magnets and magnetism

what makes a material magnetic

Magnetism is due to the movement of electrons within atoms of matter. When electrons spin around the nucleus of an atom, it causes the atom to become a tiny magnet, with north and south poles and a magnetic field. In most materials, the electrons orbiting the nuclei of the atoms are arranged in such a way that the materials have no magnetic properties. Also, in most types of matter, the north and south poles of atoms point in all different directions, so overall the matter is not magnetic. Examples of nonmagnetic materials include wood, glass, plastic, paper, copper, and aluminum. These materials are not attracted to magnets and cannot become magnets. In other materials, electrons fill the orbitals of the atoms that make up the material in a way to allow for each atom to have a tiny magnetic field, giving each atom a tiny north and south pole. There are large areas where the north and south poles of atoms are all lined up in the same direction. These areas are called magnetic domains. Generally, the magnetic domains point in different directions, so the material is still not magnetic. However, the material can be magnetized by placing it in a magnetic field. When this happens, all the magnetic domains become aligned, and the material becomes a magnet. This is illustrated in Figure 24.6. Materials that can be magnetized are called ferromagnetic materials. They include iron, cobalt, and nickel.

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temporary and permanent magnets

Materials that have been magnetized may become temporary or permanent magnets. An example of each type of magnet is described below. Both are demonstrated in Figure 24.7. If you bring a bar magnet close to pile of paper clips, the paper clips will become temporarily magnetized, as all their magnetic domains align. As a result, the paper clips will stick to the magnet and also to each other. However, if you remove the paper clips from the bar magnets magnetic field, their magnetic domains will no longer align. As a result, the paper clips will no longer be magnetized or stick together. If you stroke an iron nail with a bar magnet, the nail will become a permanent (or at least long-lasting) magnet. Its magnetic domains will remain aligned even after you remove it from the magnetic field of the bar magnet. Permanent magnets can be demagnetized, however, if they are dropped or heated to high temperatures. These actions move the magnetic domains out of alignment.

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properties of magnets

A magnet is an object that attracts certain materials such as iron. Youre probably familiar with common bar magnets, like the one in Figure 24.2. Like all magnets, this bar magnet has north and south poles and attracts objects such as paper clips that contain iron.

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magnetic poles

All magnets have two magnetic poles. The poles are regions where the magnet is strongest. The poles are called north and south because they always line up with Earths north-south axis if the magnet is allowed to move freely. (Earths axis is the imaginary line around which the planet rotates.) What do you suppose would happen if you cut the bar magnet in Figure 24.2 in half along the line between the north and south poles? Both halves would also have north and south poles. If you cut each of the halves in half, all those pieces would have north and south poles as well. Pieces of a magnet always have both north and south poles no matter how many times you cut the magnet.

magnetic force

The force that a magnet exerts on certain materials is called magnetic force. Like electric force, magnetic force is exerted over a distance and includes forces of attraction and repulsion. North and south poles of two magnets attract each other, while two north poles or two south poles repel each other.

magnetic field

Like the electric field that surrounds a charged particle, a magnetic field surrounds a magnet. This is the area around the magnet where it exerts magnetic force. Figure 24.3 shows the magnetic field surrounding a bar magnet. Tiny bits of iron, called iron filings, were placed under a sheet of glass. When the magnet was placed on the glass, it attracted the iron filings. The pattern of the iron filings shows the lines of force that make up the magnetic field of the magnet. The concentration of iron filings near the poles indicates that these areas exert the strongest force. To see an animated magnetic field of a bar magnet, go to this URL: http://elgg.norfolk.e2bn.org/jsmith112/files/68/149/ When two magnets are brought close together, their magnetic fields interact. You can see how in Figure 24.4. The drawings show how lines of force of north and south poles attract each other whereas those of two north poles repel each other. The animations at the URL below show how magnetic field lines change as two or more magnets move in relation to each other. You can take an animated quiz to check your understanding of magnetic field interactions at this URL: http://elgg.

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magnetism and materials

Magnetism is the ability of a material to be attracted by a magnet and to act as a magnet. No doubt youve handled refrigerator magnets like the ones in Figure 24.5. You probably know first-hand that they stick to a metal refrigerator but not to surfaces such as wooden doors and glass windows. Wood and glass arent attracted to a magnet, whereas the steel refrigerator is. Obviously, only certain materials respond to magnetic force.

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instructional diagrams

No diagram descriptions associated with this lesson

questions

A maglev train can hover above the track without touching it because of

a. friction.

-->  b. magnetic force.

c. static electricity.

d. none of the above

Ferromagnetic materials include

a. iron.

b. nickel.

c. cobalt.

-->  d. all of the above

If a magnet is allowed to move freely, its north and south poles will always

-->  a. line up with Earths north-south axis.

b. keep changing places.

c. point east and west.

d. repel each other.

How is magnetic force like electric force?

a. It is caused by moving electrons.

b. It generates a force field.

c. It acts over a distance.

-->  d. all of the above

Magnetic domains in a ferromagnetic material are most likely to align in the same direction when the material is

a. heated to a high temperature.

-->  b. placed near a magnet.

c. dropped on a hard surface.

d. two of the above

Some magnets have just one magnetic pole.

a. true

-->  b. false

Bringing together the north poles of two magnets demagnetizes them.

a. true

-->  b. false

A magnet will attract any material that contains iron.

-->  a. true

b. false

An atom is a tiny magnet because its electrons spin around its nucleus.

-->  a. true

b. false

Only ferromagnetic materials are affected by magnetic force.

-->  a. true

b. false

Copper and aluminum are attracted by magnets.

a. true

-->  b. false

For a ferromagnetic material to become magnetic its magnetic domains must be aligned.

-->  a. true

b. false

Iron is the only ferromagnetic material.

a. true

-->  b. false

Only temporary magnets can be demagnetized.

a. true

-->  b. false

While paper clips are clinging to a bar magnet, they are temporary magnets.

-->  a. true

b. false

The most magnetic material in nature is magnetite.

-->  a. true

b. false

An iron nail can be turned into a permanent magnet.

-->  a. true

b. false

Permanent magnets can never be demagnetized.

a. true

-->  b. false

Magnetite is a naturally occurring permanent magnet.

-->  a. true

b. false

The magnetic properties of lodestone were discovered only recently.

a. true

-->  b. false

force of attraction or repulsion exerted by a magnet

a. ferromagnetic material

b. magnet

c. magnetic domain

d. magnetic field

-->  e. magnetic force

f. magnetic pole

g. magnetism

object that attracts ferromagnetic materials

a. ferromagnetic material

-->  b. magnet

c. magnetic domain

d. magnetic field

e. magnetic force

f. magnetic pole

g. magnetism

iron, nickel, or cobalt

-->  a. ferromagnetic material

b. magnet

c. magnetic domain

d. magnetic field

e. magnetic force

f. magnetic pole

g. magnetism

north or south end of a magnet

a. ferromagnetic material

b. magnet

c. magnetic domain

d. magnetic field

e. magnetic force

-->  f. magnetic pole

g. magnetism

area around a magnet where it exerts force

a. ferromagnetic material

b. magnet

c. magnetic domain

-->  d. magnetic field

e. magnetic force

f. magnetic pole

g. magnetism

ability of a material to respond to and exert magnetic force

a. ferromagnetic material

b. magnet

c. magnetic domain

d. magnetic field

e. magnetic force

f. magnetic pole

-->  g. magnetism

area of a ferromagnetic material where the poles of atoms are aligned in the same direction

a. ferromagnetic material

b. magnet

-->  c. magnetic domain

d. magnetic field

e. magnetic force

f. magnetic pole

g. magnetism

Why are the poles of a magnet called north and south poles?

a. One pole is positive and one pole is negative.

b. The poles are at opposite ends of the magnet.

c. The poles are the coldest parts of the magnet.

-->  d. The poles line up with Earths north-south axis.

What happens if you cut a bar magnet in half between the north and south poles?

a. One half has a north pole and one half has a south pole.

b. Each half is a stronger magnet than the original magnet.

-->  c. Each half has both a north pole and a south pole.

d. The two halves are no longer magnetic.

Which statement about magnetic force is false?

a. It is exerted over a distance.

b. It affects only certain types of matter.

c. It includes forces of attraction and repulsion.

-->  d. It acts only on materials that are touching a magnet.

Which of the following materials is attracted to a magnet?

a. aluminum

b. copper

c. glass

-->  d. steel

A material that can be magnetized

a. has magnetic domains.

b. is called ferromagnetic.

c. must contain iron.

-->  d. two of the above

If you place a paper clip very close to a magnet, the paper clip

a. is attracted to the magnet.

b. moves toward the magnet.

c. becomes a temporary magnet.

-->  d. all of the above

Magnetism is caused by the

a. attraction between protons and neutrons of atoms.

b. attraction between positive and negative ions.

-->  c. movement of electrons within atoms.

d. none of the above

diagram questions

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