The A.C. Generator (Cambridge (CIE) O Level Physics): Revision Note

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

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

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

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