IGCSE Physics d.c. Motor Questions and Worked Solutions (4.5.5) | 0625

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IGCSE Physics 0625 · Topic 4.5.5

IGCSE Physics d.c. Motor Questions and Worked Solutions

50 topic-wise practice questions on The d.c. Motor (Unit 4: Electricity and Magnetism), written to the Cambridge IGCSE Physics 0625 syllabus for 2026–2028, each with a step-by-step worked solution.

50 questions16 Core24 Extended10 Challenge125 marksTextbook §4.5 · pp. 229–250
Questions tagged Core are for all candidates (Papers 1 & 3). Questions tagged Extended are Supplement content (Papers 2 & 4). Questions 41–50 are harder Challenge questions. Try each question first, then tap Show solution to check your working.

Topic guide: The d.c. Motor

A d.c. motor uses the force on a current-carrying coil in a magnetic field to produce rotation. A split-ring commutator keeps it turning in one direction.

Key formulas

turning effect ↑ with turns, current and field strengthqualitativeCore

Prerequisite knowledge

  • Force on a conductor (4.5.4)
  • Moments (1.5.2)

Exam tips & common mistakes

  • Turning effect is zero when the coil is vertical (perpendicular to the field) — momentum carries it on
  • Reversing the current or field reverses the rotation
  • Do not confuse commutator (motor) with slip rings (generator)

Key facts from the syllabus (4.5.5)

  • A current-carrying coil in a magnetic field experiences a turning effect
  • The turning effect increases with more turns, larger current and stronger field
  • Extended forces on opposite sides of the coil act in opposite directions, producing rotation
  • Extended the split-ring commutator reverses the current in the coil every half turn so it keeps rotating the same way
  • Extended brushes make sliding contact with the commutator

Textbook: §4.5 Electromagnetic effects, pp. 229–250

1
Core[1 mark]

State what happens to a current-carrying coil placed in a magnetic field.

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Method: Turning effect.

It experiences a turning effect (it may rotate)
2
Core[3 marks]

State three ways of increasing the turning effect on a coil in a d.c. motor.

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Method: Turns, current, field.

Increase the number of turns; increase the current; use a stronger magnetic field
3
Core[2 marks]

State the energy transfer in an electric motor.

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Method: Electrical → kinetic.

Electrical energy → kinetic energy (and some thermal)
4
Core[2 marks]

State the effect of reversing the current in a d.c. motor.

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Method: Reverses rotation.

The motor turns in the opposite direction
5
Core[2 marks]

Give two everyday devices containing electric motors.

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Method: Fans, drills.

e.g. electric fan, washing machine, drill, car windows
6
Core[2 marks]

A motor is connected to a battery and turns clockwise. The magnet is turned round. State the new direction of rotation.

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Method: Field reversed.

Anticlockwise
7
Core[2 marks]

A motor spins slowly. Suggest two changes to make it spin faster.

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Method: More current, stronger field.

Increase the current (higher p.d.); use stronger magnets (or more turns)
8
Core[1 mark]

Name the device that turns a d.c. motor into a generator when it is turned by hand.

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Method: Same device.

It acts as a (d.c.) generator/dynamo
9
Core[2 marks]

State the effect of increasing the current on the speed of a d.c. motor.

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Method: Faster.

It turns faster (greater turning effect)
10
Core[2 marks]

State the effect of a stronger magnet on a d.c. motor.

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Method: Faster.

Greater turning effect, so it turns faster
11
Core[2 marks]

State the effect of more turns on the coil of a d.c. motor.

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Method: Larger turning effect.

Greater turning effect
12
Core[2 marks]

State two ways to reverse the direction of rotation of a d.c. motor.

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Method: Reverse current or field.

Reverse the current or reverse the magnetic field
13
Core[2 marks]

Give one domestic use of a d.c. motor.

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Method: Toys etc.

e.g. electric toothbrush, toy car, CD drive
14
Core[2 marks]

State why a motor gets warm during use.

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Method: Resistance.

Current through the coil's resistance (and friction) produces thermal energy
15
Core[2 marks]

A d.c. motor is connected to a 3.0 V cell. The cell is replaced by a 6.0 V battery. State the effect.

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Method: More current.

It turns faster (larger current)
16
Core[2 marks]

State what happens to a motor if the current is switched off.

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Method: Slows and stops.

It slows down and stops
17
Extended[3 marks]

Explain why the coil of a motor rotates.

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Method: Opposite forces on opposite sides.

Current flows in opposite directions on the two sides of the coil, so forces act in opposite directions (up and down), producing a turning effect
18
Extended[2 marks]

State the purpose of the split-ring commutator.

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Method: Reverse current every half turn.

It reverses the current in the coil every half turn so it keeps rotating in the same direction
19
Extended[2 marks]

State the purpose of the brushes in a d.c. motor.

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Method: Contact.

They make electrical contact with the rotating commutator
20
Extended[3 marks]

Explain what would happen in a d.c. motor without a commutator.

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Method: Rocks back and forth.

The coil would turn until vertical, then the forces would turn it back; it would oscillate and stop rather than rotate
21
Extended[2 marks]

Explain why the coil continues to turn past the vertical position where the turning effect is zero.

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Method: Momentum.

Its momentum carries it past; then the commutator reverses the current
22
Extended[2 marks]

State the position of the coil when the turning effect is greatest.

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Method: Parallel to field.

When the plane of the coil is parallel to the magnetic field
23
Extended[2 marks]

State the position of the coil when the turning effect is zero.

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Method: Perpendicular to field.

When the plane of the coil is perpendicular to the field
24
Extended[2 marks]

Explain why the brushes in a motor are made of carbon.

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Method: Conducts and slides.

Carbon conducts and slides with low friction, wearing slowly
25
Extended[2 marks]

Explain why practical motors have many coils at different angles.

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Method: Smoother torque.

There is always a coil near the maximum turning position, so the motor turns smoothly and with greater turning effect
26
Extended[3 marks]

Explain why a motor coil is wound on a soft-iron core.

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Method: Concentrates the field.

The soft iron concentrates the magnetic field, increasing the turning effect
27
Extended[2 marks]

Explain the role of the commutator when the coil passes the vertical position.

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Method: Swaps contacts.

The split ring swaps which brush each end of the coil touches, so the current direction in the coil reverses
28
Extended[2 marks]

Describe the difference between slip rings and a split-ring commutator.

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Method: Continuous rings vs split ring.

Slip rings are two separate continuous rings (keep connections the same, used in a.c. generators); a split ring reverses connections every half turn (used in d.c. motors)
29
Extended[3 marks]

Explain how the split-ring commutator keeps a motor turning in one direction.

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Method: Reverses current every half turn.

Every half turn the commutator swaps the coil's connections to the brushes, reversing the current so the forces always turn the coil the same way
30
Extended[2 marks]

Describe the forces on the two sides of a motor coil.

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Method: Opposite directions.

Equal forces in opposite directions (one up, one down)
31
Extended[2 marks]

Explain why there is no turning effect when the coil is vertical (plane perpendicular to the field).

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Method: Forces in line through axle.

The forces act along the plane of the coil through the axle, giving no moment
32
Extended[3 marks]

Sketch and label a simple d.c. motor.

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Method: Labels.

Coil between N and S poles on an axle, split-ring commutator, carbon brushes connected to a d.c. supply
33
Extended[2 marks]

Explain why carbon brushes wear out and need replacing.

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Method: Friction.

They rub against the rotating commutator
34
Extended[2 marks]

Explain why curved pole pieces are used in practical motors.

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Method: Radial field.

They give a radial field so the coil is always parallel to the field lines, giving a steady maximum turning effect
35
Extended[2 marks]

Explain why many coils on a motor give a smoother rotation.

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Method: Always one near max.

There is always a coil near its maximum turning position
36
Extended[2 marks]

State what happens if the commutator is replaced by slip rings in a d.c. motor.

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Method: Oscillates.

The coil would oscillate about the vertical and not keep rotating
37
Extended[3 marks]

Use Fleming's left-hand rule to show the direction of rotation for a coil whose left side carries current into the page, in a field from left to right.

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Method: Left side force.

Left side force is downwards, right side upwards, so the coil turns anticlockwise
38
Extended[2 marks]

State the energy transfers in a d.c. motor lifting a load.

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Method: Electrical → kinetic → GPE.

Electrical → kinetic → gravitational potential (plus thermal waste)
39
Extended[2 marks]

Explain why a motor is a device using the force on a current-carrying conductor.

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Method: Coil sides are conductors.

The sides of the coil carry current in a magnetic field, so forces act on them to produce rotation
40
Extended[2 marks]

State the purpose of the axle in a motor.

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Method: Pivot.

It allows the coil to rotate and transfers the rotation to the load

Challenge questions

Harder, multi-step questions at the top end of the exam. Attempt these once you are confident with the questions above.

41
ChallengeCore[4 marks]

State three changes that increase the turning effect on a motor coil and explain why each works.

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Method: Force factors.

More turns (more wires feeling force); larger current (larger force on each wire); stronger magnetic field (larger force)
42
ChallengeCore[4 marks]

A toy motor turns slowly with one cell. Explain the effect of adding a second cell in series and of turning the magnet round.

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Method: More current, reversed field.

Two cells increase the p.d. and current, so it turns faster; turning the magnet reverses the field and the direction of rotation
43
ChallengeExtended[5 marks]

Explain in detail how a d.c. motor keeps rotating in one direction, referring to the forces on the coil and the split-ring commutator.

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Method: Commutator reversal.

Opposite sides of the coil carry current in opposite directions, so forces act up on one side and down on the other, giving a turning effect; every half turn the commutator reverses the current in the coil so the forces keep turning it the same way
44
ChallengeExtended[4 marks]

Explain why the turning effect on a motor coil varies as it rotates and why the coil continues past the vertical position.

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Method: Perpendicular distance.

The forces stay the same but their perpendicular distance from the axle changes — maximum when the coil is parallel to the field, zero when perpendicular; momentum carries it past
45
ChallengeExtended[4 marks]

Explain what would happen if the commutator in a d.c. motor were replaced by slip rings and the motor connected to d.c.

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Method: No reversal.

The current would not reverse; the coil would turn to the vertical position and oscillate about it before stopping
46
ChallengeExtended[4 marks]

A coil in a motor has its left side carrying current into the page and its right side carrying current out of the page, in a field from left to right. Determine the direction of rotation.

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Method: Left-hand rule each side.

Left side force downwards, right side force upwards: the coil rotates anticlockwise
47
ChallengeExtended[4 marks]

Explain why practical motors use several coils on a laminated soft-iron core and curved pole pieces.

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Method: Smooth, stronger.

Several coils give a smoother, more constant turning effect; the soft-iron core concentrates the field; curved pole pieces keep the field radial so the turning effect is near maximum all the time
48
ChallengeExtended[4 marks]

A 12 V motor draws 2.0 A and lifts a 6.0 N weight 1.2 m in 3.0 s. Calculate the input power, useful power and efficiency, and suggest where energy is wasted.

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Method: P = IV; P = Fd/t.

Input = 24.0 W; useful = 2.40 W; efficiency = 10.0%; wasted as heat in coil resistance, friction and sound
49
ChallengeExtended[3 marks]

Explain why a motor connected to a supply with the magnet removed will not turn.

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Method: No field.

Without a magnetic field there is no force on the current-carrying coil
50
ChallengeExtended[4 marks]

Explain why the carbon brushes are needed in a d.c. motor and what happens when they wear down too far.

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Method: Contact.

They make sliding electrical contact with the rotating commutator; when worn, contact is poor or lost, so the motor runs intermittently or stops
All IGCSE Physics topics← 4.5.4 Force on a Current-Carrying Conductor4.5.6 The Transformer →

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