Edexcel IGCSE Physics · Spec 6.6-6.7
Electromagnetic Induction & Generators
Electromagnetic induction and how generators produce electricity.
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Electromagnetic Induction & Generators
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Explained
Making electricity by moving a magnet
Electromagnetic induction is the motor effect running backwards. In a motor, a current in a magnetic field produces movement. In induction, movement in a magnetic field produces a voltage.
Move a magnet into a coil of wire and a voltage is induced across the coil. If the coil is part of a complete circuit, a current flows. Stop moving it and the voltage disappears immediately, because it is the change that matters, not the presence of the magnet.
What is actually required
Three ideas make a complete answer, and they need to be in this order.
First, there must be relative motion between the magnet and the coil. It makes no difference which one moves; moving the coil past a stationary magnet works just as well.
Second, that motion means the conductor cuts through the magnetic field lines, so the field through the coil is changing.
Third, the result is that a voltage is induced across the coil.
Making the induced voltage bigger
- Move the magnet or the coil faster.
- Use a stronger magnet.
- Use more turns on the coil.
- Add an iron core, which concentrates the field.
Reversing the direction of the motion, or turning the magnet round, reverses the direction of the induced voltage. That reversal is what makes a generator produce alternating current.
What the examiners say about this topic
A principal examiner report for Edexcel International GCSE Physics is unusually direct here. Successful candidates made clear three required ideas: that there was relative motion between the magnet and the coil, that the coil was cutting field lines, and that a voltage was induced.
It then gives a warning worth quoting. Candidates should avoid using non technical language which has been given in the question, and in this case generates electricity would not score any marks, compared with the more technical induces a voltage. Elsewhere the report repeats that generate is not sufficient for induce.
Two responses are described. One scored a single mark for the idea of cutting field lines, having used neither the term induction nor the idea of relative motion. Another described the relative motion clearly but was not specific enough with the technical language to earn either of the remaining marks.
The mark scheme accepts potential difference, p.d. or EMF in place of voltage, and condones induces a current. So there is flexibility in which technical term you choose. There is none about whether you use one.
One further contrast is worth noticing. On a motor effect question, a different mark scheme instructs the examiner to ignore cutting field. Cutting field lines is the right idea for induction and the wrong one for the motor effect, so the phrase earns a mark in one topic and nothing in the other.
The alternating current generator
A coil is rotated between the poles of a magnet. As it turns, the sides of the coil cut through the field lines, so a voltage is induced.
Every half turn, each side of the coil moves through the field in the opposite direction, so the induced voltage reverses. The output therefore alternates, and the coil is connected to the circuit through slip rings and brushes, which maintain the connection while allowing continuous rotation.
Slip rings are what distinguish a generator from a d.c. motor, which uses a split ring commutator instead. The commutator reverses the connection every half turn to keep the current flowing one way; slip rings deliberately do not, so the alternating output passes straight through.
Spinning the coil faster does two things at once: it raises the peak voltage, because the field is being cut more quickly, and it raises the frequency, because each rotation takes less time.
Spec 6.6-6.7
What you need to know
- Describe electromagnetic induction
- State what makes the induced voltage bigger
- Explain how an a.c. generator works
Active recall
Quick check
Answer each question before opening the answer.
What is electromagnetic induction?
A voltage is induced when a conductor cuts magnetic field lines, or when the field through a coil changes.
How can you increase the induced voltage in a generator?
Turn the coil faster, use a stronger magnet, or add more turns to the coil.
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