28.1 - Magnet fundamentals

Magnet: An object that creates a magnetic field and exerts a magnetic force.

All known magnets have two poles.

Just like electrical charges, magnets create fields and

exert forces. All magnets are dipoles, meaning they

have two poles: a north pole and a south pole. As

with electrical charges, magnetic opposites attract

and likes repel; a north pole attracts a south pole, and

a pair of like poles, such as two south poles, repel

each other.

A rectangular magnet like the one shown to the right

is called a bar magnet. It has a magnetic pole at each

end. If you were to bend this magnet into a “U,” you

would create a horseshoe magnet like the one the

boy is holding in the photo above. Even though you

have changed the magnet’s shape, the poles remain

at the two extremities.

The attraction and repulsion of magnetic poles can be demonstrated with a pair of bar

magnets. If you have two such magnets, position them so that their opposite poles are

adjacent. What happens? They attract each other. You can see this illustrated in

Concept 1. Now place them so their like poles are next to each other; the magnets will

repel each other. This is shown in Concept 2.

The strongest and most frequently found form of magnetism is called ferromagnetism.

Only certain types of material (iron is a notable example) exhibit this form of magnetism.

Everyday videotapes reveal a common application of ferromagnetism: A movie is

encoded as a pattern of small magnets on the videotape, and the VCR “reads” this

data. Computer hard drives function in a similar way.

You may have noticed that magnets can “stick” to surfaces that do not initially exhibit

magnetism. For instance, the exterior of your refrigerator is typically not magnetic, and

yet if it is made of a ferromagnetic metal, magnets will stick to it. A magnet sticks

because it is able to induce a temporary magnetic field in the refrigerator’s surface. The

permanent and temporary magnets then attract each other.

Another way to observe this phenomenon is to attach a metal paper clip to a magnet

and then attach a second clip to the first clip. The two clips will attract each other. When

you remove the original magnet, however, the clips will no longer attract one another

because the fields of the paper clips disappear when they are removed from the

influence of the magnet’s field.

No one has created a magnet with just one pole (a monopole). Magnetic poles always

come in pairs. If you take a bar magnet and cut it in half, you will create two magnets,

each with a south and a north pole. Cut each of those in half and you will have four

magnets, each with two poles. In principle, if you could cut the magnet into its

constituent atoms, each atom would have its own magnetic field.

This boy is holding a U magnet. The magnet suspends a few paperclips.

Magnet fundamentals

Have two poles, north and south

·Opposite poles attract

Magnet fundamentals

·Like poles repel

28.2 - Magnetic fields

Two magnets can attract or repel each other without

touching: They exert a force at a distance. Magnetic

fields surround magnets. Like an electric field, the

magnetic field is a vector field. It has a strength and a

direction at every point. The letter B represents the

magnetic field and the unit for magnetic field strength

is the tesla (T).

The photographs above show the alignment of iron

filings gathered around the poles of two pairs of

magnets held close together. The filings in both

photos align with the magnetic fields between the

poles. The photograph on the upper left shows the

filings between a south pole and a north pole, which

attract each other. The filings are connected and aligned in the same direction as the magnets because the magnetic field lines pass directly

from the north pole to the south pole. In the photograph on the upper right, two south poles are repelling each other, which causes the filings to

Iron filings line up with the magnetic field.

(^506) Copyright 2000-2007 Kinetic Books Co. Chapter 28