AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY
Electric motors
Magnetism and electromagnetism · Lesson 4 of 8
Teacher copy - includes the notes for whoever is teaching from it.
Warm-up
Answer each one, then check.
1. What is the motor effect?
The force on a current-carrying wire in a magnetic field
2. What does a motor transfer energy to?
Kinetic energy store
3. What does the left-hand rule give?
The direction of the force
4. What is a commutator?
A part that reverses the current each half turn
5. What are brushes?
Carbon contacts touching the commutator
Learning Objectives
1. Describe how a coil of wire in a magnetic field rotates.
2. Explain the role of the split-ring commutator.
3. Explain how to increase the speed of a motor.
4. Explain why the forces on the two sides of the coil are in opposite directions.
The D.c. Motor
A current-carrying coil in a magnetic field experiences forces on its two long sides that act in opposite directions, so it rotates. The split-ring commutator reverses the current every half turn to keep it turning in the same direction.
The forces are opposite because the current flows in opposite directions along the two sides of the coil.
A d.c. Motor
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The two sides of the coil feel opposite forces. |
A d.c. motor with a coil, magnets, split-ring commutator and brushes, with the forces on the two sides shown.
How a d.c. Motor Turns
Follow the steps.
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1 Current flows Through the coil via the brushes and commutator. |
2 Forces act One side of the coil is pushed up, the other down (motor effect). |
3 The coil rotates The opposite forces turn the coil. |
4 The commutator swaps The split ring reverses the current each half turn. |
5 Rotation continues The forces always turn the coil the same way. |
Increasing the Speed
Ways to make a motor faster.
|
Change |
Why it works |
|---|---|
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Increase the current |
Larger force on the coil |
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Stronger magnets |
Larger magnetic flux density |
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More turns on the coil |
Larger total force |
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Reverse the battery |
Motor spins the other way |
Why the Commutator Matters
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Explain what would happen without the split-ring commutator. |
1. Half a turn
The coil would turn to the vertical position
2. Without a commutator
The forces would then turn it back
3. Result
The coil would only oscillate and stop
Answer: The coil would not rotate continuously.
Key Terms
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d.c. motor A motor that uses direct current to make a coil rotate in a magnetic field. |
Commutator A split ring that reverses the current each half turn. |
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Brushes Carbon contacts that touch the commutator. |
Coil A loop of wire that turns in the magnetic field. |
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Rotation Turning about an axis. |
Motor effect The force on a current-carrying wire in a magnetic field. |
Your Task: Motor Build
15 minutes
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Explain each part of a model motor: coil, magnets, commutator, brushes, battery. What happens if you reverse the battery? 1. Use the left-hand rule for each side. |
A good answer shows: The motor spins in the opposite direction.
Note: Ask how to make it spin faster.
Can I...?
☐ Name the parts.
☐ Explain the forces on the coil.
☐ Explain the commutator.
☐ Describe rotation.
☐ Increase the speed.
☐ Reverse the direction.
☐ Say why opposite forces.
☐ Link to the motor effect.
Summary
✓ Coil in a field feels opposite forces on opposite sides.
✓ The split ring swaps the current direction each half turn.
✓ This keeps the coil rotating the same way.
✓ A larger current or stronger field makes it faster.
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EXAM FOCUS Describe the job of the split-ring commutator. (2 marks) It reverses the current every half turn. |
Exam Practice: Electric motors
Answer all questions. Use the mark allocation as a guide to how much to write. · 15 minutes
▸ Question 1 · 2 marks · State. State two ways of increasing the speed of rotation of a d.c. motor.
▸ Question 2 · 3 marks · Explain. Explain why the coil in a d.c. motor turns when a current flows through it.
▸ Question 3 · 3 marks · Explain. Explain the purpose of the split-ring commutator in a d.c. motor.
▸ Question 4 · 2 marks · Explain. The battery in a d.c. motor is connected the other way round. Describe and explain the effect.
▸ Question 5 · 4 marks · Explain. A motor rotates slowly. Explain two changes that would make it rotate faster.
Question 1 · 2 marks · State
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“State two ways of increasing the speed of rotation of a d.c. motor.” |
HOW TO ANSWER IT Command word: State. Worth 2 marks, so plan before writing.
Question 1 · mark scheme
2 marks available. Award a mark for each point made.
▸ One way. 1 mark
▸ Another way. 1 mark
▸ Model answer. Increase the current. Use stronger magnets. Use more turns. (any two)
Question 2 · 3 marks · Explain
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“Explain why the coil in a d.c. motor turns when a current flows through it.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 2 · mark scheme
3 marks available. Award a mark for each point made.
▸ Coil in a field carrying a current experiences forces. 1 mark
▸ Current in opposite directions in the two sides. 1 mark
▸ Opposite forces turn the coil. 1 mark
▸ Model answer. The current in the coil is at right angles to the magnetic field so each long side of the coil experiences a force. The current in the two sides is in opposite directions, so the forces are in opposite directions and turn the coil.
Question 3 · 3 marks · Explain
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“Explain the purpose of the split-ring commutator in a d.c. motor.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 3 · mark scheme
3 marks available. Award a mark for each point made.
▸ Reverses the current. 1 mark
▸ Every half turn. 1 mark
▸ Keeps the coil turning the same way. 1 mark
▸ Model answer. It reverses the direction of the current in the coil every half turn so that the forces keep turning the coil in the same direction, giving continuous rotation.
Question 4 · 2 marks · Explain
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“The battery in a d.c. motor is connected the other way round. Describe and explain the effect.” |
HOW TO ANSWER IT Command word: Explain. Worth 2 marks, so plan before writing.
Question 4 · mark scheme
2 marks available. Award a mark for each point made.
▸ Turns in the opposite direction. 1 mark
▸ Current (forces) reversed. 1 mark
▸ Model answer. The current in the coil is reversed, so the forces are reversed and the motor turns in the opposite direction.
Question 5 · 4 marks · Explain
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“A motor rotates slowly. Explain two changes that would make it rotate faster.” |
HOW TO ANSWER IT Command word: Explain. Worth 4 marks, so plan before writing.
Question 5 · mark scheme
4 marks available. Award a mark for each point made.
▸ Increase the current. 1 mark
▸ Larger force. 1 mark
▸ Stronger magnets or more turns. 1 mark
▸ Larger force or flux density. 1 mark
▸ Model answer. Increase the current, which increases the force on the coil. Use stronger magnets, which increases the magnetic flux density so the force is larger. More turns also increase the force.