microphones and loudspeakers (topic 7)

Question 1

explain the function of a microphone (1)

Answer 1

to convert sound into variations in current

Question 2

explain how a moving-coil microphone works (4)

Answer 2

• sound waves cause the diaphragm to vibrate/oscillate
• causing the coil to vibrate
• the coil cuts the magnetic field of the magnet
• a potential difference is induced

Question 3

state one way in which the magnitude of the induced potential difference could be increased without changing the incident sound

Answer 3

• increase the number of turns on the coil
• use a stronger magnet

Question 4

when a sound wave is incident on the diaphragm, it causes a current to be induced in the coil, explain why a current is induced in the coil when a sound wave is incident on the diaphragm (4)

Answer 4

• the sound wave causes the diaphragm to vibrate
• causing the moving coil to vibrate
• coil cuts the magnetic field of the magnet, or coil passes through the magnetic field of the magnet
• inducing a current in the coil, or induces a potential difference across the ends of the coil

Question 5

explain how the moving-coil loudspeaker converts an electrical signal into a sound wave (6)

Answer 5

• the current in the coil is an alternating current
• the coil experiences a force (inwards or outwards)
• every half cycle the current reverses and reverses the direction of force on the coil
• the alternating current causes the coil to vibrate
• the vibrating coil causes the cone to vibrate
• the vibrating cone causes the air molecules to vibrate
• the vibration of the air molecules produces the pressure variations in the air needed for a sound wave
• the air molecules bunch together forming compressions and spread out forming rarefactions

Question 6

a student replaces the alternating current electrical signal with a direct current, explain why the student no longer hears the music output (3)

Answer 6

• coil will experience force in one direction only
• so coil won’t vibrate
• and the cone won’t vibrate
• no sound wave produced

Question 7

state two ways in which the force on the speaker cone could be increased

Answer 7

• use a stronger magnet
• use a higher current in the coil
• add more turns to the coil

Question 8

Answer 8

pressure variations caused by compressions and rarefactions in the air molecules

Question 9

Answer 9

Question 10

at the instant shown in the diagram above, the left hand electrode is positive and the right hand electrode is negative, state the direction of the force acting on the aluminium foil

Answer 10

upwards

Question 11

explain why the aluminium foil emits a low pitched humming noise when connected to the signal generator (6)

Answer 11

• current carrying aluminium foil has a magnetic field
• magnetic field of foil interacts with the magnetic field of the magnet
• force experienced by foil (as predicted by Fleming’s left hand rule)
• (alternating current causes the) direction of force to alternate every half cycle
• causing the foil to vibrate/oscillate
• (the vibrating foil causes the) air molecules to vibrate/oscillate
• the movement of the air molecules produces the pressure variations in the air needed for a sound wave