Topic 13 – Electromagnetic induction — Edexcel GCSE Physics
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Topic 13 – Electromagnetic induction explained
Electromagnetic induction occurs when a conductor experiences a changing magnetic field, inducing a potential difference across it.
Read the full explanation
In an alternator, a coil rotates between the poles of a magnet; each half-turn the coil sides cut the field in the opposite sense, so the induced p.d. reverses direction and the output current alternates (a.c.). A dynamo instead uses a split-ring commutator: it reverses the connection to the external circuit every half-turn, so the current in the external circuit keeps one direction (d.c.), even though the induced p.d. in the coil itself still alternates. Explaining the difference means linking the rotating coil, the changing flux linkage, the induced p.d. and the commutator or slip rings to the resulting current direction.
Your focus
- Describe how a rotating coil in a magnetic field produces an induced potential difference.
- Explain why an alternator produces current that alternates in direction.
- Explain how a split-ring commutator makes a dynamo deliver direct current to an external circuit.
Topic 13 – Electromagnetic induction exam tips
Marking Points
- Describes a coil rotating in a magnetic field so that the magnetic flux linking the coil changes continuously.
- Explains that a changing magnetic flux induces a potential difference across the coil, in line with electromagnetic induction.
- States that in an alternator the induced p.d. reverses every half-turn because the coil sides cut the field in opposite directions.
- Identifies slip rings in an alternator as the connection that lets the output current alternate in direction (a.c.).
- Explains that a dynamo uses a split-ring commutator that reverses the coil-to-circuit connections every half-turn.
- States that the commutator keeps the external current flowing in one direction, giving direct current (d.c.).
- Distinguishes the alternating p.d. induced in the dynamo coil from the one-directional current delivered to the external circuit.
- Links the rate of rotation to the frequency of the alternating current in an alternator.
Examiner Tips
- 💡Name the component that determines the output type: slip rings for a.c., split-ring commutator for d.c.
- 💡Explain the mechanism step by step: rotating coil, changing flux, induced p.d., connection to circuit, resulting current direction.
- 💡Use a sketch of the coil position at maximum and zero flux linkage to support the explanation of reversal.
- 💡Keep the distinction between the induced p.d. in the coil and the current in the external circuit explicit.
Common Mistakes
- Claims that a dynamo induces a steady one-directional p.d. in the coil; correct this by noting the coil p.d. still alternates and the commutator rectifies the external current.
- Confuses slip rings with a split-ring commutator; keep slip rings for the alternator and the commutator for the dynamo.
- States that a stationary coil in a steady field generates current; require a changing flux, produced by rotation or a changing field.
- Describes the output of an alternator as direct current; link the alternating output to the reversing induced p.d. and slip rings.