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Edexcel International GCSE Physics · Section 6 Magnetism and electromagnetism · 6 (c) Electromagnetism
The Motor Effect: Motors and Loudspeakers
In this lesson you'll learn to: explain why a force acts on a current-carrying wire in a
magnetic field in three steps, check its direction with Fleming's left-hand rule, explain how a
d.c. motor keeps turning (and what the split-ring commutator does), and explain how a
loudspeaker makes a sound.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — have a play
Don't read anything yet. On tab 1, press the steps in order: switch on the current, add the magnets, let go of the wire. Watch the fingers of the left-hand rule tick off, then flip the current and do it again. On tab 2, switch the motor on in slow motion and follow one side of the coil: when does its force change direction? Then take the commutator away. On tab 3, play the loudspeaker, then try a battery instead.
Fleming's left-hand rule
✓SeCond finger: Current
✓First finger: Field (N → S)
✓ThuMb: Motion (the force)
Current
Magnets
What's happening
Speed
Coil ends
Cell
Labels
What's happening
The 6-mark answer, in order:
- The current in the wire creates a magnetic field around it.
- This field interacts with the field of the permanent magnet,
- so a force acts on the wire: up on one side of the coil, down on the other.
- The split-ring commutator reverses the current in the coil every half turn,
- so the force on each side of the coil changes direction every half turn,
- so the coil keeps rotating the same way around the axle.
From the
Pitch
Volume
What's happening
The 5-mark answer: the same three motor-effect steps, then two more.
- The current in the coil creates a magnetic field.
- It interacts with the field of the permanent magnet,
- so a force acts on the coil, and the coil and the diaphragm (cone) move.
- The current from the amplifier is a.c., so it keeps changing direction,
- so the force changes direction: the diaphragm vibrates and makes a sound wave.
Slowed right down. A real motor turns many times a second, and a loudspeaker cone vibrates hundreds of times a second.
▲ The field lines on tab 1 are drawn to scale: where they are closer together, the field really is stronger.
Seen the catapult? Now learn the three steps.
The motor, the loudspeaker and the force on a wire all use the same three motor-effect steps. Learn them once and you can answer all three questions on page 11.
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Revision notes
6.12 Why does a force act on the wire? The motor effect
1
The current makes a field
The current in the wire creates a magnetic field around it. The field lines are circles round the wire.
Right-hand grip rule: thumb along the current, fingers curl the way the field goes.2
The two fields interact
This field interacts with the field of the permanent magnet. The two fields add together.
One side: same direction, they add, strong field. Other side: opposite directions, they cancel, weak field.3
So a force acts on the wire
So a force acts on the wire. It is pushed from the strong field towards the weak field.
This is called the motor effect.⭐ Exam tip: explain, don't describe
Describe says what happens ("the wire moves to the right"). Explain says why it happens, and that is the three steps: current makes a field → the fields interact → a force acts. All three lines are worth a mark.
Reverse either the current or the magnetic field and the force reverses. Reverse both and it goes the same way as before.
6.12 The d.c. electric motor
A coil of wire sits on an axle between the poles of a magnet. A cell drives a current into the coil through two brushes, which press on a split-ring commutator: a ring cut into two halves, fixed to the end of the coil so it turns with it.
| Part | What it does |
|---|---|
| Magnet | Gives the magnetic field, from N to S, across the coil. |
| Coil (current-carrying wire) | The current in each side creates a field, so each side feels a force. |
| Split-ring commutator | Turns with the coil and reverses the current in the coil every half turn. |
| Brushes | Stay still and press on the commutator, connecting the turning coil to the cell. |
| Axle | The rod the coil turns around. |
- The current goes one way along one side of the coil and the other way along the other side, so the force is up on one side and down on the other. That turns the coil.
- After half a turn, the sides have swapped places. Without help, each force would now push the coil back.
- But the commutator's two halves have swapped brushes too, so the current in the coil reverses. The force on each side changes direction, and the coil keeps rotating the same way.
⭐ Make the motor turn faster
A bigger current, a stronger magnet, or more turns on the coil all give a bigger force. Reverse the current (turn the cell round) or swap the magnets and the motor spins the other way.
6.12 The loudspeaker
A loudspeaker is the motor effect again. A coil sits around the centre pole of a permanent magnet and is glued to a paper or plastic cone (the diaphragm). The coil is connected to an amplifier.
- The current in the coil creates a magnetic field, which interacts with the magnet's field, so a force acts on the coil and the coil and cone move.
- The amplifier gives a.c., which keeps changing direction, so the force keeps changing direction.
- The cone vibrates back and forth. It pushes and pulls on the air, making a sound wave.
⭐ The motor and the speaker: one difference
In the motor, the split-ring commutator reverses the current every half turn. In the loudspeaker, there is no commutator: the a.c. from the amplifier reverses the current itself. A battery (d.c.) would push the cone out once and it would stay there: no vibration, no sound.
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How to write it in the exam
The force on a wire, the motor and the loudspeaker all start with the same three links. Learn them as one sentence, then add the extra lines for the motor or the speaker.
The three motor-effect links
The current in the wire creates a magnetic field around it.
This field interacts with the field of the permanent magnet,
so a force acts on the wire.
what is therethe fieldsthe result
Exam-style question · direction · 1 mark
A wire carrying a current into the page sits between a N pole on the left and a S pole on the right. State the direction of the force on the wire.
- Downwards (1). First finger to the right (N → S), second finger into the page, thumb points down.
Where marks go missing: using the right hand. The right hand is for the field round a wire. Forces on wires use the left hand.
Exam-style question · the motor · 3 marks
Explain why the coil of a d.c. motor keeps turning in the same direction.
- The split-ring commutator reverses the current in the coil every half turn (1)
- so the force on each side of the coil changes direction every half turn (1)
- so the coil keeps rotating the same way around the axle (1)
Where marks go missing: "the commutator changes the direction of the coil". It changes the direction of the current.
Exam-style question · the loudspeaker · 2 marks
Explain why a loudspeaker connected to a battery makes no sound.
- A battery gives d.c., which flows in one direction only, so the force on the coil is always in the same direction (1)
- so the cone moves once and stays there: it does not vibrate, so no sound wave is made (1)
Exam-style question · factors · 2 marks
Give two ways to make a d.c. motor turn faster.
- Any two of: a bigger current · a stronger magnet · more turns on the coil (1 + 1)
Check your answer has all 3 links whenever it says "explain why a force acts": the current's field, the interaction, the force.
📘 Now do it in your workbook
Find the force on the wire and explain it, label the six parts of the d.c. motor and explain how it works, then explain how a loudspeaker makes a sound.
Edexcel iGCSE Physics Workbook · MAGNETISM & ELECTROMAGNETISM · page 11The full workbook covers the whole of Section 6 Magnetism and electromagnetism, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Page 11 of the workbook has the force on a wire, the d.c. motor to label and explain, and the loudspeaker to explain, with full answers at the back.
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