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Energy in waves (Amplitude and Frequency)

  1. Aug 11, 2011 #1
    ---------- About Mechanics waves ---------

    We know that in mechanics waves, energy is proportion to Amplitude^2.
    So we can write equation as E = kA^2 , where k is a constant.

    Is it possible to find this constant (k) ?
    Is k different in other waves (meaning it is depended on other values) or it is constant to all mechanics waves ?

    ---------- About Electromagnetics waves ---------

    Does "Energy proportion to Amplitude^2" also apllied to ElectroMagnetics waves ?

    Is the result of photoelectric effect meaning that energy of EM waves is not proportion to amplitude but only frequency ?

    Will the calculation be accurated if I calculate energy from Amplitude^2 in EM waves ?

    Thank in advance.
     
  2. jcsd
  3. Aug 11, 2011 #2
  4. Aug 11, 2011 #3

    Drakkith

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    Does amplitude have any meaning for a photon?
     
  5. Aug 11, 2011 #4
    h taking the place of A^2 as the smallest measurable density.

    The energy of any wave is proportional to the product of the density(amplitude) and the frequency.
     
  6. Aug 11, 2011 #5

    Drakkith

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    Yes but for an EM wave does that mean that the amplitude is always set to a certain value?
     
  7. Aug 11, 2011 #6
    No, not at all. The particle view attempts to break down the EM wave into a collection of particles. The Amplitude is now the density of particles, each particle having the minimum measurable amplitude and only differing in frequency.

    It is just a point of view.
     
  8. Aug 11, 2011 #7

    Drakkith

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    So the amplitude of a single photon is set then?
     
  9. Aug 11, 2011 #8
    I was talking about a collection of photons, that is what an EM wave is right?
     
  10. Aug 11, 2011 #9
    Yup, it is the smallest measurable amplitude of an EM wave. Plank's constant
     
  11. Aug 11, 2011 #10

    Drakkith

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    Not really. An EM wave to me means a photon. But I could be incorrect.

    Ok, so Planks constant, AKA the amplitude, helps defines the energy of a particle because a higher frequency would have more waves per second, more peaks or whatever?
     
  12. Aug 11, 2011 #11
    The frequency is the velocity of the source charge, or the rate of change of the wave function. Combine this with the amplitude^2 to get your energy.

    In the particle view, the particles have the lowest measurable amplitude, empirically found to be plank's constant.
     
  13. Aug 11, 2011 #12
    A photon is the quanta of an EM wave.

    Edit: it is nothing like a wave.
     
  14. Aug 11, 2011 #13

    Drakkith

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    Hrmm. I am unsure about this. A photon is quantized, but I think that the photon IS the EM wave.
     
  15. Aug 11, 2011 #14
    Thank everyone.

    But I think I am missing some important point of Photoelectric effect. Isn't that means amplitude is not proportion to energy ?
     
  16. Aug 11, 2011 #15

    Drakkith

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    That is correct. The change in wavelength determines the amount of change in energy of an EM wave. (Photon) Double the frequency (half the wavelength) and you double the energy.
     
  17. Aug 11, 2011 #16
    somebody tell me what is the value of "k".
    It is constant (or) varies with materials?
     
  18. Aug 11, 2011 #17
    Anyway, Wavelength and Amplitude are different.
     
  19. Aug 12, 2011 #18
    k would be your frequency. The material is not usually taken into account with mechanical waves when determining the energy transferred through the material, it is usually taken to be some material that is good at transferring energy. If you want to look into inefficiencies in the material first you take the mechanical wave energy and then you apply inefficiencies in some complex manner.
     
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