Note-A-Rific: Domains

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Magnetic field lines of a bar magnet form a pattern much like that for the electric field (look at the images on page 677)

· Also, the attraction and repulsion between poles resembles the forces between charges.

· An important difference is that a positive or negative electric charge can easily be isolated, but so far the isolation of a single magnetic pole (north or south, but not both) has not been done.

o If a bar magnet is cut in half you get two new magnets.

o If the cutting is repeated, more magnets are produced, each with a north and a south pole.

o Physicists have tried various ways to isolate a single magnetic pole without success.

o This is an active research field today since certain theories suggest they should exist.

Microscopic examination reveals that a magnet is actually made up of tiny regions known as “domains”

· They are at most about 1mm in length or width.

· Each domain behaves like a tiny magnet with a north and a south pole.

· In an unmagnetized piece of iron, these domains are arranged randomly.

· The magnetic effects of the domains cancel each other out, so this piece of iron is not a magnet.

· In a magnet, the domains are basically lined up in one direction.

A magnet can be made from an unmagnetised piece of iron by placing it in a strong magnetic field.

· In this case the domains may actually rotate slightly so they line up more to the magnet’s field.

· More commonly, the borders of domains move so that the ones which already line up with the magnetic field grow in size.

This explains how a magnet can pick up unmagnetised pieces of iron like paper clips.

· The magnet's field causes a slight alignment of the domains in the unmagnetised object.

· The object becomes a temporary magnet with its north pole facing the south pole of the permanent magnet.

An iron magnet can remain magnetized for a long time, and is referred to as a "permanent magnet."

· If you drop a magnet on the floor or strike it with a hammer, you may jar the domains into randomness.

· The magnet can lose some or all of its magnetism.

· Heating a magnet can cause a loss of magnetism, since raising the temperature increases the random thermal motion of the atoms which tends to randomize the domains.

o Above a certain temperature, known as the Curie temperature (1043 K for iron), a magnet cannot be made at all.

But where are the domains coming from.

· Even though a magnet might be at rest, the electrons in its atoms are in constant motion.

· The electrons basically move as though they are in little orbits around the nucleus.

· Soon we will be studying how a moving charge (current) creates its own magnetic field.

· More important, electrons spin on their own axis like tops, also making a magnetic field.

· Enough of these electrons moving the same way make a domain.