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GCSE Physics Revision

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GCSE Physics revision

Microphones (HT only)

Induced potential, transformers and the National Grid (physics only) (HT only)

AQA 4.7.3.3
Your specification

AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Explain how a moving-coil microphone uses the generator effect to convert sound-pressure variations into variations in electrical current.

MOST 🎯🎯

Apply the scientific explanation of microphones to a relevant example.

SOME 🎯🎯🎯

Analyse a new situation involving microphones and explain the scientific reasoning.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

How a Loudspeaker Works – Overview

  • A loudspeaker converts electrical energy into sound energy using the motor effect.
  • An alternating potential difference (alternating PD) is applied to a coil of wire inside the loudspeaker.
  • The coil sits within the magnetic field of a permanent magnet, so when current flows, two magnetic fields interact.

The Motor Effect in a Loudspeaker

  • When the magnetic field from the current-carrying coil crosses the magnetic field of the permanent magnet, a force is produced — this is the motor effect.
  • Because the PD is alternating, the current changes direction, so the force on the coil also changes direction repeatedly.
  • The changing force causes the coil to move back and forth, vibrating the attached speaker cone (diaphragm).

Producing Sound from a Loudspeaker

  • The vibrating speaker cone (diaphragm) creates compressions and rarefactions in the surrounding air, producing a sound wave.
  • The frequency of the sound wave produced is equal to the frequency of the alternating potential difference supplied to the coil.
  • Humans can hear sound waves with frequencies between approximately 20 Hz and 20,000 Hz.

Understanding Frequency in Loudspeakers

  • Frequency is measured in hertz (Hz) and represents the number of waves produced per second.
  • A high frequency means the waves are closer together, producing a higher-pitched sound.
  • A low frequency means the waves are more spread out, producing a lower-pitched sound.

How a Microphone Works – Overview

  • A microphone converts sound energy into electrical energy using the generator effect — the opposite process to a loudspeaker.
  • Sound waves entering the microphone cause pressure variations (compressions and rarefactions) that make the diaphragm vibrate.
  • The diaphragm is attached to a coil of wire that is positioned around a permanent magnet.

The Generator Effect in a Microphone

  • When the diaphragm vibrates, it causes the attached coil of wire to move within the magnetic field of the permanent magnet.
  • A conductor moving through a magnetic field induces a potential difference — this is the generator effect.
  • Because the coil moves back and forth, the induced potential difference alternates in direction, producing an alternating PD.

Linking Sound Frequency to Electrical Frequency in a Microphone

  • The frequency of the alternating potential difference induced in the microphone's coil is equal to the frequency of the incoming sound wave.
  • This means the electrical signal produced is an accurate representation of the original sound wave's frequency.

Comparing Loudspeakers and Microphones

  • A loudspeaker uses the motor effect: electrical energy (alternating PD) mechanical vibration sound wave.
  • A microphone uses the generator effect: sound wave mechanical vibration induced alternating PD (electrical energy).
  • Both devices rely on the interaction between a coil of wire and a permanent magnet, but the energy transfer occurs in opposite directions.

Key Components Shared by Both Devices

  • Both a loudspeaker and a microphone contain a diaphragm (speaker cone) that vibrates to either produce or detect sound.
  • Both devices use a coil of wire positioned within the field of a permanent magnet to convert between mechanical and electrical energy.
  • The alternating nature of the potential difference in both devices is essential — it ensures the direction of force or induced current changes in sync with the sound wave.

Key Terms to Remember

  • Alternating potential difference (alternating PD): a voltage that repeatedly changes direction, like a wave.
  • Motor effect: a force is produced when a current-carrying conductor is placed in a magnetic field.
  • Generator effect (electromagnetic induction): a potential difference is induced when a conductor moves through a magnetic field.