Loudspeakers and headphones.pptx

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Loudspeakers and headphones

Magnetism and electromagnetism

Lesson 5 of 8

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Warm-up

Magnetism and electromagnetism

Lesson 5 of 8

Answer each one, then check.

1

What is sound?

2

What is frequency?

3

What is a.c.?

4

What is the motor effect?

5

What is a diaphragm?

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Warm-up - Answers

Magnetism and electromagnetism

Lesson 5 of 8

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

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Learning Objectives

Magnetism and electromagnetism

Lesson 5 of 8

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.

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AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Magnetism and electromagnetism

Lesson 5 of 8

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.

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A Loudspeaker

Magnetism and electromagnetism

Lesson 5 of 8

The cone moves at the frequency of the current.

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How a Loudspeaker Makes Sound

Magnetism and electromagnetism

Lesson 5 of 8

Follow the steps.

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.

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Changing the Sound

Magnetism and electromagnetism

Lesson 5 of 8

What controls the sound?

Current

Sound

Higher frequency a.c.

Higher pitch

Larger a.c. amplitude

Louder sound (larger force, greater vibration)

Zero current

No sound

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Headphones

Magnetism and electromagnetism

Lesson 5 of 8

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.

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Key Terms

Magnetism and electromagnetism

Lesson 5 of 8

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.

Alternating current

A current that repeatedly changes direction.

Frequency

The number of vibrations each second.

Amplitude

The size of the vibration.

Diaphragm

A thin vibrating membrane.

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Magnetism and electromagnetism

Lesson 5 of 8

YOUR TASK

Speaker Soak

10 minutes

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.

WHAT A GOOD ANSWER SHOWS

a.c. → coil → motor effect → cone vibrates → sound. The force reverses each time the current reverses.

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Can I...?

Magnetism and electromagnetism

Lesson 5 of 8

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.

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Summary & Exam Focus

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.

EXAM FOCUS

Explain why the cone in a loudspeaker vibrates. (3 marks)

Alternating current in the coil, motor effect.

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