Magnetism: Electric Motors and Loudspeakers

Electric Motors

An electric motor uses magnetism to convert electrical energy into mechanical energy.
The operation of an electric motor relies on a coil of wire, known as the armature, that spins within a magnetic field when an electric current is passed through it.
The spinning coil is supported by an axle and sits between two polarised magnets.
The motor effect causes the coil to spin. As current flows through the coil at right angles to a magnetic field, a force acts on it, causing it to turn.
At the correct instances, a device called a commutator reverses the direction of the electric flow. This ensures that the coil continues to spin in the same direction despite the change in magnetic force.
The size of the current, the strength of the magnetic field, and the number of turns on the coil, can all affect the turning force and hence the rotation speed of the electric motor.
The direction of the spinning of the motor can be determined by Fleming’s left-hand rule.

Loudspeakers also utilise magnetism to convert electrical energy, but instead into sound energy.
Like motors, loudspeakers employ an electromagnet and a permanent magnet, operating together.
A diaphragm (cone) is attached to the coil, and as the coil moves due to the magnetic forces, it causes the diaphragm to vibrate.
These vibrations push and pull air particles, creating pressure variations or sound waves that we perceive as sound.
The frequency of the electricity passing through the coil dictates the frequency at which the diaphragm vibrates, hence the pitch of the sound.
Likewise, the amplitude of the electricity influences the volume of the sound. Higher current results in greater magnetic forces, larger vibrations, and hence louder sound.