• If a coil of wire is rotated inside a magnetic field by an external force, an emf 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

Alternators

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

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

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

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

  

  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)