Alternators dynamos and microphones - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Alternators, dynamos and microphones

Magnetism and electromagnetism

Lesson 7 of 8

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Warm-up

Magnetism and electromagnetism

Lesson 7 of 8

Answer each one, then check.

1

What is a.c.?

2

What is d.c.?

3

What is the generator effect?

4

What is the UK mains frequency?

5

What is a slip ring?

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up - Answers

Magnetism and electromagnetism

Lesson 7 of 8

1

What is a.c.?

Alternating current

2

What is d.c.?

Direct current

3

What is the generator effect?

p.d. induced by a changing magnetic field

4

What is the UK mains frequency?

50 Hz

5

What is a slip ring?

A continuous ring that keeps contact with a brush

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Learning Objectives

Magnetism and electromagnetism

Lesson 7 of 8

1

Describe how an alternator generates an alternating current.

2

Describe how a dynamo generates direct current.

3

Interpret potential difference–time graphs from a.c. and d.c. generators.

4

Describe how a microphone converts sound to an electrical signal (Higher).

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Magnetism and electromagnetism

Lesson 7 of 8

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.

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Alternator and Dynamo Graphs

Magnetism and electromagnetism

Lesson 7 of 8

Slip rings give a.c.; a commutator gives d.c.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Alternator or Dynamo?

Magnetism and electromagnetism

Lesson 7 of 8

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.

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Changing the Output

Magnetism and electromagnetism

Lesson 7 of 8

Learn what changes the graph.

Change

Effect on the graph

Faster rotation

Larger amplitude and higher frequency

Stronger magnet

Larger amplitude

More turns

Larger amplitude

Slower rotation

Smaller amplitude and lower frequency

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Reading an a.c. Graph

Magnetism and electromagnetism

Lesson 7 of 8

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

1

Equation

f = 1 ÷ T

2

Substitute

f = 1 ÷ 0.020

3

Answer

f = 50 Hz

ANSWER

50 Hz

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How a Microphone Works (Higher)

Magnetism and electromagnetism

Lesson 7 of 8

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

1

Sound waves

Make a diaphragm vibrate

2

Coil

Attached to the diaphragm, moves in the field of a magnet

3

Generator effect

Induces an a.c. p.d. in the coil with the same frequency as the sound

ANSWER

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

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Key Terms

Magnetism and electromagnetism

Lesson 7 of 8

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.

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Magnetism and electromagnetism

Lesson 7 of 8

YOUR TASK

Generator Graphs

12 minutes

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.

WHAT A GOOD ANSWER SHOWS

Sine wave; twice as fast gives twice the amplitude and twice the frequency.

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Can I...?

Magnetism and electromagnetism

Lesson 7 of 8

Describe an alternator.

Describe a dynamo.

Name slip rings.

Name the commutator.

Sketch a.c. and d.c. graphs.

Change the output.

Describe a microphone.

Calculate frequency.

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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)

Alternator a.c., dynamo d.c.

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Exam Practice: Alternators, dynamos and microphones

Magnetism and electromagnetism

Lesson 7 of 8

Answer all questions. Use the mark allocation as a guide to how much to write. · 15 minutes

Question 1 · 2 marks · State

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

Question 2 · 3 marks · Explain

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

Question 3 · 3 marks · Explain

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

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Exam Practice: Alternators, dynamos and microphones (cont.)

Magnetism and electromagnetism

Lesson 7 of 8

Question 4 · 2 marks · Calculate

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

Question 5 · 4 marks · Explain

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

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Question 1 · 2 marks · State

Magnetism and electromagnetism

Lesson 7 of 8

“

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

HOW TO ANSWER IT

Command word: State. Worth 2 marks, so plan before writing.

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Question 1 · mark scheme

Magnetism and electromagnetism

Lesson 7 of 8

2 marks available. Award a mark for each point made.

Alternator a.c.

1 mark

Dynamo d.c.

1 mark

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Question 2 · 3 marks · Explain

Magnetism and electromagnetism

Lesson 7 of 8

“

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

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

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Question 2 · mark scheme

Magnetism and electromagnetism

Lesson 7 of 8

3 marks available. Award a mark for each point made.

Alternating or repeating pattern

1 mark

Positive and negative

1 mark

Reverses every half turn

1 mark

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Question 3 · 3 marks · Explain

Magnetism and electromagnetism

Lesson 7 of 8

“

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

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

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Question 3 · mark scheme

Magnetism and electromagnetism

Lesson 7 of 8

3 marks available. Award a mark for each point made.

Larger amplitude

1 mark

Higher frequency

1 mark

Shorter period / more waves in same time

1 mark

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Question 4 · 2 marks · Calculate

Magnetism and electromagnetism

Lesson 7 of 8

“

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

HOW TO ANSWER IT

Command word: Calculate. Worth 2 marks, so plan before writing.

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Question 4 · mark scheme

Magnetism and electromagnetism

Lesson 7 of 8

2 marks available. Award a mark for each point made.

Substitution

1 mark

50 Hz

1 mark

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Question 5 · 4 marks · Explain

Magnetism and electromagnetism

Lesson 7 of 8

“

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

HOW TO ANSWER IT

Command word: Explain. Worth 4 marks, so plan before writing.

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Question 5 · mark scheme

Magnetism and electromagnetism

Lesson 7 of 8

4 marks available. Award a mark for each point made.

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

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