The Generator Effect (Oxford AQA IGCSE Physics)

The Generator Effect

Inducing a potential difference in a circuit

• The process of generating a potential difference in a conductor using a magnetic is called electromagnetic induction, or the generator effect

• There are two methods of inducing a potential difference in a conductor:

  • Moving a conductor within a fixed magnetic field

  • Placing a conductor in a changing magnetic field

• If the conductor is part of a complete circuit then the induced potential difference produces a current in that circuit

Moving the electrical conductor

• When a conductor (such as a wire) is moved perpendicular to the direction of the field lines of a magnetic field (which are fixed) the wire cuts through the field lines

  • A potential difference is induced in the circuit which creates the current

  • An ammeter detects the current in the circuit

Moving an electrical conductor in a magnetic field

Moving the magnetic field

• As the magnet moves through a fixed coil, the field lines cut through the turns on the coil

• This generates a potential difference in the coil and induces a current

Moving the magnetic field relative to the conductor

AC generator & DC dynamo

• The generator effect can be used to:

  • Generate ac in an alternator

  • Generate dc in a dynamo

• A simple alternator is a type of generator that uses mechanical work to produce an alternating current

Diagram of an alternator

• A rectangular coil is forced (e.g. by rising steam or wind rotating a turbine) 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 ammeter

• When the coil rotates continuously in the same direction:

  • The coil cuts through the magnetic field lines, so 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 as long as the coil keeps turning in the same direction

• The induced potential difference and the current alternate because they repeatedly change direction / polarity

Alternating current produced by alternator

• A dynamo is a direct current (d.c.) 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

Diagram of a dynamo

• 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)

Direct current produced by dynamo