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How can a magnet and a coil be used to generate electricity?

Describe electromagnetic induction and how a generator and transformer make use of it

Describe how moving a magnet near a coil induces a voltage, how a simple generator produces electricity, and the basic idea of a transformer at N(A)-Level.

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to describe electromagnetic induction (using a changing magnetic field to produce a voltage), to explain how a simple generator works, and to describe the basic idea of a transformer. The big idea is the reverse of the motor effect: instead of a current making movement, movement makes a current.

The answer

Electromagnetic induction

If you move a magnet into or out of a coil of wire, a voltage is produced across the coil. If the coil is part of a complete circuit, this voltage drives a current. This effect is called electromagnetic induction, and the voltage is said to be induced.

The key point is that the magnetic field through the coil must be changing. Movement is what causes the change:

  • Push the magnet in, and a voltage is induced one way.
  • Pull the magnet out, and the voltage is induced the other way.
  • Hold the magnet still, and there is no change, so no voltage is induced.

Making the induced voltage bigger

The induced voltage is larger if the magnetic field through the coil changes faster or by more. You can increase it by:

  • moving the magnet faster;
  • using a stronger magnet;
  • using a coil with more turns.

The simple generator

A generator turns kinetic energy into electrical energy using induction. A coil is spun in a magnetic field (or a magnet is spun near a coil). As the coil rotates, the magnetic field through it keeps changing, so a voltage is continually induced. This drives a current in the connected circuit.

Spinning the coil faster makes the field change more quickly, giving a larger voltage and more frequent changes. A simple generator produces a current that reverses direction each half turn (alternating current).

Transformers

A transformer uses induction to change the size of an alternating voltage. It has two coils wound on the same iron core. An alternating current in the first coil makes a constantly changing magnetic field in the core, which induces a voltage in the second coil.

Transformers are how the power grid raises voltage for efficient long-distance transmission and lowers it again to a safe level for homes. They work only with alternating current, because a steady current would not give a changing field.

Examples in context

Example 1. Power stations. In a power station, a turbine (driven by steam, water or wind) spins a generator. The spinning coil in a magnetic field induces a large alternating voltage, which is the source of the mains electricity in homes. The original energy may be chemical, nuclear or from moving water, but induction is the final step that makes the electricity.

Example 2. A bicycle dynamo. A small dynamo on a bicycle wheel contains a magnet that spins as the wheel turns, near a coil. The changing field induces a voltage that lights the bicycle lamp. The faster you pedal, the faster the magnet spins, the larger the induced voltage, and the brighter the lamp.

Try this

  • Cue. State what the meter reads when a magnet is held still inside a coil, and why. [2 marks] Zero, because the magnetic field through the coil is not changing, so no voltage is induced.

  • Cue. State two ways to increase the voltage induced when a magnet is moved into a coil. [2 marks] Move the magnet faster and use more turns on the coil (a stronger magnet also works).

  • Cue. State the energy change that takes place in a generator. [1 mark] Kinetic energy is transferred to electrical energy.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksA bar magnet is pushed into a coil connected to a sensitive meter. (a) State what the meter shows. (b) State two ways to make the reading bigger. (c) State what happens when the magnet is held still inside the coil.
Show worked answer →

(a) The meter shows a reading: a voltage (and current) is induced in the coil while the magnet moves.

(b) Any two: move the magnet faster, use a stronger magnet, or use a coil with more turns.

(c) When the magnet is held still, the meter reads zero, because there is no movement and so no change in the magnetic field through the coil.

What markers reward: a voltage induced while the magnet moves, two valid ways to increase it, and zero reading when there is no movement (no change).

Original4 marks(a) Explain how a simple generator produces a voltage. (b) State what happens to the induced voltage if the coil spins faster. (c) State one everyday use of electromagnetic induction.
Show worked answer →

(a) A coil is turned in a magnetic field. As it rotates, the magnetic field through the coil keeps changing, which induces a voltage across the coil. This drives a current in a connected circuit.

(b) Spinning the coil faster makes the field change more quickly, so the induced voltage is larger.

(c) Any sensible use, for example generating mains electricity in a power station, a bicycle dynamo, or transformers in the power grid.

What markers reward: a coil turning in a field with a changing field inducing a voltage, faster spin giving a larger voltage, and a valid everyday use.

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