Revision Notes

4.6.3 A.C. Generator

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A.C. Generator: Basics

  • A generator looks very similar to a motor, but instead of connecting it to a power supply, the coil is spun by some mechanical process which then produces electricity


AC generator, IGCSE & GCSE Physics revision notes

When a coil is spun in a magnetic field, a voltage is induced between the ends of the coil


  • As the coil rotates, it cuts through the field lines
  • This induces an EMF between the end of the coil
    (which could then create a current)
  • The size of this EMF could be increased by:
    • Turning the coil faster
    • Increasing the strength of the magnetic field
    • Adding more turns to the coil
  • Slip rings, attached to the ends of the coil, transfer the current to metal brushes whilst allowing the coil to rotate freely


Exam Tip

When a conductor cuts through field lines, it induces an EMF.

Motors and generators look very similar, but they do very different things. When tackling a question on either of them, make sure you are writing about the right one!

Remember to say ‘add more turns to the coil’ and not ‘add more coils’ – if you give the second one it will be marked wrong.


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The Output

  • The A.C. generator creates an alternating current, varying in size and direction as the coil rotates
    • The induced EMF is greatest when the coil is horizontal, as in this position it cuts through the field at the fastest rate
    • The EMF is smallest when the coil is vertical, as in this position it will not be cutting through field lines


Generator output, IGCSE & GCSE Physics revision notes

Diagram showing how the current varies with the position of the coil


Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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