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Physics · Magnetism and electromagnetism

Alternators, dynamos and microphones

Describe how alternators and dynamos generate alternating and direct current, interpret potential difference–time graphs, and explain how a microphone works (Physics only; microphones Higher tier only).

  • 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 a.c.?

    Show answerHide answer

    Alternating current

  2. 2

    What is d.c.?

    Show answerHide answer

    Direct current

  3. 3

    What is the generator effect?

    Show answerHide answer

    p.d. induced by a changing magnetic field

  4. 4

    What is the UK mains frequency?

    Show answerHide answer

    50 Hz

  5. 5

    What is a slip ring?

    Show answerHide answer

    A continuous ring that keeps contact with a brush

Learning Objectives

  1. 1Describe how an alternator generates an alternating current.
  2. 2Describe how a dynamo generates direct current.
  3. 3Interpret potential difference–time graphs from a.c. and d.c. generators.
  4. 4Describe how a microphone converts sound to an electrical signal (Higher).

GENERATORS

An alternator rotates a coil in a magnetic field using slip rings and brushes to produce an alternating potential difference. A dynamo uses a split-ring commutator so the output is direct current (p.d. in one direction).

The output increases with faster rotation, more turns or a stronger field. Faster rotation also increases the frequency.

Alternator or Dynamo?

Alternator

  • Coil rotates in a magnetic field.
  • Slip rings and brushes.
  • Output: alternating p.d., changing direction each half turn.

Dynamo

  • Coil rotates in a magnetic field.
  • Split-ring commutator and brushes.
  • Output: direct p.d., always in one direction.

Changing the Output

Learn what changes the graph.

  • Faster rotation

    Effect on the graph: Larger amplitude and higher frequency

  • Stronger magnet

    Effect on the graph: Larger amplitude

  • More turns

    Effect on the graph: Larger amplitude

  • Slower rotation

    Effect on the graph: Smaller amplitude and lower frequency

Reading an a.c. Graph

The graph shows a sine wave with period 0.020 s. Find the frequency.

Show the solutionHide the solution
  1. 1 Equation \(f = 1 \div T\)
  2. 2 Substitute \(f = 1 \div 0.020\)
  3. 3 Answer \(f = 50\) Hz

Answer50 Hz

How a Microphone Works (Higher)

Explain how a moving-coil microphone changes sound into an electrical signal.

Show the solutionHide the solution
  1. 1 Sound waves Make a diaphragm vibrate
  2. 2 Coil Attached to the diaphragm, moves in the field of a magnet
  3. 3 Generator effect Induces an a.c. p.d. in the coil with the same frequency as the sound

AnswerSound vibrates the diaphragm, the coil moves in a magnetic field, and a p.d. is induced.

Generator Graphs

Sketch the p.d.–time graph for an alternator. Then sketch it when the coil spins twice as fast.

1. Count the number of cycles.

A good answer shows: Sine wave; twice as fast gives twice the amplitude and twice the frequency.

Can I...?

  1. 1Describe an alternator.
  2. 2Describe a dynamo.
  3. 3Name slip rings.
  4. 4Name the commutator.
  5. 5Sketch a.c. and d.c. graphs.
  6. 6Change the output.
  7. 7Describe a microphone.
  8. 8Calculate frequency.

Summary & Exam Focus

  • Alternator: slip rings, a.c.
  • Dynamo: split ring, d.c.
  • Faster rotation increases the p.d. and the frequency.
  • Microphone: sound makes a coil move in a field, inducing an a.c.

Exam focus

Explain the difference between the output of an alternator and a dynamo. (2 marks) (2 marks)

Alternator a.c., dynamo d.c.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Alternator
A generator that produces alternating current.
Dynamo
A generator that produces direct current.
Slip rings
Rings that keep the coil connected to the brushes without reversing the output.
Commutator
A split ring that reverses the connections each half turn.
Microphone
A device that converts sound into an electrical signal.
Brush
A carbon contact that touches a ring.

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 the type of current produced by (a) an alternator and (b) a dynamo.

Mark scheme — 2 marks available

  • Alternator a.c. — 1 mark
  • Dynamo d.c. — 1 mark

Model answer

(a) Alternating current. (b) Direct current.

2. Exam question Explain 3 marks Easier

An alternator rotates at a constant speed. Describe how the output potential difference changes with time.

Mark scheme — 3 marks available

  • Alternating or repeating pattern — 1 mark
  • Positive and negative — 1 mark
  • Reverses every half turn — 1 mark

Model answer

It is an alternating p.d. that changes from positive to negative and back in a regular sine wave. It reverses every half turn of the coil.

3. Exam question Explain 3 marks Easier

The speed of rotation of the coil in an alternator is doubled. Describe two changes to the potential difference–time graph.

Mark scheme — 3 marks available

  • Larger amplitude — 1 mark
  • Higher frequency — 1 mark
  • Shorter period / more waves in same time — 1 mark

Model answer

The amplitude doubles (larger p.d.) and the frequency doubles (shorter period).

4. Exam question Calculate 2 marks Easier

A potential difference–time graph of an alternator shows one complete wave lasts 0.020 s. Calculate the frequency.

Mark scheme — 2 marks available

  • Substitution — 1 mark
  • 50 Hz — 1 mark

Model answer

\(f = 1 \div T = 1 \div 0.020 = 50\) Hz

5. Exam question Explain 4 marks Easier

Explain how a moving-coil microphone converts sound waves into an electrical signal.

Mark scheme — 4 marks available

  • Sound makes the diaphragm vibrate — 1 mark
  • Coil moves in a magnetic field — 1 mark
  • Generator effect / induced p.d. — 1 mark
  • a.c. with the same frequency as the sound — 1 mark

Model answer

Sound waves make the diaphragm vibrate. The coil attached to it moves in the field of a magnet. By the generator effect, a potential difference is induced in the coil. It has the same frequency as the sound, so an a.c. signal is produced.

6. Multiple choice 1 mark Easier

An alternator produces...

  1. A direct current only
  2. B sound
  3. C light
  4. D alternating current Correct

Why: Slip rings.

7. Multiple choice 1 mark Core

A dynamo produces...

  1. A direct current Correct
  2. B alternating current
  3. C static
  4. D sound

Why: Commutator.

8. Multiple choice 1 mark Core

Faster rotation makes the frequency...

  1. A decrease
  2. B increase Correct
  3. C stay the same
  4. D become zero

Why: More rotations a second.

9. Multiple choice 1 mark Core

A period of 0.05 s gives a frequency of...

  1. A 0.05 Hz
  2. B 5 Hz
  3. C 20 Hz Correct
  4. D 50 Hz

Why: 1 ÷ 0.05.

10. Multiple choice 1 mark Stretch

A microphone uses the...

  1. A motor effect
  2. B greenhouse effect
  3. C Doppler effect
  4. D generator effect Correct

Why: Coil moves in a field.