Grade 12 Physical Sciences · Electricity and Magnetism

Describe generators and motors

Describe how generators and motors work, their energy changes and parts like slip rings, commutators and brushes (a motor changes electrical to mechanical energy).

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Electrodynamics: Motors and generators PhysicsMiss Martins Maths and Science

AC and DC generators and GRAPHS | Electrodynamics Grade 12 PhysicsMiss Martins Maths and Science

Electrodynamics Motors SUMMARY Grade 12 PhysicsMiss Martins Maths and Science

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Question 1.9 · November 2025 · Paper 1 · National (DBE) · 2 marks

Various options are provided as possible answers to the following questions. Each question has only ONE correct answer. Choose the answer and write only the letter (A–D) next to the question numbers (1.1 to 1.10) in the ANSWER BOOK, e.g. 1.11 E.

1.9 How does the commutator in a DC motor ensure that the coil rotates continuously in ONE direction?(2)

A By reducing friction
B By converting AC to DC
C By reversing the direction of the current in the coil
D By maintaining electrical contact between the external and the internal circuits

Work it out on paper first. Then mark yourself with the official memo, one tick at a time.

More past paper questions: describe generators and motors

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3. Question 9.1.3 · May/June 2025 · Paper 1 · National (DBE) · 3 marks
9.1 The diagram below represents a simplified version of an AC generator.

The induced current flows from X to Y in the coil when the coil is rotated clockwise, as shown in the diagram below.

Answer the questions that follow.
9.1.3 Sketch a graph of induced current versus time for ONE complete rotation of the coil from the horizontal position, as shown in the diagram above.(3)
Show the memo (3 ticks)Memo
1. ✓
  Starts at maximum currentThe AC graph starts with maximum current (positive or negative maximum) at $t = 0$ .
  1 mark · Describe generators and motors
2. ✓
  One complete cycleThe graph is drawn for one cycle / one complete wavelength.
  1 mark · Describe generators and motors
3. ✓
  Correct shapeCorrect shape: a cosine graph.
  1 mark · Describe generators and motors
4. Question 9.4 · November 2025 · Paper 1 · National (DBE) · 3 marks
An AC generator operates a fan. The directions of the rotation of the coil and the induced current are shown in the simplified diagram below.
The voltage versus time graph for this generator is given below.
9.4 Redraw this graph in the ANSWER BOOK and label it as A. On the same set of axes, draw the graph for ONE rotation of the coil when the speed of rotation is doubled. Label this as graph B.(3)
Show the memo (3 ticks)Memo
1. ✓
  AmplitudeThe amplitude of graph B is bigger than that of graph A.
  1 mark · Describe generators and motors
2. ✓
  PeriodThe period of B is half of A (one cycle ends at $2$ s).
  1 mark · Describe generators and motors
3. ✓
  ShapeCorrect shape and one cycle (cosine graph): B starts at its maximum, like A.
  1 mark · Describe generators and motors
5. Question 9.7 · November 2024 · Paper 1 · National (DBE) · 2 marks
The graph below shows how the induced current in a generator varies with time.
The graph below shows how the induced current varies with time after a change was made to the operation of the generator.
9.7 Fully describe the change that was made.(2)
Show the memo (1 tick)Memo
1. ✓
  Change madeSpeed of rotation halved
  Accept: Frequency was halved / Period was doubled
  2 marks · Describe generators and motors
6. Question 9.6 · May/June 2024 · Paper 1 · National (DBE) · 4 marks
The simplified diagram below represents an AC generator with a coil rotating clockwise. X and Y are two points in the external circuit.
The generator induced current at a frequency of $20$ Hz. The coil started rotating from the initial position, as shown in the diagram above.
9.6 Sketch a graph of induced current versus time for two complete rotations of the coil.

Indicate the following on the graph:
• The time taken for two rotations
• The maximum current induced by the generator(4)
Show the memo (4 ticks)Memo
1. ✓
  Two complete cyclesThe graph shows two complete cycles of the alternating current.
  1 mark · Describe generators and motors
2. ✓
  Time for two rotationsGraph stops at $0, 1$ s OR one cycle in $0, 05$ s.
  1 mark · Describe generators and motors
3. ✓
  Maximum current indicatedMaximum current ( $2, 94$ / $2, 95$ A) as a positive or negative value correctly indicated.
  1 mark · Describe generators and motors
4. ✓
  Correct shape (cosine graph)The graph starts at a maximum (positive or negative) value: a cosine graph.
  1 mark · Describe generators and motors

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