Generators & Dynamos (Edexcel IGCSE Physics)

Spec point — 6.16 · describe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field, and describe the factors that affect the size of the induced voltage

Generators & dynamos

• When a coil of wire is rotated within a magnetic field, an electric current can be generated

  • This is known as the generator effect
  • This is how electricity is produced

• The generator effect can be used to:

  • Generate a.c in an generator
  • Generate d.c in a dynamo

Generators

• A simple alternator is a type of generator that produces an alternating current (a.c.)

An alternator is a rotating coil in a magnetic field with slip rings

• A rectangular coil is forced to spin in a uniform magnetic field

• The coil is connected to a circuit which contains 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

• When the coil turns in one direction:

  • The pointer deflects first one way, then the opposite way, and then back again
  • This is because the coil cuts through the magnetic field lines and an alternating potential difference is induced in the coil
  • The alternating potential difference produces an alternating current(a.c.)

• An alternating current may also be produced when a magnet rotates within a stationary coil

  • Both methods operate on the principle that p.d. is induced when a coil experiences a changing external magnetic field

• 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 type of generator that produces a direct current
• 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

Exam Hint 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!

You are also expected to know that alternating current can be produced when a coil rotates in a magnetic field, or when a magnet rotates within a coil. The key is relative motion between the coil and the magnet.