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Wave intensity question

  1. May 1, 2009 #1
    1. The problem statement, all variables and given/known data
    Q. Explain why the intensity of a sound or light wave is proportional to A^2, where A is the amplitude of the wave.


    2. Relevant equations
    S = (1/μ_0)ExB ... Poynting vector
    S = (1/μ_0)EB
    B = E/c
    E = Asin(kx - ωt)


    3. The attempt at a solution
    Hi everyone,

    So, for light, I used the pointing vector as above, substituting in B = E/c to get
    S = (1/cμ_0)E^2

    and then E = Asin(kx - ωt) to get
    S = (1/cμ_0)A^2sin^2(kx - ωt)

    And I used I = S_average

    So I = (1/cμ_0)[A^2sin^2(kx - ωt)]_avg

    From which we can conclude that I α A^2.

    Is this correct?

    As regards sound, I really don't know what to do, as it's not an electromagnetic wave - is it still possible to use the Poynting vector?
    Please help!
     
  2. jcsd
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