Why Does Placing a Plate Over a Horseshoe Magnet Affect its Magnetic Field?

[GRB 080319B]

After a demonstration, my professor placed a metal plate over the two ends of a horseshoe magnet in order to prevent it from attracting other materials. I was wondering if this has to do with there being some type of "magnetic resistance" between the poles, where it is easier for the field to be setup inside the conductor than outside of it, so there would be less outside field to attraction. I asked my professor, and he explained to the effect that the magnetic field lines travel the shortest distance from pole to pole (most of the field lines are being channeled through the plate). I was wondering, why wouldn't the field lines travel the shortest distance in air, without the plate present? Is there a law that explains how the magnetic field becomes enclosed through a conductor? Any help will be greatly appreciated.

 

[Antiphon]

What he put on the magnet is called a keeper. The horseshoe magnet would loose strength over time without the keeper.

The keeper is made of something called soft iron. It has a high mu but is not easily magnetized.

Material with a high mu draws the magnetism through itself much more easily than air. With the keeper stuck to the magnetost of the magneti is no longer apparent which might make him think it there to keep the magnet from attracting things.

 

[cesiumfrog]

The horseshoe magnet would loose strength over time without the keeper.

Why?

 

[pallidin]

Why?

Yeah, I did a brief Google search and all the sites merely said something like " a soft iron keeper is used to complete the magnetic path of the poles, thus increasing the magnets longevity"

None of the "few" sites I went to actually explained why this helps.

 

[SpectraCat]

Why?

Entropy ... over time there would be a thermal randomization of the magnetic domains defining the poles, which would lead to weakening of the magnetic field.

I am not completely sure, but I think the soft iron retards this thermalization by increasing the local magnetic field at the poles. A stronger field tends to keep the domains better aligned, reducing the likelihood of randomization event at a given temperature.

An imprecise but perhaps instructive analogy is to imagine a continuous "current" circulating through the poles ... any time the "current" encounters a "resistance", it has to use up some energy to get past it, weakening the magnetic field. So you want the "resistance" between the poles to be as low as possible. Materials with high magnetic permeability (like iron) have low "resistance", and thus are good for "completing the magnetic circuit".

 

[cesiumfrog]

I think the soft iron retards this thermalization by increasing the local magnetic field at the poles. A stronger field tends to keep the domains better aligned, reducing the likelihood of randomization event at a given temperature.

That sounds plausible, focussing and concentrating the field to more strongly force co-magnetisation.

imprecise but perhaps instructive analogy is to imagine a continuous "current" circulating [along the field lines] ... any time the "current" encounters a "resistance", it has to use up some energy to get past it, weakening the magnetic field.

No, I don't think that relates in any way to the actual physics, and it feeds the common misconception of elementary fields performing work just by exerting static forces.