The d.c. Motor and the Loudspeaker

The d.c. motor turns electrical energy into rotation using the motor effect.

How it works:

1. A coil sits in a magnetic field between the poles of a permanent magnet.
2. Current flows through the coil. By Fleming's left-hand rule the force on one side is upwards and on the other downwards, because the current runs in opposite directions on the two sides.
3. This pair of opposite forces creates a turning effect, and the coil rotates.

The problem, and the part examiners actually ask about. After half a turn, the side that was on the left is now on the right. The forces would now turn the coil back the way it came, and it would simply rock back and forth.

The split-ring commutator solves this. It is a ring cut into two halves, connected to the ends of the coil and pressed against fixed carbon brushes. Every half turn the halves swap brushes, which reverses the current in the coil. Because the current reverses at exactly the moment the coil passes the vertical, the forces stay in the same rotational sense and the coil keeps turning one way.

Making a motor turn faster: increase the current, use a stronger magnet, add more turns to the coil, or wind the coil on a soft iron core.

Reversing a motor: reverse the current or reverse the magnetic field — but not both, since two reversals cancel.

The loudspeaker uses the same effect to make sound.

1. An alternating current from the amplifier flows through a coil attached to the base of a paper cone.
2. The coil sits in the field of a permanent magnet, so it experiences a force.
3. Because the current alternates, the force reverses direction many times a second, so the coil moves in and out.
4. The cone attached to it vibrates, pushing and pulling the air into compressions and rarefactions — which is a sound wave.

The frequency of the a.c. sets the pitch, and its amplitude sets the loudness. The loudspeaker is therefore a direct conversion of an electrical signal into a longitudinal wave.

Example: a motor connected to a.c. without a commutator vibrates instead of rotating, because the current reverses on its own schedule rather than in step with the coil's position. The commutator exists to synchronise the reversal with the rotation.