A student is using a signal generator connected to a loudspeaker to produce sound waves in a room. Explain what is meant by a sound wave, describing how the wave transfers energy through the air and distinguishing it from a transverse wave.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sound is produced when a loudspeaker cone vibrates back and forth, creating disturbances in the surrounding air.
Model answer (5 marks)
A sound wave is a longitudinal wave.
1. The particles of air vibrate parallel to the direction in which the wave travels.
2. This vibration creates alternating regions of compression, where particles are pushed together, and rarefaction, where particles are spread apart.
3. Energy is carried away from the loudspeaker in the direction the wave propagates; the air particles themselves only oscillate about their equilibrium positions and do not move with the wave.
4. In contrast, a transverse wave has particles that vibrate perpendicular to the direction of energy transfer, whereas in a longitudinal wave the vibration is parallel to the wave direction.
1. The particles of air vibrate parallel to the direction in which the wave travels.
2. This vibration creates alternating regions of compression, where particles are pushed together, and rarefaction, where particles are spread apart.
3. Energy is carried away from the loudspeaker in the direction the wave propagates; the air particles themselves only oscillate about their equilibrium positions and do not move with the wave.
4. In contrast, a transverse wave has particles that vibrate perpendicular to the direction of energy transfer, whereas in a longitudinal wave the vibration is parallel to the wave direction.
Examiner tips
- Use the word "longitudinal" early to show understanding of wave type.
- Show the compression/rarefaction cycle to demonstrate knowledge of wave structure.
- Explain that particles vibrate about a fixed point, not travel with the wave.
- Contrast particle motion with that of a transverse wave to answer the comparison part.
Common mistakes
- Confusing longitudinal with transverse wave terminology.
- Saying particles move with the wave instead of oscillating about a fixed point.
- Omitting the compression/rarefaction description.
Mark scheme (5 marks)
- Sound is a longitudinal wave
- The particles of air vibrate parallel to (in the same direction as) the direction of energy transfer / wave travel
- The wave produces regions of compression (particles pushed closer together) and rarefaction (particles spread further apart)
- Energy is transferred in the direction the wave travels / from the loudspeaker outwards, but the air particles themselves do not travel — they only vibrate about a fixed point
- In a transverse wave the particles vibrate at right angles (perpendicular) to the direction of energy transfer / wave travel, whereas in a longitudinal wave they vibrate parallel
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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