AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY
Loudspeakers and headphones
Magnetism and electromagnetism · Lesson 5 of 8
Teacher copy - includes the notes for whoever is teaching from it.
Warm-up
Answer each one, then check.
1. What is sound?
A longitudinal wave
2. What is frequency?
Number of waves per second
3. What is a.c.?
Alternating current
4. What is the motor effect?
Force on a current-carrying wire in a magnetic field
5. What is a diaphragm?
A thin flexible cone or membrane
Learning Objectives
1. Describe the structure of a moving-coil loudspeaker.
2. Explain how a loudspeaker converts electrical signals into sound.
3. Explain how the frequency and loudness of the sound depend on the a.c.
Loudspeakers
In a loudspeaker an alternating current flows through a coil in a magnetic field. The motor effect makes the coil, and the cone attached to it, move back and forth at the same frequency as the current, producing sound waves.
Loudspeakers and headphones use the motor effect; microphones (next lessons) use the generator effect.
A Loudspeaker
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The cone moves at the frequency of the current. |
A loudspeaker with a coil and magnet making a cone vibrate to produce sound.
How a Loudspeaker Makes Sound
Follow the steps.
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1 a.c. flows An alternating current from the amplifier flows through the coil. |
2 The coil is in a field The permanent magnet provides a magnetic field. |
3 Motor effect A force acts on the coil, changing direction as the current changes direction. |
4 The cone vibrates The coil moves the cone back and forth at the same frequency. |
5 Sound waves The vibrating cone makes longitudinal sound waves in the air. |
Changing the Sound
What controls the sound?
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Current |
Sound |
|---|---|
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Higher frequency a.c. |
Higher pitch |
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Larger a.c. amplitude |
Louder sound (larger force, greater vibration) |
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Zero current |
No sound |
Headphones
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Explain how headphones work in the same way. |
1. Small coil
A small coil and magnet inside
2. a.c. signal
Changing current moves the coil
3. Diaphragm
A small diaphragm vibrates and creates sound
Answer: Headphones are tiny loudspeakers.
Parts of a Moving-coil Loudspeaker
Each part has one job.
|
Part |
Job |
|---|---|
|
Permanent magnet |
Provides a strong, steady magnetic field |
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Coil |
Carries the alternating current from the amplifier |
|
Cone or diaphragm |
Attached to the coil; pushes on the air |
|
Suspension |
Holds the cone in place and returns it to the centre |
|
Frame |
Holds everything rigidly in position |
Loudspeaker and Microphone
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LOUDSPEAKER |
MICROPHONE |
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▸ Electrical signal in, sound out. ▸ Uses the motor effect. ▸ Current in the coil causes a force. ▸ Coil moves the cone. |
▸ Sound in, electrical signal out. ▸ Uses the generator effect. ▸ Movement of the coil induces a p.d. ▸ Sound moves the diaphragm and coil. |
Frequency, Amplitude and the Sound
The sound is an exact copy of the electrical signal.
▸ Pitch. The cone completes one back-and-forth vibration for each cycle of the a.c. A 440 Hz current makes the cone vibrate 440 times a second, giving the note A.
▸ Loudness. A larger current gives a larger force, so the cone moves further each way and makes sound waves with a bigger amplitude.
▸ Why a.c.?. A direct current would push the cone one way and hold it there; it would only click when switched on and off.
Key Terms
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Loudspeaker A device that uses the motor effect to convert electrical signals into sound. |
Cone The part of the loudspeaker that vibrates to make sound. |
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Alternating current A current that repeatedly changes direction. |
Frequency The number of vibrations each second. |
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Amplitude The size of the vibration. |
Diaphragm A thin vibrating membrane. |
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Amplifier A device that increases the size of an electrical signal. |
Pitch How high or low a sound is, set by the frequency. |
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Permanent magnet A magnet that keeps its magnetism, giving a steady field. |
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Your Task: Speaker Soak
10 minutes
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Write a chain of events from the amplifier to the sound reaching your ear. Then explain why the cone vibrates at the same frequency as the current. 1. Follow the chain. |
A good answer shows: a.c. → coil → motor effect → cone vibrates → sound. The force reverses each time the current reverses.
Note: Ask what happens to the sound if the a.c. is stronger.
Can I...?
☐ State the parts.
☐ Name the effect used.
☐ Explain the coil movement.
☐ Explain the cone.
☐ Link frequency and pitch.
☐ Link amplitude and loudness.
☐ Describe headphones.
☐ Give the energy transfer.
Summary
✓ Alternating current flows through a coil near a magnet.
✓ The motor effect gives a force that changes direction with the current.
✓ The coil and cone vibrate at the same frequency.
✓ The cone makes sound waves in the air.
✓ A loudspeaker uses the motor effect; a microphone uses the generator effect.
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EXAM FOCUS Explain why the cone in a loudspeaker vibrates. (3 marks) Three linked marks: an alternating current flows in the coil, which is in a magnetic field; the motor effect gives a force on the coil; the force changes direction as the current alternates, so the coil and cone move back and forth. |
Exam Practice: Loudspeakers and headphones
Answer all questions. Use the mark allocation as a guide to how much to write. · 14 minutes
▸ Question 1 · 2 marks · State. What type of current flows through the coil of a loudspeaker? Describe how the cone moves.
▸ Question 2 · 4 marks · Explain. Explain how a loudspeaker produces sound.
▸ Question 3 · 2 marks · Explain. The frequency of the alternating current is increased. State the effect on the sound.
▸ Question 4 · 2 marks · Explain. The amplitude of the current is increased. State the effect on the sound and explain.
▸ Question 5 · 3 marks · Explain. Explain why the current in a loudspeaker must be alternating rather than direct.
Question 1 · 2 marks · State
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“What type of current flows through the coil of a loudspeaker? Describe how the cone moves.” |
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.
▸ Alternating current. 1 mark
▸ Cone vibrates back and forth. 1 mark
▸ Model answer. An alternating current. The cone moves back and forth (vibrates).
Question 2 · 4 marks · Explain
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“Explain how a loudspeaker produces sound.” |
HOW TO ANSWER IT Command word: Explain. Worth 4 marks, so plan before writing.
Question 2 · mark scheme
4 marks available. Award a mark for each point made.
▸ a.c. in a coil in a magnetic field. 1 mark
▸ Motor effect force. 1 mark
▸ Force changes direction so cone vibrates. 1 mark
▸ Sound waves produced. 1 mark
▸ Model answer. An alternating current flows in the coil, which is in the field of a permanent magnet. The motor effect gives a force on the coil that reverses as the current reverses. The coil and cone vibrate at the frequency of the current. The cone produces longitudinal sound waves.
Question 3 · 2 marks · Explain
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“The frequency of the alternating current is increased. State the effect on the sound.” |
HOW TO ANSWER IT Command word: Explain. Worth 2 marks, so plan before writing.
Question 3 · mark scheme
2 marks available. Award a mark for each point made.
▸ Higher pitch / higher frequency. 1 mark
▸ Cone vibrates faster. 1 mark
▸ Model answer. The pitch of the sound rises because the cone vibrates faster.
Question 4 · 2 marks · Explain
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“The amplitude of the current is increased. State the effect on the sound and explain.” |
HOW TO ANSWER IT Command word: Explain. Worth 2 marks, so plan before writing.
Question 4 · mark scheme
2 marks available. Award a mark for each point made.
▸ Louder. 1 mark
▸ Larger force or larger vibration. 1 mark
▸ Model answer. The sound is louder because a larger current gives a larger force so the cone vibrates with a larger amplitude.
Question 5 · 3 marks · Explain
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“Explain why the current in a loudspeaker must be alternating rather than direct.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 5 · mark scheme
3 marks available. Award a mark for each point made.
▸ d.c. gives a steady force in one direction. 1 mark
▸ Cone only moves once/stays displaced. 1 mark
▸ a.c. reverses the force so the cone vibrates. 1 mark
▸ Model answer. A direct current would give a force in one direction, so the cone would only move once and stay there. An alternating current reverses the force so the cone moves back and forth and produces sound.