• The?generator effect?can be used to:
  • Generate?a.c?in an?generator
  • 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

An alternator is a rotating coil in a magnetic field with 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.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

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

   

• 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)

   

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)

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