AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY
Loudspeakers and headphones
Magnetism and electromagnetism · Lesson 5 of 8
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.
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. |
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.
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.
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EXAM FOCUS Explain why the cone in a loudspeaker vibrates. (3 marks) Alternating current in the coil, motor effect. |