7.1 Permanent and induced magnetism, magnetic forces and fields — Learn

Prerequisites: Section 2.1, especially contact and non-contact forces; the idea that a force can act on an object without touching it.

By the end of this section you can: describe the attraction and repulsion between the poles of two magnets; tell a permanent magnet from an induced magnet; describe the magnetic field of a magnet, including its direction and where it is strongest; and explain how a compass gives evidence that the Earth's core is magnetic.

1. Poles and magnetic forces

A magnet has two places where the magnetic force is strongest. These are its poles: a north pole and a south pole.

When two magnets are brought close together, they exert a force on each other.

• Two like poles repel each other. A north pole pushes another north pole away.
• Two unlike poles attract each other. A north pole pulls a south pole towards it.

Magnetic forces are non-contact forces. The magnets do not need to touch. The force acts through the space around them.

Worked example 1. The north pole of one bar magnet is brought close to the north pole of a second bar magnet. Describe the force between them, and give a reason for your answer.

A model answer is:

• The two magnets repel each other. The two poles are like poles, and like poles repel.
• The force acts without contact, so the magnets push each other apart before they touch.

2. Magnetic materials

Only some materials are attracted by a magnet.

• Magnetic materials include iron, steel, cobalt and nickel. A magnet pulls on these materials.
• Copper, aluminium, plastic and wood are not magnetic materials. A magnet does not pull on them.

A magnet always attracts a magnetic material. It never repels one. Repulsion happens only between two magnets.

Worked example 2. A steel paper clip is placed near a bar magnet. A copper coin is placed near the same magnet. Explain why the paper clip is attracted and the coin is not.

A model answer is:

• Steel is a magnetic material, so the magnet attracts the paper clip.
• Copper is not a magnetic material, so the magnet does not attract the coin.

3. Permanent and induced magnets

A permanent magnet produces its own magnetic field all the time. A bar magnet is a permanent magnet. It still attracts magnetic materials when nothing else is nearby.

An induced magnet is a material that becomes a magnet only when it is placed in a magnetic field. The field lines up the magnetic parts inside the material. Two things follow.

• Induced magnetism always causes a force of attraction.
• When the material is taken out of the field, it loses most or all of its magnetism quickly.

So a steel paper clip is a permanent magnet only while it sits in the field of another magnet. Take the magnet away, and the clip stops attracting other clips.

Watch out. An induced magnet cannot repel a magnet. It is always attracted, because the field it makes lines up so that the nearest poles are unlike. Re-check by asking whether the object is a permanent magnet or a magnetic material in a field.

4. Magnetic fields

The region around a magnet where a force acts on another magnet or on a magnetic material is called the magnetic field. You cannot see the field, but you can plot it with a compass.

Two facts about a magnetic field matter most.

• The field is strongest at the poles. It gets weaker as you move further from the magnet.
• The direction of the field at a point is the direction of the force that would act on a north pole placed at that point. Outside the magnet, the field lines go from the north pole to the south pole. The lines are drawn closer together where the field is stronger.

The figure shows the field of a bar magnet. You can move a plotting compass around the magnet and watch the needle.

Predict first. Do not move the sliders yet. Write down your answers.

1. At a point beside the magnet, will the compass needle point along the field, or at right angles to it?
2. Where will the field be strongest: close to a pole, or far from the magnet?

Test. Move the compass to a point near the north pole and read the direction. Then move it to a point halfway between the poles, and then to a point far from the magnet.

Explain. The needle of a plotting compass is a small magnet. Why does it line up along the field line at each point? Write your answer in two or three sentences.

5. Plotting a field with a compass

You can find the shape of a magnetic field with a plotting compass.

1. Put the magnet on a sheet of paper and mark its outline.
2. Put the compass near one pole. Mark the two ends of the needle.
3. Move the compass so that its south end sits where the north end was. Mark the new position of the north end.
4. Repeat until you reach the other pole. Join the marks with a smooth curve.
5. Start again from a different point, so that you plot several field lines.

Each joined line is a field line. An arrow on the line shows the direction from the north pole to the south pole.

6. The Earth's magnetic field

A compass needle points north. The needle is a small bar magnet, so it must be sitting in a magnetic field. That field belongs to the Earth.

The compass points in the same way wherever you stand on the Earth's surface, so the Earth must have a magnetic field with poles near its geographic poles. The only material that can make such a large field is a large amount of magnetic material inside the Earth. This is evidence that the core of the Earth must be magnetic. The core is made mainly of iron and nickel.

Worked example 3. Explain how the behaviour of a compass gives evidence that the core of the Earth must be magnetic.

A model answer is:

• A compass needle is a small bar magnet, and it always points north.
• A compass needle lines up only when it is in a magnetic field, so the Earth must have a magnetic field.
• The Earth's field must come from magnetic material deep inside the planet. This is evidence that the core is magnetic.

Progress check

Return to the chapter problem. The crane magnet picks up the steel scrap, and it leaves the aluminium and copper behind. Explain why the steel is attracted, and why the aluminium is not. Then explain why the steel falls when the current is switched off.

You should now be able to

• describe the attraction and repulsion between the poles of two magnets;
• list magnetic materials and non-magnetic materials;
• tell a permanent magnet from an induced magnet;
• describe the magnetic field of a magnet, its direction, and where it is strongest;
• plot a magnetic field with a compass;
• explain how the behaviour of a compass gives evidence that the Earth's core is magnetic.