7.4 Microphones and speakers — Practise with help

Work each problem first. Open a hint only if you are stuck, and the full working only to check.

Warm-up: remember

Answer these from memory. They use ideas from Section 7.3.

Describe the generator effect in one sentence.
A transformer changes 240 V to 12 V. Is it a step-up transformer or a step-down transformer?

Answers

A moving conductor, or a changing magnetic field, induces a potential difference across the conductor, which drives a current in a complete circuit.
It is a step-down transformer, because the secondary potential difference is smaller than the primary potential difference.

If question 1 went wrong, look again at Section 7.3. If question 2 went wrong, check which coil has the larger potential difference.

Practise with help

1. Explain how a moving-coil microphone converts a sound wave into a varying current.

Hint

Follow the energy: sound arrives, the diaphragm moves, the coil moves in a field.

Next step

Say which effect is used when a coil moves in a magnetic field, then say what that effect produces.

Full working

Sound waves make the diaphragm of the microphone vibrate. The diaphragm is attached to a coil, so the coil moves backwards and forwards in the magnetic field. The coil is moving relative to the field, so a potential difference is induced across its ends. This is the generator effect. The induced potential difference drives a current that varies as the coil moves, so the microphone produces a varying current that copies the sound wave.

2. Explain how a moving-coil loudspeaker converts a varying current into sound.

Hint

Start with the current in the coil. The coil sits in a magnetic field.

Next step

Name the motor effect, then say what the changing force does to the cone and to the air.

Full working

A varying current flows through the coil of the loudspeaker. The coil sits in the magnetic field of a magnet, so the current experiences a force. This is the motor effect. As the current changes, the size and direction of the force change, so the coil moves backwards and forwards and pushes the cone. The moving cone makes pressure variations in the air, which are a sound wave.

3. Compare a moving-coil microphone with a moving-coil loudspeaker. State the effect each device uses.

Hint

Compare the input and the output of each device.

Next step

Say what goes into each device and what comes out, then name the effect for each.

Full working

A microphone takes in the pressure variations of a sound wave and puts out variations in a current. It uses the generator effect, because a coil moves in a magnetic field.

A loudspeaker takes in variations in a current and puts out the pressure variations of a sound wave. It uses the motor effect, because a current in a magnetic field experiences a force.

Each device does the opposite of the other.

4. A loudspeaker coil has 0.10 m of wire inside the magnetic field. The flux density is 0.30 T, and the current is 0.40 A. Calculate the force on the coil.

Hint

The coil is at right angles to the field. Use the force equation from the motor effect.

Next step

Write $F = BIl$ , then put in $B = 0.30$ T, $I = 0.40$ A and $l = 0.10$ m.

Full working

\begin{aligned} F &= B I l \\ &= 0.30 \times 0.40 \times 0.10 \\ &= 0.012\,\text{N} \end{aligned}