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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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Warm-up

Answer each one, then check.

  1. 1

    What is a.c.?

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    Alternating current

  2. 2

    What is d.c.?

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    Direct current

  3. 3

    What is the generator effect?

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    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.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 State 2 marks

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

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    Model answer

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

    Mark scheme

    • Alternator a.c. — 1 mark
    • Dynamo d.c. — 1 mark
  2. Question 2 Explain 3 marks

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

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    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.

    Mark scheme

    • Alternating or repeating pattern — 1 mark
    • Positive and negative — 1 mark
    • Reverses every half turn — 1 mark
  3. Question 3 Explain 3 marks

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

    Show answerHide answer

    Model answer

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

    Mark scheme

    • Larger amplitude — 1 mark
    • Higher frequency — 1 mark
    • Shorter period / more waves in same time — 1 mark
  4. Question 4 Calculate 2 marks

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

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    Model answer

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

    Mark scheme

    • Substitution — 1 mark
    • 50 Hz — 1 mark
  5. Question 5 Explain 4 marks

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

    Show answerHide answer

    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.

    Mark scheme

    • 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

Quick check

  1. An alternator produces...

    1. Adirect current only
    2. Bsound
    3. Clight
    4. Dalternating current
    Show answerHide answer

    D: alternating current

    Slip rings.

  2. A dynamo produces...

    1. Adirect current
    2. Balternating current
    3. Cstatic
    4. Dsound
    Show answerHide answer

    A: direct current

    Commutator.

  3. Faster rotation makes the frequency...

    1. Adecrease
    2. Bincrease
    3. Cstay the same
    4. Dbecome zero
    Show answerHide answer

    B: increase

    More rotations a second.

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

    1. A0.05 Hz
    2. B5 Hz
    3. C20 Hz
    4. D50 Hz
    Show answerHide answer

    C: 20 Hz

    1 ÷ 0.05.

  5. A microphone uses the...

    1. Amotor effect
    2. Bgreenhouse effect
    3. CDoppler effect
    4. Dgenerator effect
    Show answerHide answer

    D: generator effect

    Coil moves in a field.

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