Like all other materials, permanent magnets are made up
of atoms that have electrons orbiting a nucleus of protons and neutrons.
In moving around the nucleus, these electrons create miniscule magnetic
fields. In most materials, these tiny fields all point in different random
directions, so the bulk material does not have a magnetic field.
But in permanent magnets, the fields are all lined up together,
and so the material is magnetized. Materials, like
iron, that can be magnetized, are called ferromagnetic.
There are two other types of magnetic materials: If a nonferromagnetic
material is attracted by a magnet, it is called paramagnetic.
The atoms in an paramagnet line up in the direction of an external
field. If a nonferromagnetic material is repelled by a magnet, it
is called diamagnetic. The atoms in a diamagnet line
up against an external field.
Magnetic Field Lines
Permanent magnets—and electromagnets—have positive and
negative poles, often called “north” and “south,” respectively.
Like electric field lines, magnetic field lines go from the positive,
or north, pole, toward the negative, or south, pole. For example,
the magnetic field of a bar magnet looks like this:
A horseshoe-shaped magnet creates a magnetic field like
It is possible to do a nifty experiment to see these magnetic
field lines by scattering iron fillings around a permanent magnet—the
filings will move to trace the lines.
The Earth’s Magnetic Field
The Earth itself acts like a huge bar magnet. The presence
of a magnetic field about the Earth allows us to use compasses that
point northward, and creates a spectacular aurora over the northern
and southern skies. But the magnetism of the Earth is quite complicated, and
is still an active subject of research for geologists, so let us
turn to the simpler cases of idealized charges and constant magnetic