Electric Motors and Loudspeakers

(Higher Tier only) Everything in this lesson is Higher Tier content.

(Separate Physics only) The loudspeaker section is also Separate Physics content — it is not in the Combined Science course.

Both the electric motor and the loudspeaker are direct applications of the motor effect: a current-carrying conductor in a magnetic field feels a force. If you are unsure of Fleming's left-hand rule or the idea of a force on a current-carrying wire, review those first, because everything here builds on them.

The motor effect turns electrical energy into movement. In a motor that movement is a spinning coil; in a loudspeaker it is a vibrating cone.

Device	Type of current	What the coil does	Useful output
Motor	Direct current	Spins continuously	Rotation
Loudspeaker	Alternating current	Vibrates in and out	Sound waves

(Higher Tier only)

An electric motor has a rectangular coil of wire mounted so it can spin on an axle, sitting between the poles of a permanent magnet. When a current flows, the motor effect acts on the coil.

The key idea is that the two long sides of the coil carry current in opposite directions:

• On one side the current flows one way, so the force (by Fleming's left-hand rule) pushes that side up.
• On the other side the current flows the opposite way, so the force pushes that side down.

Two equal forces acting in opposite directions on opposite sides of the coil create a turning effect (a moment) that rotates the coil.

One side rising while the other falls spins the coil around its axle. This rotation is the useful output of the motor, used in everything from fans and drills to electric cars.

(Higher Tier only)

A problem appears once the coil reaches the vertical position: if nothing changed, the forces would then push the coil back the way it came, and it would rock rather than spin. The split-ring commutator solves this.

The commutator is a metal ring split into two halves, turning with the coil and pressed against fixed contacts called brushes. Every half turn, the gaps in the ring line up with the brushes and the two halves swap over.

The split-ring commutator reverses the direction of the current in the coil every half turn, so the force on each side always acts to push the coil round the same way.

This keeps the rotation continuous and in one direction. Two things control the motor:

• Speed of rotation: increase the current or use a stronger magnetic field to make it turn faster.
• Direction of rotation: reverse the current or reverse the magnetic field (swap the poles) to make it spin the other way. Reversing both together leaves the direction unchanged.

(Higher Tier only) (Separate Physics only)

A loudspeaker (and a pair of headphones) converts an electrical signal into sound using the motor effect. Its parts are a permanent magnet, a coil of wire that can slide over one pole of the magnet, and a paper or plastic cone attached to the coil.

The signal from the amplifier is an alternating current whose pattern matches the sound to be produced. Because the coil sits in the field of the permanent magnet, this current makes the coil feel a motor-effect force:

1. When the current flows one way, the force pushes the coil (and the cone) outward.
2. When the current reverses, the force pushes the coil and cone inward.
3. As the alternating current cycles, the cone vibrates back and forth in step with it.

(Higher Tier only) (Separate Physics only)

The vibrating cone is what actually makes the sound. As it moves out and in, it pushes on the air in front of it:

• Moving outward squashes the air, creating a region of higher pressure (a compression).
• Moving inward lets the air spread out, creating a region of lower pressure (a rarefaction).

These pressure variations travel outward through the air as a longitudinal sound wave, copying the pattern of the alternating current.

So the chain of energy is: electrical signal → motor-effect force on the coil → vibration of the cone → pressure variations in the air → sound wave. The frequency of the alternating current sets the frequency (pitch) of the sound, and a larger current gives a bigger vibration and a louder sound. A microphone does the reverse of this, which is the generator effect.

1. Not explaining why the coil turns

Full marks need the idea that the two sides carry current in opposite directions, so the forces on them act in opposite directions, giving a turning effect. Just saying "there is a force" is not enough.

2. Getting the commutator's job wrong

The split-ring commutator reverses the current in the coil every half turn to keep it spinning the same way. It does not supply the current or reverse the magnet.

3. Reversing both current and field

Reversing the current alone, or the field alone, reverses the direction of rotation. Reversing both at once cancels out and leaves the direction unchanged.

4. Saying a loudspeaker uses the generator effect

A loudspeaker uses the motor effect (current in, movement out). The generator effect (movement in, current out) is what a microphone uses. Do not swap them.

5. Forgetting the current must be alternating

A loudspeaker cone must vibrate back and forth, so it needs an alternating current. A steady direct current would push the cone to one position and hold it there, producing no continuous sound.