Conceptual Physics

In many materials, the spin of each electron cancels out that of another electron in the

same atom with which it is “paired,” resulting in no net magnetic field. Ferromagnetism

results from the net magnetic field created by unpaired electron spins. For example,

each iron atom contains four electrons with uncancelled spins, giving the iron atom a

net dipole moment.

In ferromagnetic materials, the spins of the electrons of one atom interact with the spins

of electrons of neighboring atoms. This interaction, called exchange coupling, is

quantum mechanical in nature. As a result of exchange coupling, the magnetic dipoles

of atoms within a ferromagnetic material tend to align in the same direction.

When the magnetic dipole moments of ferromagnetic atoms align, the result is a

magnetic domain, a region in a material where there is a net magnetic field. Magnetic

domains can be seen under powerful microscopes, often with the aid of a technique

called magnetic force microscopy. In the color micrograph above, domains are given

false colors to be more easily seen in the image. Typical domains have a diameter

about one-third that of a human hair. This can be best seen in the small inset

micrograph that shows the magnetic domains in a sample of carbon steel. Even small

amounts of magnetic materials contain vast numbers of domains. A domain may

contain 10^12 to 10^15 atoms, but since a cubic centimeter of iron contains about 2.5×10^19

atoms, there are still a large number of domains.

There is a good question here: Why are not all iron objects magnets? The reason is that

although domains have magnetic fields, they point in random directions unless they

have been subjected to an external magnetic field. This is shown in Concept 2. The

random nature of their directions means there is no net magnetic field in the substance.

However, when the ferromagnetic material is placed in an external magnetic field, a

process occurs that results in the material gaining its own overall magnetic field. This

happens because the external field exerts a torque on the magnetic dipoles.

The magnetic field overcomes the tendency of the dipoles to stay aligned with their

neighbors, and causes them to align with the field. Unaligned dipoles bordering

domains that are aligned with the field steadily become “converted” to the new

orientation. This causes the domains aligned with the external field to expand. This is

shown in Concept 3, where the domains shown in Concept 2 have combined to form

three domains.

Since there is a tendency for the dipoles to remain aligned, when the external magnetic

field is removed, the domains (and the dipoles that make them up) remain aligned. This

means the material now has its own magnetic field. In short, a magnet has been

created.

An external magnetic field can cause non-ferromagnetic materials to develop their own

magnetic field. These other forms of magnetism are called diamagnetism and

paramagnetism. However, ferromagnets are distinct and quite useful for two reasons.

First, a ferromagnet retains its magnetic field after the external field is removed. A

refrigerator magnet, or the information stored on a VCR tape, both rely on the longevity

of ferromagnets. The other forms of magnetism disappear after the external magnetic

field is removed.

Second, the magnetic field created by ferromagnetic materials is typically orders of

magnitude stronger than that found in the other forms of magnetism in most materials.

This makes ferromagnets useful for a wide range of everyday applications.

Random domains

Unaligned domains mean no net

magnetic field

In external magnetic field

In external magnetic field, B 0

·domains aligned to external field grow

Magnetic field removed

Substance maintains magnetic field

28.7 - Magnetic fields and charged particles

Magnetic fields exert forces on moving, electrically

charged particles.

This phenomenon makes for a good demonstration in

a physics class. A current-carrying wire is placed near

a magnet. The magnet exerts a force on the electrons

moving in the wire, which causes the wire to move

toward or away from the magnet. When the current is

turned off, the magnetic field stops exerting a force on

the wire.

The images on the screens of traditional televisions

and computers are the result of electrons being

accelerated by an electric field, subsequently being

“steered” by magnetic fields, and then striking the

screen to create light of different colors at specific

locations. If you were to place a magnet close to such a system, you would distort the image. However, we do not recommend you do this, as it

could cause expensive or irreversible damage!

TV tube. A gun (far left) accelerates electrons across a potential difference.

Electromagnets (brown) steer the moving charges to light up screen pixels.

(^510) Copyright 2000-2007 Kinetic Books Co. Chapter 28