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Name: Tim
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Question:
When a magnetic material is placed in a magnetic field, what do the magnetic field lines do?



Replies:
First, keep in mind that field lines are just a model that people use, they are a great way to picture in your head where the magnetic field is getting stronger or which way it is pointing. When you put a piece of magnetic material such as iron into a magnetic field, then it is like the field lines bend in, to try to go through the material. So the field lines sort of look like a funnel, or hour-glass shape, coming in narrow to try to go through the block of iron, then when they are out, fanning back out.

Steve Ross


Dear Tim,

Magnetic field lines always indicate the strength and direction of the magnetic field. If a magnetic material (I assume you mean ferromagnetic, which has the largest effect) is placed in the field the magnetic field is concentrated in the ferromagnetic material, so the magnetic field lines will be concentrated in the ferromagnetic material.

The exact configuration of the field lines of course depends on the details of the geometry of the objects producing the magnetic field and of the ferromagnetic material. In general, however, a ferromagnetic material concentrates and strengthens the magnetic field.

There are also paramagnetic and diamagnetic materials. Paramagnetic materials act like ferromagnetic materials, but with a much smaller effect. Diamagnetic materials act in the opposite manner, tending to spread out the field lines and weakening the magnetic field.

The entire subject is quite complicated and not easy to analyze.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


The lines try to crowd together to fit as many of them as practical through the piece of material. To magnetic field lines, distance in air is like walking uphill, hard work, and a passive ferro-magnetic material is like an easy shortcut that pushes back much less.

What limits this gathering and crowding tendency is that the lines dislike being dense in air, close-together side-by-side. They always try to spread out while in air, because the energy tied up per unit volume goes as the square of the magnetic field-line density, times the permeability of the material. Air has a permeability of 1, and iron maybe >10,000. certainly >100. So flux-lines will crowd themselves at least 10 times denser to get into iron, often 100 times denser.

And of course field-lines do not like going long distances through air to reach this easy pathway through iron. No profit in that. So if an iron bar is oriented parallel to the filed lines, the lines do smooth swerving curves from nearby volumes into dense zones entering each end of the bar, then stream straight down the length of the bar. The density beside the middle of the bar is less than it would be without the iron, because many of those lines are taking the shortcut through the iron instead.

Jim Swenson



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