Motor Effect, Loudspeakers and Generators (Paper 2)

Explained

Moving charges, loudspeakers and generators

The force on a current carrying wire has a cause one level down. A current is moving charge, and a charge moving across a magnetic field experiences a force. The wire is pushed because the charges inside it are pushed, and they drag the wire with them.

The force only appears when the motion is not parallel to the field. A charge moving along the field lines feels nothing at all.

How a loudspeaker works

A loudspeaker turns an electrical signal into sound using the motor effect.

• A coil of wire sits in the field of a permanent magnet, attached to a paper cone.
• An alternating current passes through the coil.
• The current direction reverses many times each second, so by the left hand rule the force on the coil reverses at the same rate.
• The coil and cone vibrate backwards and forwards, pushing on the air and creating a sound wave.

The frequency of the sound matches the frequency of the alternating current, and a larger current gives a larger amplitude of vibration, which means a louder sound. If a question asks how to make it louder or higher pitched, those are the two levers.

Generators run the effect backwards

A motor uses a current in a field to produce movement. A generator uses movement in a field to produce a current. It is the same physics read in the other direction, and saying so is often the cleanest way to answer a comparison question.

Moving a wire across a magnetic field, or changing the field through a coil, induces a potential difference. If the circuit is complete, a current flows.

The two generator arrangements

An a.c. generator uses slip rings. Each end of the coil stays connected to the same ring throughout, so as the coil turns the current in the external circuit reverses every half turn. The output is alternating current.

A d.c. generator uses a split ring commutator instead. The connection swaps every half turn, at the same moment the current in the coil would otherwise reverse, so the output always flows the same way. The result is direct current, though it varies in size rather than being steady.

Slip rings for alternating, split ring for direct. The commutator is the same component that makes a d.c. motor turn continuously, doing the equivalent job in reverse.

Increasing the output

Turn the coil faster, use a stronger magnet, or add more turns to the coil. All three increase the induced potential difference, and turning faster also increases the frequency of the alternating output.