The A.C. Generator (Cambridge O Level Physics)

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Katie M

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Katie M

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Simple A.C Generators

  • The generator effect can be used to generate a.c in an alternator
  • A simple alternator is a type of generator that produces an alternating current

Structure of an Alternator

new-7-3-2-alternator

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

  • A rectangular coil 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
    • The slip rings and brushes provide a continuous connection between the coil and the meter

  • 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 an EMF, 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 EMF in the coil repeatedly changes its direction
    • This continues on as long as the coil keeps turning in the same direction

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

Examiner Tip

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! A motor takes in electricity and turns it into motion. A generator takes in motion and generates electricity.

You might be expected to give explanations of how these two things happen - make sure that you understand their subtle differences!

Graphs for A.C. Generators

  • The A.C. generator creates an alternating current, varying in size and direction as the coil rotates
  • Potential difference (also called e.m.f.) is induced when coil rotates in the external magnetic field

new-7-3-3-ac-graphs

Alternating e.m.f. with corresponding positions of the coil relative to the field

  • When the number of field lines through the coil is at a maximum, induced e.m.f. is at a minimum
    • In positions 1 and 3 in the diagram below, the number of field lines through the coil is at a maximum and induced e.m.f. is zero in this position
  • When the number of field lines through the coil is at a minimum, induced e.m.f. is at a maximum
    • In positions 2 and 4, no field lines pass through the centre of the coil

Examiner Tip

For your exam, you need to be aware that an alternating current can be produced by:

  • A coil rotating in a magnetic field
  • A magnet rotating within a coil

Both will induce an e.m.f. in the coil as they both ensure the coil will experience a changing magnetic field.

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.