AQA GCSE Physics

Revision Notes

7.3.2 Applications of the Generator Effect

Higher Tier Only

Applications of the Generator Effect

  • The generator effect can be used to:
    • Generate a.c in an alternator
    • Generate d.c in a dynamo

Alternator

  • 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

ac-alternator-output, IGCSE & GCSE Physics revision notes

A.C output from an alternator – the current is both in the positive and negative region of the graph

Dynamos

  • 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 notesA 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
  • 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)

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

Bicycle Dynamo

  • A bicycle dynamo is used to supply electricity to bicycle lights whilst in motion
  • It consists of a rotating magnet placed inside (or next to) a coil
  • The magnet is rotated by its connection to the bicycle inside the coil
    • This is sometimes called the friction wheel and the axle / spindle
  • The magnetic field lines cut through the sides of the coil
    • This induces a potential difference in the coil
  • Since the magnetic field is constantly changing direction as it rotates, so does the output potential difference
    • This means the output current is also changing direction
  • Therefore, a bicycle dynamo, unlike a normal dynamo, produces alternating current (a.c)

Bicycle Dynamo, downloadable IGCSE & GCSE Physics revision notes

A bicycle dynamo consists of a magnet rotating in a coil due to the motion of the wheels

Exam Tip

Motors and generators look very similar (as do microphones and loudspeakers), but they do very different things.

When tackling a question on either of them, make sure you are writing about the right one!

You might be expected to give the above explanations – make sure that you understand their subtle differences!

Author: Katie

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.
Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top