OCR AS Physics

Revision Notes

4.9.7 Stationary Waves

Stationary Waves

  • Stationary waves, or standing waves, are produced by the superposition of two waves of the same frequency and amplitude travelling in opposite directions
  • This is usually achieved by a travelling wave and its reflection
    • The superposition produces a wave pattern where the peaks and troughs do not move
  • Stationary waves store energy, unlike progressive waves which transfer energy

Stationary wave formation, downloadable AS & A Level Physics revision notes

Formation of a stationary wave on a stretched spring fixed at one end

  • Stationary waves can be represented by various mediums

Stretched String

  • Vibrations caused by stationary waves on a stretched string produce sound
    • This is how stringed instruments, such as guitars or violins, work
  • This can be demonstrated by a length of string under tension fixed at one end and vibrations made by an oscillator:

Stationary wave string, downloadable AS & A Level Physics revision notes

Stationary wave on a stretched string

 

  • As the frequency of the oscillator changes, standing waves with different numbers of minima (nodes) and maxima (antinodes) form

Microwaves

  • A microwave source is placed in line with a reflecting plate and a small detector between the two
  • The reflector can be moved to and from the source to vary the stationary wave pattern formed
  • By moving the detector, it can pick up the minima (nodes) and maxima (antinodes) of the stationary wave pattern

 

Stationary wave microwave, downloadable AS & A Level Physics revision notes

Using microwaves to demonstrate stationary waves

Air Columns

  • The formation of stationary waves inside an air column can be produced by sound waves
    • This is how musical instruments, such as clarinets and organs, work
  • This can be demonstrated by placing a fine powder inside the air column and a loudspeaker at the open end
  • At certain frequencies, the powder forms evenly spaced heaps along the tube, showing where there is zero disturbance as a result of the nodes of the stationary wave

Air column stationary waves, downloadable AS & A Level Physics revision notes

Stationary wave in an air column

 

  • In order to produce a stationary wave, there must be a minima (node) at one end and a maxima (antinode) at the end with the loudspeaker

Exam Tip

Always refer back to the experiment or scenario in an exam question e.g. the wave produced by a loudspeaker reflects at the end of a tube. This reflected wave, with the same frequency, overlaps the initial wave to create a stationary wave.

Graphical Representation of a Stationary Wave

  • A stationary wave is formed when two waves travelling in opposite directions along the same line overlap with each other
  • The waves must have:
    • The same speed
    • The same frequency (or wavelength)
    • A similar amplitude
  • As a result of superposition, a resultant wave is produced

Formation of stationary waves (1), downloadable AS & A Level Physics revision notes

Nodes and antinodes are a result of destructive and constructive interference respectively

  • A stationary wave is made up of nodes and antinodes
  • At the nodes:
    • The waves are in anti-phase meaning destructive interference occurs
    • This causes the two waves to cancel each other out and there is no vibration
  • At the antinodes:
    • The waves are in phase meaning constructive interference occurs
    • This causes the waves to add together and the vibration is at maximum amplitude
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