AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER
Alternators, dynamos and microphones
Magnetism and electromagnetism · Lesson 7 of 8
Teacher copy - includes the notes for whoever is teaching from it.
Warm-up
Answer each one, then check.
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
Learning Objectives
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).
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 and Dynamo Graphs
|
Slip rings give a.c.; a commutator gives d.c. |
An alternator producing a sine-wave p.d. and a dynamo producing a pulsating one-direction p.d.
Alternator or Dynamo?
|
ALTERNATOR |
DYNAMO |
|
▸ Coil rotates in a magnetic field. ▸ Slip rings and brushes. ▸ Output: alternating p.d., changing direction each half turn. |
▸ 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.
|
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 |
Reading an a.c. Graph
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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
How a Microphone Works (Higher)
|
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.
Key Terms
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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. |
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. |
A good answer shows: Sine wave; twice as fast gives twice the amplitude and twice the frequency.
Note: Ask learners to label the period.
Can I...?
☐ 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.
Summary
✓ 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. |
Exam Practice: Alternators, dynamos and microphones
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.
▸ 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.
Question 1 · 2 marks · State
|
“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.
Question 1 · mark scheme
2 marks available. Award a mark for each point made.
▸ Alternator a.c.. 1 mark
▸ Dynamo d.c.. 1 mark
▸ Model answer. (a) Alternating current. (b) Direct current.
Question 2 · 3 marks · Explain
|
“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.
Question 2 · mark scheme
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
▸ 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.
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.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 3 · mark scheme
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
▸ Model answer. The amplitude doubles (larger p.d.) and the frequency doubles (shorter period).
Question 4 · 2 marks · Calculate
|
“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.
Question 4 · mark scheme
2 marks available. Award a mark for each point made.
▸ Substitution. 1 mark
▸ 50 Hz. 1 mark
▸ Model answer. f = 1 ÷ T = 1 ÷ 0.020 = 50 Hz
Question 5 · 4 marks · Explain
|
“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.
Question 5 · mark scheme
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
▸ 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.