Magnetism and Electromagnetism — AQA Combined Science: Trilogy

This unit covers permanent and induced magnets, magnetic fields, electromagnets and the motor effect.

Permanent and induced magnets

• A permanent magnet produces its own magnetic field all the time. It has a north and a south pole.
• An induced magnet is a material that becomes magnetic when placed in a magnetic field; it loses most of its magnetism when removed.

Magnetic materials are iron, steel, cobalt and nickel.

Forces between poles

• Like poles repel, unlike poles attract — these are non-contact forces.
• Induced magnetism always causes a force of attraction.

Magnetic fields

A magnetic field is the region around a magnet where a force acts on another magnet or magnetic material.

• Field lines run from north to south outside the magnet.
• The field is strongest where the field lines are closest together — at the poles.
• A compass contains a small magnet; it lines up with the field. A compass pointing north (when away from other magnets) is evidence that the Earth has its own magnetic field, generated by its core.

Electromagnetism

When a current flows through a wire it creates a magnetic field around the wire. The field is made of concentric circles around the wire; its strength increases with the current and decreases with distance from the wire.

Solenoids

Shaping a wire into a coil (a solenoid) increases the strength of the magnetic field, because the fields from each turn add together. The field inside a solenoid is strong and uniform, like a bar magnet. Adding an iron core makes an electromagnet, which is even stronger.

Electromagnets are useful because they can be turned on and off and their strength can be varied — used in scrapyard cranes, electric bells, relays and loudspeakers.

The motor effect

When a current-carrying conductor is placed in a magnetic field, the field of the wire and the field of the magnet interact, exerting a force on the wire. This is the motor effect.

• The force is greatest when the wire is at 90° to the magnetic field, and zero when parallel to it.
• The direction of the force is found using Fleming's left-hand rule (thumb = motion/force, first finger = field, second finger = current).
• Reversing the current or the field reverses the direction of the force.

Force on a conductor (Higher Tier)

The size of the force is: $$F = B \times I \times L$$ (force = magnetic flux density × current × length of wire in the field). Magnetic flux density (B) is measured in tesla (T).

Electric motors

The motor effect is used in an electric motor. A current flows through a coil in a magnetic field; the two sides of the coil experience forces in opposite directions, creating a turning effect that makes the coil rotate. A split-ring commutator reverses the current every half turn so the coil keeps spinning the same way. Loudspeakers and headphones also use the motor effect to convert electrical signals into sound.

Exam tips

• Distinguish permanent and induced magnets (induced magnetism is temporary and always attractive).
• Describe magnetic fields: lines from N to S, strongest at the poles.
• Explain how to strengthen an electromagnet: more current, more turns, an iron core.
• For the motor effect, state that the force is greatest at 90° and use Fleming's left-hand rule for direction.
• Be able to explain how a simple electric motor produces continuous rotation (forces on each side + commutator).