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Physics · Magnetism and electromagnetism
Electric motors
Explain how a d.c. electric motor works using the motor effect and the split-ring commutator (Higher tier only; Physics only).
Warm-up
Answer each one, then check.
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1
What is the motor effect?
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The force on a current-carrying wire in a magnetic field
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2
What does a motor transfer energy to?
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Kinetic energy store
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3
What does the left-hand rule give?
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The direction of the force
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4
What is a commutator?
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A part that reverses the current each half turn
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5
What are brushes?
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Carbon contacts touching the commutator
Learning Objectives
- 1Describe how a coil of wire in a magnetic field rotates.
- 2Explain the role of the split-ring commutator.
- 3Explain how to increase the speed of a motor.
- 4Explain 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
The two sides of the coil feel opposite forces.
How a d.c. Motor Turns
Follow the steps.
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1
Current flows
Through the coil via the brushes and commutator.
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2
Forces act
One side of the coil is pushed up, the other down (motor effect).
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3
The coil rotates
The opposite forces turn the coil.
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4
The commutator swaps
The split ring reverses the current each half turn.
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5
Rotation continues
The forces always turn the coil the same way.
Increasing the Speed
Ways to make a motor faster.
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Increase the current
Why it works: Larger force on the coil
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Stronger magnets
Why it works: Larger magnetic flux density
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More turns on the coil
Why it works: Larger total force
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Reverse the battery
Why it works: Motor spins the other way
Why the Commutator Matters
Explain what would happen without the split-ring commutator.
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- 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
AnswerThe coil would not rotate continuously.
Motor Build
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.
Can I...?
- 1Name the parts.
- 2Explain the forces on the coil.
- 3Explain the commutator.
- 4Describe rotation.
- 5Increase the speed.
- 6Reverse the direction.
- 7Say why opposite forces.
- 8Link to the motor effect.
Summary & Exam Focus
- 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.
Exam focus
Describe the job of the split-ring commutator. (2 marks) (2 marks)
It reverses the current every half turn.
Key terms
The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.
- 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.
- Brushes
- Carbon contacts that touch the commutator.
- Coil
- A loop of wire that turns in the magnetic field.
- Rotation
- Turning about an axis.
- Motor effect
- The force on a current-carrying wire in a magnetic field.
Practice questions
Have a go at each one before you open its answer.
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Question 1 State 2 marks
State two ways of increasing the speed of rotation of a d.c. motor.
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Model answer
Increase the current. Use stronger magnets. Use more turns. (any two)
Mark scheme
- One way — 1 mark
- Another way — 1 mark
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Question 2 Explain 3 marks
Explain why the coil in a d.c. motor turns when a current flows through it.
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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.
Mark scheme
- 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
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Question 3 Explain 3 marks
Explain the purpose of the split-ring commutator in a d.c. motor.
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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.
Mark scheme
- Reverses the current — 1 mark
- Every half turn — 1 mark
- Keeps the coil turning the same way — 1 mark
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Question 4 Explain 2 marks
The battery in a d.c. motor is connected the other way round. Describe and explain the effect.
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Model answer
The current in the coil is reversed, so the forces are reversed and the motor turns in the opposite direction.
Mark scheme
- Turns in the opposite direction — 1 mark
- Current (forces) reversed — 1 mark
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Question 5 Explain 4 marks
A motor rotates slowly. Explain two changes that would make it rotate faster.
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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.
Mark scheme
- Increase the current — 1 mark
- Larger force — 1 mark
- Stronger magnets or more turns — 1 mark
- Larger force or flux density — 1 mark
Quick check
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The part that reverses the current each half turn is the...
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A: commutator
Split ring.
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The forces on the two sides of a motor coil are...
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B: in opposite directions
Current is opposite.
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Brushes are usually made of...
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C: carbon
Conducting and slippery.
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A stronger magnet makes a motor...
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D: faster
Bigger force.
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Reversing the battery makes the motor...
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A: turn the other way
Forces reverse.
Downloads
Free to keep, print and annotate.
- Electric motors.pptx Built from the lesson script on 30 September 2026. View
- Electric motors - Completed Notes.docx The full notes for the lesson, to revise from. Built from the lesson script on 30 September 2026. View
- Electric motors - Exam Questions.docx Built from the lesson script on 30 September 2026. View
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