Magnets and magnetic fields

What you'll learn

• How magnetic poles attract and repel, and how to draw magnetic field patterns.
• The difference between permanent magnets and induced magnets.
• Why a compass gives evidence that the Earth has a magnetic field.
• How electric currents make magnetic fields, and — on Higher Tier — how this links to motors.

Magnetic poles and forces

A magnet is an object that produces a magnetic field. A pole is one end of a magnet where the magnetic effect is strongest. Bar magnets have a north pole and a south pole.

• Like poles are the same type: north–north or south–south. They repel.
• Unlike poles are different types: north–south. They attract.

Permanent and induced magnets

Magnetic materials include iron, steel, nickel and cobalt. If a piece of iron is attracted to a permanent magnet, the iron becomes an induced magnet while it is near the magnet. The end nearest the permanent magnet becomes the opposite pole, so the force is attractive.

Induced magnetism is often temporary. For example, soft iron loses most of its magnetism when the magnetic field is removed.

Magnetic fields and field lines

A magnetic field line is a model used to show the field. The arrow on a field line shows the direction a north pole would be pushed. Around a bar magnet, field lines outside the magnet go from north to south.

The strength of the field changes from place to place. Field lines that are closer together mean a stronger magnetic field. The field is strongest near the poles of a magnet.

These diagrams show the patterns around one bar magnet, attracting unlike poles, and repelling like poles.

Investigating magnetic fields

You can investigate a magnetic field using plotting compasses. A plotting compass is a small compass that lines up with the magnetic field at its position.

To map a field, place the compass near the magnet, mark the direction the north end points, move the compass along, and join the marks into smooth field lines. Iron filings can show the shape of the field, but plotting compasses show the direction more clearly.

The Earth as a magnet

A normal compass points roughly north because the Earth has a magnetic field. The compass needle lines up with this field.

A geographic pole is based on the Earth’s rotation axis. A magnetic pole is based on the Earth’s magnetic field. These are not in exactly the same place.

A dipping compass can tilt up and down as well as rotate horizontally. Its behaviour gives evidence that the Earth’s core produces a magnetic field, because the field has a direction through the Earth as well as around it.

Magnetic field around a current-carrying wire

A current is the rate of flow of electric charge, measured in amperes (A). A conductor is a material that allows charge to flow through it easily, such as copper.

When a current flows through a straight wire, it creates a magnetic field around the wire. The field lines are circles centred on the wire.

The direction is found using the right-hand grip rule:

• Point your right thumb in the direction of the conventional current.
• Your curled fingers show the direction of the magnetic field.

If you reverse the current, the magnetic field direction reverses. The field is stronger when the current is larger, and weaker further away from the wire.

The same idea applies to a coil of wire called a solenoid.

Solenoids and electromagnets

A solenoid is a coil of wire with many turns. When current flows through it, the magnetic fields from the turns add together. This makes a stronger, more useful magnetic field, especially inside the coil.

An electromagnet is a magnet made using an electric current. It can be switched on and off. You can strengthen an electromagnet by:

• increasing the current;
• increasing the number of turns of wire;
• adding an iron core inside the solenoid.

Higher Tier: the motor effect

On Higher Tier, you also need to describe the motor effect. This is when a current-carrying conductor experiences a force because it is in a magnetic field.

The force happens because the magnetic field around the current-carrying conductor interacts with the magnetic field from the magnet. The magnet and the conductor exert forces on each other. In a “jumping wire” demonstration, the wire jumps when the current is switched on.

The force is at right angles to both:

• the current in the conductor;
• the magnetic field.

If you reverse the current, the force reverses. If you reverse the magnetic field, the force also reverses.

Higher Tier: Fleming’s left-hand rule

Fleming’s left-hand rule is used to find the direction of the force in the motor effect.

Use your left hand:

• Thumb = force or motion.
• First finger = magnetic field, from north to south.
• Second finger = conventional current.

Higher Tier: calculating the force on a conductor

For Higher Tier, you need to apply the equation for the force on a current-carrying conductor at right angles to a magnetic field. OCR’s focus here is applying the relationship correctly, so make sure you can choose it and use SI units.

$$F=BIl$$

where:

• $F$ is force in newtons (N);
• $B$ is magnetic flux density in tesla (T);
• $I$ is current in amperes (A);
• $l$ is the length of conductor in the magnetic field in metres (m).

Higher Tier: how electric motors rotate

An electric motor transfers electrical energy into kinetic energy using the motor effect.

In a simple coil between magnetic poles, current flows up one side of the coil and down the other side. Because the currents are in opposite directions, the forces on the two sides are also in opposite directions. This pair of forces creates a turning effect, so the coil rotates.

You do not need detailed motor structure for this Gateway Combined Science point, but you should understand the basic cause of rotation: opposite forces on opposite sides of a current-carrying coil.