⬆️ One side
With the field running left to right, the current into the page on this side gives a force downwards.
Electromagnetism » Electric Motors and Loudspeakers
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 6.12
- How a simple d.c. electric motor turns, part by part
- What the split-ring commutator and brushes do
- How to make a motor spin faster, or the other way round
- How a loudspeaker uses the same force to make sound
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Unlock This CourseFrom a pushed wire to a spinning coil
In the lesson on the motor effect you saw that a wire carrying a current in a magnetic field is pushed sideways. One push is not much use. An electric motor uses that push over and over to make something spin, as in a fan, a washing machine or an electric toy car. Here we look at the simple d.c. motor, which runs on direct current from a battery.
Key terms:
- Coil: a loop (or many loops) of insulated wire that can spin.
- Split-ring commutator: a metal ring cut into two halves, fixed to the coil so that it turns with it.
- Brushes: two small blocks of carbon that press lightly against the split ring and carry the current to it.
The parts of a simple d.c. motor
| Part | What it does |
|---|---|
| Permanent magnets | Two magnets with unlike poles facing, giving a field from north to south across the gap |
| Coil | A rectangular coil of wire between the poles, free to turn on an axle |
| Split-ring commutator | Two half rings on the axle, each joined to one end of the coil |
| Brushes | Touch the two halves of the ring and connect them to the battery |
| Battery or d.c. supply | Drives the current round the coil |
To draw it in the exam: draw a N pole on the left and a S pole on the right, a rectangular coil between them, the split ring on the axle below the coil, a brush touching each half, and a battery joined to the brushes.
How the coil turns
The coil has two long sides, one next to each pole. Current flows one way along one side and the opposite way along the other side, because it goes round the loop. Use the left-hand rule from the last lesson on each side.
⬇️ The other side
The current comes out of the page on this side, so the force is upwards.
One side is pushed up and the other is pushed down. These opposite forces on the two sides make the coil turn.
Keeping it turning: the commutator
Without the commutator the coil would turn only a quarter of a turn and stop. When the coil reaches the vertical position, the forces would start to pull it back the way it came.
The split-ring commutator fixes this. Every half turn, each half of the ring moves round to touch the other brush. This reverses the current in the coil. The force on each side therefore reverses at just the right moment, and the coil keeps turning in the same direction.
The brushes stay still and the ring spins against them. The brushes carry the current into the spinning coil without twisting the wires.
Describe it in order
1. Current flows through the brushes and the split ring into the coil. 2. The two sides of the coil are in the magnetic field, with current flowing in opposite directions. 3. The forces on the two sides are opposite, so the coil turns. 4. Each half turn, the split ring swaps contact with the brushes and reverses the current. 5. The forces keep pushing the coil the same way round.
Changing the motor
| Change | Effect |
|---|---|
| Larger current | Bigger force, so it turns faster |
| More turns on the coil | Bigger force, so it turns faster |
| Stronger magnets (stronger field) | Bigger force, so it turns faster |
| Reverse the current (swap the battery connections) | Motor turns the opposite way |
| Reverse the field (swap the magnets round) | Motor turns the opposite way |
Worked example
A student wants a toy motor to spin faster and then to run backwards.
Faster: increase the current, add more turns to the coil, or use stronger magnets. Each one makes the force on the coil bigger.
Backwards: swap the two wires on the battery so the current is reversed. The forces reverse and the coil turns the other way.
Loudspeakers
A loudspeaker uses the same force on a current-carrying wire, but to make a vibration instead of a spin. Its parts are:
- a permanent magnet, which gives a magnetic field;
- a coil of wire placed in that field, which is free to move in and out;
- a paper cone fixed to the coil.
An alternating current from an amplifier flows through the coil. It keeps changing direction, so the force on the coil keeps changing direction too. The coil is pushed forwards, then backwards, over and over. The cone moves with it, so the cone vibrates. The vibrating cone pushes on the air and makes sound waves.
Learn this
Loudspeaker: alternating current in the coil, so a changing force in the magnetic field, so the cone vibrates, so sound waves are made. The cone vibrates at the same frequency as the alternating current.
Common mistakes
Common mistakes
- Saying the commutator makes the motor turn. It reverses the current every half turn so the coil keeps going the same way.
- Forgetting that the two sides of the coil have opposite currents, so the forces are opposite.
- Saying a loudspeaker uses a direct current. It uses an alternating current, which is why the cone moves both ways.
- Writing that the magnet moves in a loudspeaker. The permanent magnet stays still; the coil and cone move.
Exam-style question
A simple d.c. electric motor has a coil between two permanent magnets.
(a) Describe the job of the split-ring commutator. [2 marks]
(b) State two ways to make the motor turn faster. [2 marks]
(c) Describe how a loudspeaker produces sound. [3 marks]
Model answer
(a) It reverses the current in the coil every half turn (1). This keeps the forces pushing the coil in the same direction, so it keeps turning one way (1).
(b) Increase the current (1). Use stronger magnets, or add more turns to the coil (1).
(c) An alternating current flows in a coil placed in the field of a permanent magnet (1). The force on the coil keeps changing direction, so the coil and the cone fixed to it vibrate (1). The vibrating cone makes sound waves in the air (1).
Exam tip
For a "describe the job" question, say what the part does and why that matters. "It reverses the current" earns one mark; "so the coil keeps turning the same way" earns the second.