Loudspeakers (physics only) (HT only) — AQA GCSE Physics
Test yourself on Loudspeakers (physics only) (HT only) with AQA GCSE practice questions.
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Loudspeakers (physics only) (HT only) explained
A moving-coil loudspeaker turns an electrical signal into sound using the motor effect.
Read the full explanation
A coil is wound on a former sitting in the gap of a cylindrical permanent magnet, so the field runs straight across the wire. An alternating current in the coil puts a force on it, and because of the way the field is arranged that force pushes the coil in or out along the axis. Reverse the current and the force reverses, so an alternating signal makes the coil move in and out at the frequency of the signal, and a larger current gives a larger force and a bigger movement. The coil is fixed to a paper cone, the cone pushes on the air next to it, and the air is squashed into compressions and pulled apart into rarefactions. That is a sound wave with the same frequency as the current.
Students should be able to explain how a moving-coil loudspeaker and headphones work.
Questions on this show a cross-section: a cylindrical magnet with the coil sitting in the narrow gap between its north and south poles, the coil fixed to the narrow end of a paper cone, and the cone held at its rim by a flexible surround. Reading that diagram is half the answer. The field in the gap runs across the coil, so the force on the coil acts along the axis of the cone, in or out, and the flexible surround lets the coil and cone move freely and spring back. Headphones are the same device made small: a light coil and a thin diaphragm instead of a large cone, driven by a much smaller current, which is why they can be run straight from a phone. In both, the frequency of the sound matches the frequency of the current, and the amplitude of the movement sets the loudness.
Your focus
- Trace the signal path from an alternating current in a circuit to a sound wave travelling in air.
- Explain why reversing the current reverses the direction of the force on the coil.
- Relate the size and the frequency of the current to the loudness and the pitch of the sound produced.
Show all 7 objectives
- Identify the coil, magnet, cone and surround on a cross-section of a moving-coil loudspeaker.
- Explain why the coil moves along the axis of the cone instead of rotating.
- Compare a pair of headphones with a loudspeaker, naming what is the same and what is scaled down.
- Predict what a listener hears if the current in the coil is made steady rather than alternating.
Loudspeakers (physics only) (HT only) exam tips
Marking Points
- one mark for a varying or alternating current passing through the coil
- one mark for the coil experiencing a force because it is in the field of the permanent magnet
- one mark for reversing the current reversing the force, so the coil vibrates
- one mark for the vibrating coil making the cone vibrate
- one mark for the cone producing compressions and rarefactions, the pressure variations of a sound wave
- one mark for identifying the coil sitting in the gap between the poles of the permanent magnet
- one mark for the force on the coil acting along the axis, in or out, so the coil moves rather than turns
- one mark for the cone or diaphragm being attached to the coil and moving with it
- one mark for linking the frequency of the current to the frequency of the sound produced
- one mark for linking the size of the current to the amplitude of the vibration and so to the loudness
Examiner Tips
- 💡This is usually a six mark levels question; write the chain in order, current, force, coil, cone, air, sound.
- 💡Use the words compression and rarefaction, which turn the cone moves into a description of a sound wave.
- 💡If the question changes the signal, say what happens to both the loudness and the pitch.
- 💡Label the coil, magnet, cone and surround on the given figure, then use those labels in your sentences.
- 💡Mention one thing that changes the loudness and one that changes the pitch; questions often ask for both.
- 💡A steady direct current holds the cone in one position and makes no sound; say so if you are asked.
Common Mistakes
- saying the coil is attracted and repelled as a magnet, instead of naming the force on a current-carrying wire
- stopping at the cone vibrating, with no mention of pressure variations in the air
- describing the sound produced as a transverse wave, when sound in air is longitudinal
- explaining the generator effect, which is the microphone, rather than the motor effect
- saying that a bigger current gives a higher note, when it gives a louder one
- labelling the cone as the part that carries the current, when it is the coil
- saying the coil rotates like a motor coil, when the arrangement of the field makes it move in and out
- treating headphones as a different mechanism rather than a small moving-coil loudspeaker
- leaving the permanent magnet out of the explanation, so there is no field for the current to interact with