How does a
permanent magnet work?
Some materials have a feature known as
ferromagnetism. The prefix "ferro"
refers to Iron, which is one such material.
Ferromagnetic materials have the ability to
"remember" the magnetic fields they have
been subjected to.
An atom consists of a
number of negatively charged electrons,
orbiting around a positively charged
nucleus. These electrons also possess a
quantity known as spin, which is
roughly analogous to a spinning top. The
combination of orbital and spin motions is
called the angular momentum of the
electron. Angular momentum is perhaps most
easily understood in the case of the Earth:
The earth spins about a central axis, which
means it at has an angular momentum around
that axis. The planets also have an angular
momentum as they revolve about the sun.
Now, the angular momentum
of an electron is a vector quantity, meaning
it has direction. The motion of the electron
produces a current, which in turn generates
a tiny magnetic field in the direction given
by the angular momentum. Thus an atom can
behave like a dipole, meaning "two
poles". The direction of the orbital and
spin angular momentum of the electron
determine the direction of the magnetic
field for the electron and the entire atom,
thus giving it "north" and "south" poles.
Different atoms have different arrangements
of electrons into their orbits, and thus
have different angular momenta and dipolar
properties.
A ferromagnetic material
is composed of many microscopic magnets
known as domains. Each domain is a
region of the magnet, consisting of numerous
atomic dipoles, all pointing in the same
direction. A strong magnetic field will
align the domains of a ferromagnet, or in
other words, magnetize it. Once the
magnetic field is removed, the domains will
remain aligned, resulting in a permanent
magnet. This effect is known as
hysteresis.
Few materials are
actually ferromagnetic; however, all
substances have a diamagnetic nature.
Diamagnetism means that the molecules within
a substance will align themselves to an
external magnetic field. The external
magnetic field induces currents within the
material, which in turn result in an
internal magnetic field in the opposite
direction. This effect is usually quite
small and disappears when the external
magnetic field is removed.
Some materials are
paramagnetic. This is the case when the
orbital and spin motions of the electrons in
a material do not fully cancel each other,
so that the individual atoms act like
magnetic dipoles. These dipoles are randomly
oriented, but will align themselves to an
external magnetic field. However, when the
field is removed, the material is no longer
magnetized. Again, this effect is typically
small. Neither diamagnetic nor paramagnetic
materials exhibit magnetic domains.
The atomic behavior of
magnetic materials is actually considerably
more complicated than this, as it relies on
the theory of quantum mechanics.
Quantum mechanics is the theory of physics
used to describe the behavior of tiny
particles such as electrons; like
electromagnetic theory, it is complex and
involves advanced mathematics.