7.4 Microphones and speakers — Practise
Work through these on your own. Keep your working neat, and keep the units with your numbers. Every question here is for Higher tier.
Both the questions and the answers below come from a set that is checked by computer every time the book is built.
- H Explain how a moving-coil microphone converts the pressure variations in a sound wave into variations in a current.
- H Explain how a moving-coil loudspeaker converts a varying current into a sound wave.
- H Compare a moving-coil microphone with a moving-coil loudspeaker, and state the effect each device uses.
- H A current flows through the coil of a loudspeaker, and the cone moves forward. Describe what happens to the cone when the current is reversed, and give a reason.
- H A microphone produces no current when the sound stops. Explain why the coil must keep moving for a current to be induced.
- H A loudspeaker coil has 0.040 m of wire inside the magnetic field. The flux density is 0.25 T and the current is 0.80 A. Calculate the force on the coil.
- H Explain how a pair of moving-coil headphones produces sound from a varying current.
- H Explain the part played by the diaphragm in a moving-coil microphone.
- H A student says a loudspeaker works by the generator effect. Explain why the student is wrong, and name the correct effect.
- H A loudspeaker coil sits at right angles to a magnetic field. Say which equation you will use to find the force on the coil, and name the quantities you need.
Answers and concise solutions
1. How a microphone works
- The pressure variations in the sound wave 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 moves relative to the field, so a potential difference is induced across it. This is the generator effect, and it drives a varying current that copies the sound wave.
2. How a loudspeaker works
- The varying current flows through a coil in a magnetic field, so the coil experiences a force. This is the motor effect.
- As the current changes, the size and direction of the force change, so the coil and the cone move backwards and forwards.
- The moving cone pushes the air, making pressure variations that are a sound wave.
3. Microphone compared with loudspeaker
- A microphone takes in sound and puts out a varying current.
- A loudspeaker takes in a varying current and puts out sound.
- The microphone uses the generator effect, because a coil moves in a magnetic field. The loudspeaker uses the motor effect, because a current in a magnetic field feels a force.
4. What the reversed current does
- The cone moves backwards instead of forwards.
- The force on the coil depends on the direction of the current, so reversing the current reverses the force.
5. Why the coil must move
- A potential difference is induced only when the magnetic field around the conductor changes.
- The coil must move relative to the magnet to change the field through it. When the sound stops, the coil stops moving and the current stops.
6. Force on the coil
Answer: .
7. How headphones work
- The varying current flows through a coil in a magnetic field, so the coil experiences a changing force.
- The coil pushes a small diaphragm, which moves backwards and forwards.
- The moving diaphragm makes pressure variations in the air, which are heard as sound.
8. The job of the diaphragm
- The diaphragm catches the pressure variations of the sound wave and vibrates with them.
- The diaphragm is attached to the coil, so it makes the coil move in the magnetic field. That movement induces the potential difference.
9. The correct effect
- A loudspeaker starts with a current and makes movement and sound. The generator effect does the opposite: it starts with movement and makes a current.
- A loudspeaker works by the motor effect, because a current in a magnetic field experiences a force.
10. Choosing the method
- Use F = BIl, the force on a current-carrying conductor in a magnetic field.
- I need the magnetic flux density, the current in the coil, and the length of the coil in the field.