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Homework Help: Find electric and magnetic field amplitudes in an electromagnetic wave

  1. Oct 1, 2011 #1
    [Solved] Find electric and magnetic field amplitudes in an electromagnetic wave

    1. The problem statement, all variables and given/known data

    Find the electric and magnetic field amplitudes in an electromagnetic wave that has an average energy density of 1 J/m^3

    2. Relevant equations

    u = Energy density
    u = (1/2)(e0)(E^2) + (1/2)(B^2/u0)
    or u = (e0)(E^2)
    or u = (B^2)/u0

    E = cB, c = speed of light

    3. The attempt at a solution

    Let's find E first:
    u = e0*E^2
    E = sqrt(u/e0) = sqrt(1/(8.85*10^-12)) = 336146 V/m
    336146 V/m is apparently wrong for E. Let's try B instead

    u = (B^2)/u0
    B = sqrt(u*u0) = 0.001121 T
    Now let's try E = cB to find E
    E = (3*10^8)(0.001121) = 336300 V/m

    But both E and B are wrong. What am I missing? Percent error tolerance for a correct answer is 10%.
     
    Last edited: Oct 1, 2011
  2. jcsd
  3. Oct 1, 2011 #2

    SammyS

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    I think your answers are for the RMS values of the fields, rather than for the peak values. The peak values are generally used for the amplitude of a wave.

    Try multiplying your answers by √(2) .
     
  4. Oct 1, 2011 #3
    Edit: This is the equation I used, I have Uav = 1 J/m^3

    dEHaO.png

    Edit: Oh, I see. Thanks. :)
     
    Last edited: Oct 1, 2011
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