CIE AS Physics (9702) 2019-2021

Revision Notes

8.1.3 Wave Intensity

Wave Intensity

  • Progressive waves transfer energy
  • The amount of energy passing through a unit area per unit time is the intensity of the wave
  • Therefore, the intensity is defined as power per unit area

 

Intensity and power equation, downloadable AS & A Level Physics revision notes

Intensity is equal to the power per unit area

  • The area the wave passes through is perpendicular to the direction of its velocity
  • The intensity of a progressive wave is also proportional to its amplitude squared and frequency squared

 

Intensity and amplitude equation, downloadable AS & A Level Physics revision notes

Intensity is proportional to the amplitude2 and frequency2

  • This means that if the frequency or the amplitude is doubled, the intensity increases by a factor of 4 (22)

Spherical waves

  • A spherical wave is a wave from a point source which spreads out equally in all directions
  • The area the wave passes through is the surface area of a sphere: 4πr2
  • As the wave travels further from the source, the energy it carries passes through increasingly larger areas as shown in the diagram below:

Intensity of a spherical wave, downloadable AS & A Level Physics revision notes

Intensity is proportional to the amplitude squared

 

  • Assuming there’s no absorption of the wave energy, the intensity I decreases with increasing distance from the source
  • Note the intensity is proportional to 1/r2
    • This means when the source is twice as far away, the intensity is 4 times less
  • The 1/r2 relationship is known in physics as the inverse square law

Worked example

Worked example - intensity (1), downloadable AS & A Level Physics revision notes
Worked example - intensity (2), downloadable AS & A Level Physics revision notes

Exam Tip

The key concept with intensity is that it has an inverse square relationship with distance (not a linear one). This means the energy of a wave decreases very rapidly with increasing distance

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