A.C Generator (OCR A Level Physics)

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A.C Generator

  • If a coil of wire is rotated inside a magnetic field by an external force, an e.m.f. will be generated in the wire which causes current to flow within the coil
  • The generator effect can be used to:
    • Generate a.c. in an alternator
    • Generate d.c. in a dynamo


  • A simple alternator is a type of generator that converts mechanical energy to electrical energy in the form of alternating current

Alternator, downloadable IGCSE & GCSE Physics revision notes

An alternator is a rotating coil in a magnetic field connected to commutator rings

  • A rectangular coil that is forced to spin in a uniform magnetic field
  • The coil is connected to a centre-reading meter by metal brushes that press on two metal slip rings (or commutator rings)
    • The slip rings and brushes provide a continuous connection between the coil and the meter

  • When the coil turns in one direction:
    • The pointer defects first one way, then the opposite way, and then back again
    • This is because the coil cuts through the magnetic field lines and a potential difference, and therefore current, is induced in the coil

  • The pointer deflects in both directions because the current in the circuit repeatedly changes direction as the coil spins
    • This is because the induced potential difference in the coil repeatedly changes its direction
    • This continues on as long as the coil keeps turning in the same direction

  • The induced potential difference and the current alternate because they repeatedly change direction


  • A dynamo is a direct-current generator
  • A simple dynamo is the same as an alternator except that the dynamo has a split-ring commutator instead of two separate slip rings

The electric motor, IGCSE & GCSE Physics revision notes

A dynamo is a rotating coil in a magnetic field connected to a split ring commutator

  • As the coil rotates, it cuts through the field lines
    • This induces a potential difference between the end of the coil
  • The split ring commutator changes the connections between the coil and the brushes every half turn in order to keep the current leaving the dynamo in the same direction
    • This happens each time the coil is perpendicular to the magnetic field lines

    dc-dynamo-output, IGCSE & GCSE Physics revision notes

    D.C output from a dynamo - the current is only in the positive region of the graph

  • Therefore, the induced potential difference does not reverse its direction as it does in the alternator
  • Instead, it varies from zero to a maximum value twice each cycle of rotation, and never changes polarity (positive to negative)
    • This means the current is always positive (or always negative)

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Joanna obtained her undergraduate degree in Natural Sciences from Cambridge University and completed her MSc in Education at Loughborough University. After a decade of teaching and leading the physics department in a high-performing academic school, Joanna now mentors new teachers and is currently studying part-time for her PhD at Leicester University. Her passions are helping students and learning about cool physics, so creating brilliant resources to help with exam preparation is her dream job!

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