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EM waves -- Energy Calculation

  1. May 22, 2017 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    I didnt understand much the question. Should I use, ##υ=εE^2## ? Then I ll take rms value of that E.But I am not sure that is that the max value.Is it max value ? I didnt understand part (B)
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  3. May 22, 2017 #2


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  4. May 22, 2017 #3
    we didnt see this subject yet..So I am trying to solve the question using just book , and I didnt quite understand.We have energy density and we need to find rms value of E.I wrote like ##u=εE^2## cause it also contains the energy of B-field.So this value is actually used for a instant time.But I think I can take is as a max value of E then ##E_{rms}=E_{max}/(\sqrt 2)##

    I didnt understand part b cause I didnt understand what the question wants from us.
  5. May 22, 2017 #4


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    First, you missed the factor of ##\frac{1}{2}## in the energy density.
    Part (b) asks you to find distance ##r## from a 7.5 kW radio transmitter so that the energy density in a sphere of radius ##r## has the value 4×10-14 J/m3.
  6. May 22, 2017 #5
    I solved thanks
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