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Homework Help: Energy Density vs Total Energy

  1. Jan 25, 2007 #1
    Here is a detail that makes me doubt:

    1. The problem statement, all variables and given/known data
    I am given the equation for a standing wave with a dependence in 't' and 'z' only... I am told that the wave is propagating in a confined space (2*2*2) m
    Q1. Calculate the energy density for n=1,2,3. I didn't have any pb with this.
    Q2. Calculate the total energy for n=1,2,3. (!!!)

    2. Relevant equations

    3. The attempt at a solution
    Ok, I can't find anything in my lecture notes about total energy for waves. So I went back to the definition of energy density (h) which the amount of energy per unit of volume.
    Therefore if I multiply the volume by h I should get the energy (right?)
    The volume here is 8 m^3 BUT since the wave in question is only in 1 direction (z) do I still have to multiply by V or only by L=2m instead (for this particular case)

    Thanks for your answer(s)
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jan 26, 2007 #2


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    Homework Helper

    Well, since the integrated function has no "x" and "y" dependence, then the integration wrt them should be trivial, right ?
  4. Jan 28, 2007 #3
    trivial... maybe not or perhaps I would not be posting...

    The wave equation has no dependence in 'x' and 'y' indeed,however I am not integrated that function to get the total energy.
    I got the energy density (Question 1) which is h = rho * (A*Omega/2)^2
    A being the amplitude of the wave. The formula is correct according to my book (which does not mention total energy of sound waves btw)

    If I check the units of 'h' I get J/m^3 therefore I should multiply by a volume to get the total energy (in J) trivial right? I guess that why I posted, I was surprised to get an energy density in J/m^3 calculated from a wave that's propagating in 1 direction only...
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