Explain why the amplitude of a transverse wave decreases as it travels through a medium, even when no energy is transferred to neighbouring media.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A transverse mechanical wave travels along a long, uniform string. An observer notes that the amplitude of the wave decreases with distance from the source, despite the string being well insulated from any surrounding medium.
Model answer (4 marks)
A transverse wave carries energy from the source. The energy per unit length of a wave is proportional to the square of its amplitude, so a decrease in energy must give a decrease in amplitude. Even in a perfectly insulated string the particles of the string are not perfectly elastic; internal friction or damping forces act between neighbouring particles. These forces convert mechanical energy into thermal energy of the string. As the wave propagates, this energy loss reduces the energy available to the oscillating particles, so their maximum displacement – the amplitude – becomes smaller. This loss of energy by conversion to heat is called absorption or attenuation and is distinct from reflection or transmission losses.
Examiner tips
- Show the energy–amplitude relationship first; then explain internal friction as the source of loss; finally link loss of energy to reduced amplitude.
- Use the term ‘absorption/attenuation’ to match the mark scheme.
- Mention that the string is insulated to rule out external losses.
Mark scheme (4 marks)
- The wave transfers energy from the source outward / the energy of the wave is related to the square of its amplitude.
- Energy is absorbed/dissipated by the medium itself through internal friction or damping forces between particles of the medium.
- As energy is removed from the wave, the amplitude decreases because the oscillating particles have less energy and so reach a smaller maximum displacement.
- This process is called absorption/attenuation and is distinct from reflection or transmission losses; it results from the conversion of wave (mechanical) energy into thermal (internal) energy of the medium.
Key terms in this question
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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