Cambridge O Level Physics · Syllabus 5054 · Waves
Amplitude
What is Amplitude?
Amplitude is the maximum distance a point on the wave moves from its mean (equilibrium) position; it is measured from the middle to a crest, not from crest to trough.
This definition is part of the Waves chapter in Cambridge O Level Physics.
Amplitude in context
A wave is a disturbance that travels, carrying energy away from a source while the material it passes through only oscillates about fixed positions. Describe the disturbance with wavelength, frequency and amplitude; link them with \(v = f\lambda\). Every wave can reflect, refract and diffract. Light is an electromagnetic wave, so mirrors, blocks, prisms, fibres and lenses are all the same three behaviours applied to rays. Sound is a longitudinal mechanical wave, so it needs a medium, its pitch follows frequency and its loudness follows amplitude, and its echoes measure distance.
A wave is a disturbance that travels through space or a material, transferring energy from a source to a receiver without transferring matter. The particles of the medium oscillate about fixed equilibrium positions and end up where they began. Describe the wave with its wavelength, frequency and amplitude, and link them to the speed with \(v = f\lambda\). Waves of every kind can be reflected at a plane surface, refracted when their speed changes, and diffracted at a gap or an edge.
Sound is produced by a vibrating source and travels as a longitudinal mechanical wave: the particles of the medium oscillate back and forth parallel to the direction of energy transfer, creating compressions (particles closer together, pressure above normal) and rarefactions (particles further apart, pressure below normal). Because it needs particles to pass the disturbance on, sound cannot travel through a vacuum. Its frequency sets the pitch, its amplitude sets the loudness, and its waveform shape sets the quality or timbre.
Common mistakes with Amplitude
- 2. “Amplitude is the distance from crest to trough.” Why wrongThat distance spans the full swing, from one extreme to the other, so it is twice the maximum displacement from the middle. CorrectAmplitude is measured from the equilibrium (mean) line to a crest, or from the mean line to a trough. Crest-to-trough is \(2a\). Say this“Amplitude is the maximum distance from the mean position.” CheckA trace shows a crest 6 cm above a trough. State the amplitude. (3 cm.)
- 5. “Refraction occurs because the amplitude changes.” Why wrongAmplitude controls how much energy the wave carries. It has no influence on direction; a very quiet and a very loud sound refract identically. CorrectRefraction is caused by a change of speed. A wavefront arriving obliquely pivots because one end slows before the other. Say this“Refraction occurs because the wave changes speed at the boundary.” CheckState the one quantity whose change causes refraction.
- 19. “Pitch depends on amplitude.” Why wrongPlay the same guitar string harder: it sounds louder, not higher. The note is unchanged. CorrectPitch is set by frequency. Higher frequency gives higher pitch. Say this“A higher frequency gives a higher pitch.” CheckTwo traces have the same height but one has twice as many waves. Which sound is higher in pitch?
- 20. “Loudness depends on frequency.” Why wrongThe mirror image of the previous error. A high whistle can be barely audible and a low drum deafening. CorrectLoudness is set mainly by amplitude. Greater amplitude gives a louder sound. Say this“A larger amplitude gives a louder sound.” CheckWhich measurement on an oscilloscope trace tells you the loudness?
- 22. “Ultrasound means very loud sound.” Why wrongIt confuses the two axes of a trace. “Ultra” here refers to frequency, not amplitude. A dog whistle is ultrasonic and very quiet. CorrectUltrasound is sound of frequency above 20 kHz, beyond the upper limit of human hearing. Its loudness is a separate matter entirely. Say this“Ultrasound is sound with a frequency higher than 20 kHz.” CheckIs a 30 kHz sound necessarily loud? Explain.