A longitudinal wave has vibrations parallel to the direction of transfer of energy by the wave.
The wave can be represented on a graph showing the variation of the displacement of the particles with distance from the source.
Which point on the graph is the centre of a compression?
Reference: June 2018 Paper 11 Q22
Answer: point B
The graph represents the relationship between the displacement of particles and their respective distances from the source in a longitudinal wave.
Note that in a longitudinal wave, the direction of the particles' vibrations is parallel to the direction of the transfer of energy by the wave, i.e. the particles are displaced parallel to the direction of the wave.
A, B, C and D are points of different distances from the source. When their displacement values are positive, these particles move away from the source and vice versa.
Compression occurs when particles close in on each other, and rarefaction occurs when such particles move away from each other. In the case of this A-level Physics question, these points can be found by observing each particle's displacement value.
Within the distance from source 0 to just before particle B, particles are moving away from the source as their displacement is positive. In this case, point A is moving away from the source.
On the other hand, within the distance from just after point B to just before point D, particles are moving towards the source as their displacement is negative. In this case, point C is moving toward the source.
Since A moves away from the source and C moves toward the source, both these points consequently move toward point B as it is between them. As such, B becomes the centre of compression.
Additional note - centre of rarefaction:
Conversely from the above explanation, the particles in the distance from just after point B to before point D are moving towards the source (hence away from point D). The particles in the distance from after point D to just before displacement value 0 are moving away from the source (hence away from point D as well). As such, point D is the centre of rarefaction.
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