Motors and Generators

A speaker consists of a moveable coil attached to a large cone. The coil vibrates at the same frequency as the alternating current signal and the cone turns this movement into sound.

• An electric drill uses a motor to produce movement.
• A wind turbine uses the energy in the wind to spin a generator to produce electrical energy.

electical energy ------------ motor ------------> kinetic energy

kinetic energy --------- generator ----------> electical energy

Each device also wastes some energy. The motor produces mainly heat energy and the wind turbine produces mainly sound energy as waste energy.

The important discovery of how to produce electricity from movement was made by Michael Faraday.

He moved a wire, which was connected to a meter, through a magnetic field. The current created, known as the induced current, is only produced when the wire is moving. The wire must also be moving in the right direction which is cutting across the field lines.

The current is very small but can be increased by moving the wire more quickly and using a stronger field.

The induced current can be increased by making more of the wire cut across the field lines. This can be done by making the wire into a coil. A coiled wire longer than its width is known as a solenoid.

Factors that affect the size of the current are:

• The thickness of the wire
• The resistance of the wire
• The length of the wire

We often also refer to the induced voltage or potential difference that is created across the ends of the coil.

A bicycle dynamo can be used to generate alternating current. The current or voltage produced can be plotted as a sine wave. The x-axis will be labelled as time. The number of times per second that the current alternates is called the frequency and is measured in Hertz. If the dynamo is turned more slowly the waves are lower and further apart.

The coil in an alternating current generator would have a large number of turns in order to induce a large voltage. The induced voltage depends on wires cutting through the magnetic field lines of the magnets. Some generators work differently and have the magnet rotating inside a stationary coil. Electrical contacts are made using brushes and slip rings. Mains electricity is generated at a frequency of 50 hertz and a peak voltage of about 330v.