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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).

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  • 6 key terms
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Teacher resources

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Student handouts

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Warm-up

Answer each one, then check.

  1. 1

    What is the motor effect?

    Show answerHide answer

    The force on a current-carrying wire in a magnetic field

  2. 2

    What does a motor transfer energy to?

    Show answerHide answer

    Kinetic energy store

  3. 3

    What does the left-hand rule give?

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    The direction of the force

  4. 4

    What is a commutator?

    Show answerHide answer

    A part that reverses the current each half turn

  5. 5

    What are brushes?

    Show answerHide answer

    Carbon contacts touching the commutator

Learning Objectives

  1. 1Describe how a coil of wire in a magnetic field rotates.
  2. 2Explain the role of the split-ring commutator.
  3. 3Explain how to increase the speed of a motor.
  4. 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.

How a d.c. Motor Turns

Follow the steps.

  1. 1 Current flows

    Through the coil via the brushes and commutator.
  2. 2 Forces act

    One side of the coil is pushed up, the other down (motor effect).
  3. 3 The coil rotates

    The opposite forces turn the coil.
  4. 4 The commutator swaps

    The split ring reverses the current each half turn.
  5. 5 Rotation continues

    The forces always turn the coil the same way.

Increasing the Speed

Ways to make a motor faster.

  • Increase the current

    Why it works: Larger force on the coil

  • Stronger magnets

    Why it works: Larger magnetic flux density

  • More turns on the coil

    Why it works: Larger total force

  • 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. 1 Half a turn The coil would turn to the vertical position
  2. 2 Without a commutator The forces would then turn it back
  3. 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...?

  1. 1Name the parts.
  2. 2Explain the forces on the coil.
  3. 3Explain the commutator.
  4. 4Describe rotation.
  5. 5Increase the speed.
  6. 6Reverse the direction.
  7. 7Say why opposite forces.
  8. 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.

Questions and answers

10 questions set on this lesson, with the mark schemes and model answers open.

1. Exam question State 2 marks Easier

State two ways of increasing the speed of rotation of a d.c. motor.

Mark scheme — 2 marks available

  • One way — 1 mark
  • Another way — 1 mark

Model answer

Increase the current. Use stronger magnets. Use more turns. (any two)

2. Exam question Explain 3 marks Easier

Explain why the coil in a d.c. motor turns when a current flows through it.

Mark scheme — 3 marks available

  • 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.

3. Exam question Explain 3 marks Easier

Explain the purpose of the split-ring commutator in a d.c. motor.

Mark scheme — 3 marks available

  • 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.

4. Exam question Explain 2 marks Easier

The battery in a d.c. motor is connected the other way round. Describe and explain the effect.

Mark scheme — 2 marks available

  • 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.

5. Exam question Explain 4 marks Easier

A motor rotates slowly. Explain two changes that would make it rotate faster.

Mark scheme — 4 marks available

  • 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.

6. Multiple choice 1 mark Easier

The part that reverses the current each half turn is the...

  1. A commutator Correct
  2. B resistor
  3. C diode
  4. D fuse

Why: Split ring.

7. Multiple choice 1 mark Core

The forces on the two sides of a motor coil are...

  1. A in the same direction
  2. B in opposite directions Correct
  3. C zero
  4. D perpendicular to the coil

Why: Current is opposite.

8. Multiple choice 1 mark Core

Brushes are usually made of...

  1. A copper plate
  2. B iron
  3. C carbon Correct
  4. D plastic

Why: Conducting and slippery.

9. Multiple choice 1 mark Core

A stronger magnet makes a motor...

  1. A slower
  2. B stop
  3. C no change
  4. D faster Correct

Why: Bigger force.

10. Multiple choice 1 mark Stretch

Reversing the battery makes the motor...

  1. A turn the other way Correct
  2. B stop
  3. C speed up
  4. D explode

Why: Forces reverse.