !]

Gareth