Magnetic Material and Field Lines
When a magnetic material is placed in a magnetic
field, what do the magnetic field lines do?
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.
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
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.
Click here to return to the Physics Archives
Update: June 2012