Products

The earliest records date back to ancient times, particularly in the region of Asia Minor called Magnesia—the name of this region is the source of words like a magnet.

Magnetic rocks found in Magnesia, which is now part of western Turkey, stimulated interest during ancient times.

When humans first discovered magnetic rocks, they likely found that certain parts of these rocks attracted bits of iron or other magnetic rocks more strongly than other parts.

These areas are called the poles of a magnet.

A magnetic pole is the part of a magnet that exerts the strongest force on other magnets or magnetic material, such as iron.

For example, the poles of the bar magnet shown are where the paper clips are concentrated.

A bar magnet with paper clips attracted to the two poles.

If a bar magnet is suspended so that it rotates freely, one pole of the magnet will always turn toward the north, with the opposite pole facing south.

This discovery led to the compass, which is simply a small, elongated magnet mounted so that it can rotate freely.

An example of a compass is shown.

The pole of the magnet that orients northward is called the north pole, and the opposite pole of the magnet is called the south pole.

A compass is an elongated magnet mounted in a device that allows the magnet to rotate freely.

The discovery that one particular pole of a magnet orients northward, whereas the other pole orients southward allowed people to identify the north and south poles of any magnet.

It was then noticed that the north poles of two different magnets repel each other, and likewise for the south poles.

Conversely, the north pole of one magnet attracts the south pole of other magnets.

This situation is analogous to that of electric charge, where like charges repel and unlike charges attract.

In magnets, we simply replace the charge with a pole: Like poles repel and unlike poles attract.

This is summarized in, which shows how the force between magnets depends on their relative orientation.

Depending on their relative orientation, magnet poles will either attract each other or repel each other.

Consider again the fact that the pole of a magnet that orients northward is called the north pole of the magnet.

If unlike poles attract, then the magnetic pole of Earth that is close to the geographic North Pole must be a magnetic south pole! Likewise, the magnetic pole of Earth that is close to the geographic South Pole must be a magnetic north pole.

This situation is depicted, in which Earth is represented as containing a giant internal bar magnet with its magnetic south pole at the geographic North Pole and vice versa.

If we were to somehow suspend a giant bar magnet in space near Earth, then the north pole of the space magnet would be attracted to the south pole of Earth’s internal magnet.

This is in essence what happens with a compass needle: Its magnetic north pole is attracted to the magnet south pole of Earth’s internal magnet.

Earth can be thought of as containing a giant magnet running through its core.

The magnetic south pole of Earth’s magnet is at the geographic North Pole, so the north pole of magnets is attracted to the North Pole, which is how the north pole of magnets got their name.

Likewise, the south pole of magnets is attracted to the geographic South Pole of Earth.

What happens if you cut a bar magnet in half? Do you obtain one magnet with two south poles and one magnet with two north poles? The answer is no: Each half of the bar magnet has a north pole and a south pole.

You can even continue cutting each piece of the bar magnet in half, and you will always obtain a new, smaller magnet with two opposite poles.

As shown, you can continue this process down to the atomic scale, and you will find that even the smallest particles that behave as magnets have two opposite poles.

In fact, no experiment has ever found an object with a single magnetic pole, from the smallest subatomic particle such as electrons to the largest objects in the universe such as stars.

Because magnets always have two poles, they are referred to as magnetic dipoles—di means two.

Below, we will see that magnetic dipoles have properties that are analogous to electric dipoles.

All magnets have two opposite poles, from the smallest, such as subatomic particles, to the largest, such as stars.

Watch Physics

Introduction to Magnetism

This video provides an interesting introduction to magnetism and discusses, in particular, how electrons around their atoms contribute to the magnetic effects that we observe.

Grasp Check

Toward which magnetic pole of Earth is the north pole of a compass needle attracted?

The north pole of a compass needle is attracted to the north magnetic pole of Earth, which is located near the geographic North Pole of Earth.

The north pole of a compass needle is attracted to the south magnetic pole of Earth, which is located near the geographic North Pole of Earth.

The north pole of a compass needle is attracted to the north magnetic pole of Earth, which is located near the geographic South Pole of Earth.

The north pole of a compass needle is attracted to the south magnetic pole of Earth, which is located near the geographic South Pole of Earth.

Only certain materials, such as iron, cobalt, nickel, and gadolinium, exhibit strong magnetic effects.

Such materials are called ferromagnetic, after the Latin word Ferrum for iron.

Other materials exhibit weak magnetic effects, which are detectable only with sensitive instruments.

Not only do ferromagnetic materials respond strongly to magnets—the way iron is attracted to magnets—but they can also be magnetized themselves—that is, they can be induced to be magnetic or made into permanent magnets.

A permanent magnet is simply a material that retains its magnetic behavior for a long time, even when exposed to demagnetizing influences.

An unmagnetized piece of iron is placed between two magnets, heated, and then cooled, or simply tapped when cold.

The iron becomes a permanent magnet with the poles aligned as shown: Its south pole is adjacent to the north pole of the original magnet, and its north pole is adjacent to the south pole of the original magnet.

Note that attractive forces are created between the central magnet and the outer magnets.

When a magnet is brought near a previously unmagnetized ferromagnetic material, it causes local magnetization of the material with unlike poles closest, as in the right side

This causes an attractive force, which is why unmagnetized iron is attracted to a magnet.

What happens on a microscopic scale is illustrated.

Regions within the material called domains to act like small bar magnets.

Within domains, the magnetic poles of individual atoms are aligned.

Each atom acts like a tiny bar magnet.

Domains are small and randomly oriented in an unmagnetized ferromagnetic object.

In response to an external magnetic field, the domains may grow to millimeter size, aligning themselves, as shown in Figure 7(b) .

This induced magnetization can be made permanent if the material is heated and then cooled, or simply tapped in the presence of other magnets.

An unmagnetized piece of iron—or other ferromagnetic material—has randomly oriented domains.

(b) When magnetized by an external magnet, the domains show greater alignment, and some grow at the expense of others.

Individual atoms are aligned within domains;

each atom acts like a tiny bar magnet.

Conversely, a permanent magnet can be demagnetized by hard blows or by heating it in the absence of another magnet.

Increased thermal motion at higher temperatures can disrupt and randomize the orientation and size of the domains.

There is a well-defined temperature for ferromagnetic materials, which is called the Curie temperature, above which they cannot be magnetized.

The Curie temperature for iron is 1,043 K (770°C), which is well above room temperature.

There are several elements and alloys that have Curie temperatures much lower than room temperature and are ferromagnetic only below those temperatures.

Refrigerator Magnets

We know that as magnetic poles repel and unlike poles attract.

See if you can show this for two refrigerator magnets.

Will the magnets stick if you turn them over? Why do they stick to the refrigerator door anyway? What can you say about the magnetic properties of the refrigerator door near the magnet? Do refrigerator magnets stick to metal or plastic spoons? Do they stick to all types of metal?

You have one magnet with the north and south poles labeled.

How can you use this magnet to identify the north and south poles of other magnets?

If the north pole of a known magnet is repelled by a pole of an unknown magnet on bringing them closer, that pole of the unknown magnet is its north pole;

otherwise, it is its south pole.

If the north pole of the known magnet is attracted to a pole of an unknown magnet on bringing them closer, that pole of the unknown magnet is its north pole;

otherwise, it is its south pole.

We have thus seen that forces can be applied between magnets and between magnets and ferromagnetic materials without any contact between the objects.

This is reminiscent of electric forces, which also act over distances.

Electric forces are described using the concept of the electric field, which is a force field around electric charges that describes the force on any other charge placed in the field.

Likewise, a magnet creates a magnetic field around it that describes the force exerted on other magnets placed in the field.

As with electric fields, the pictorial representation of magnetic field lines is very useful for visualizing the strength and direction of the magnetic field.

the direction of magnetic field lines is defined to be the direction in which the north pole of a compass needle points.

If you place a compass near the north pole of a magnet, the north pole of the compass needle will be repelled and point away from the magnet.

Thus, the magnetic field lines point away from the north pole of a magnet and toward its south pole.

The black lines represent the magnetic field lines of a bar magnet.

The field lines point in the direction that the north pole of a small compass would point, as shown at left.

Magnetic field lines never stop, so the field lines actually penetrate the magnet to form complete loops, as shown at right.

Magnetic field lines can be mapped out using a small compass.

The compass is moved from point to point around a magnet, and at each point, a short line is drawn in the direction of the needle

Joining the lines together then reveals the path of the magnetic field line.

Another way to visualize magnetic field lines is to sprinkle iron filings around a magnet.

The filings will orient themselves along the magnetic field lines, forming a pattern such as that shown on the right

Using a Compass to Map Out the Magnetic Field

Begin by dragging the compass around the bar magnet to see in which direction the magnetic field points.

Note that the strength of the magnetic field is represented by the brightness of the magnetic field icons in the grid pattern around the magnet.

Use the magnetic field meter to check the field strength at several points around the bar magnet.

You can also flip the polarity of the magnet, or place Earth on the image to see how the compass orients itself.

Now use the magnetic field meter to answer the following question: Near the magnet, where is the magnetic field strongest and where is it weakest? Don’t forget to check inside the bar magnet.

The magnetic field is strongest at the center and weakest between the two poles just outside the bar magnet.

The magnetic field lines are densest at the center and least dense between the two poles just outside the bar magnet.

The magnetic field is strongest at the center and weakest between the two poles just outside the bar magnet.

The magnetic field lines are least dense at the center and densest between the two poles just outside the bar magnet.

The magnetic field is weakest at the center and strongest between the two poles just outside the bar magnet.

The magnetic field lines are densest at the center and least dense between the two poles just outside the bar magnet.

The magnetic field is weakest at the center and strongest between the two poles just outside the bar magnet and the magnetic field lines are least dense at the center and densest between the two poles just outside the bar magnet.

Magnetic field lines can be drawn by moving a small compass from point to point around a magnet.

At each point, draw a short line in the direction of the compass needle.

Joining the points together reveals the path of the magnetic field lines.

Another way to visualize magnetic field lines is to sprinkle iron filings around a magnet, as shown at right.

When two magnets are brought close together, the magnetic field lines are perturbed, just as happens for electric field lines when two electric charges are brought together.

Bringing two north poles together—or two south poles—will cause repulsion, and the magnetic field lines will bend away from each other.

which shows the magnetic field lines created by the two closely separated north poles of a bar magnet.

When opposite poles of two magnets are brought together, the magnetic field lines join together and become denser between the poles.

When two north poles are approached together, the magnetic field lines repel each other and the two magnets experience a repulsive force.

The same occurs if two south poles are approached together.

(b) If opposite poles are approached together, the magnetic field lines become denser between the poles and the magnets experience an attractive force.

Like the electric field, the magnetic field is stronger where the lines are denser.

Thus, between the two north poles, the magnetic field is very weak because the density of the magnetic field is almost zero.

A compass placed at that point would essentially spin freely if we ignore Earth’s magnetic field.

Conversely, the magnetic field lines between the north and south poles are very dense, indicating that the magnetic field is very strong in this region.

A compass placed here would quickly align with the magnetic field and point toward the south pole on the right.

Misconception Alert

The density of the magnetic field lines in Figure 20.12 indicates the magnitude of the force that would be applied to a small test magnet placed in this field.

The density does not indicate the force between the two magnets that create the field.

The magnitude of the force between the two magnets is the same in both cases.

This can be understood by imagining that you place one of the magnets in the field of the other magnet.

This situation is symmetrical: The magnetic fields look the same—other than direction—for both situations shown.

Because the magnets are of equal strength, they perturb the magnetic field of the opposite magnet, which is why the magnetic field must be probed by a small magnetic such as, a compass.

Note that magnets are not the only things that make magnetic fields.

Early in the nineteenth century, people discovered that electrical currents cause magnetic effects.

The first significant observation was by the Danish scientist Hans Christian Oersted (1777–1851), who found that a compass needle was deflected by a current-carrying wire.

This was the first significant evidence that the movement of electric charges had any connection with magnets.

An electromagnet is a device that uses electric current to make a magnetic field.

These temporarily induced magnets are called electromagnets.

Electromagnets are employed for everything from a wrecking yard crane that lifts scrapped cars to control the beam of a 90-km-circumference particle accelerator to the magnets in medical-imaging machines.

The instrument for magnetic resonance imaging (MRI).

The device uses a cylindrical-coil electromagnet to produce for the main magnetic field.

The patient goes into the tunnel on the gurney.

The magnetic field created by an electric current in a long straight wire is shown.

The magnetic field lines form concentric circles around the wire.

The direction of the magnetic field can be determined using the right-hand rule.

This rule shows up in several places in the study of electricity and magnetism.

Applied to a straight current-carrying wire, the right-hand rule says that, with your right thumb pointed in the direction of the current, the magnetic field will be in the direction in which your right fingers curl, as shown.

Part of the content in this article is reproduced from other media for the purpose of transmitting more information and does not mean that this website agrees with its views or confirms the authenticity of its content. It shall not bear direct responsibility and joint liability for the infringement of such works.

If there is any infringement, bad information, error correction, and other issues in the content of this page, please contact us at info@albmaterials.com

Link to this article: https://www.albmagnets.com/blog/characteristics-of-magnets-worksheet-answers.html

Previous: Ceramic Magnet Grades

Next: Cheap Fridge Magnets