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Homework Help: Energy per unit area of EM wave

  1. Apr 20, 2009 #1


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

    The E field in an EM wave has a peak of 22.8 mV/m. What is the average rate at which this wave carries energy across unit area per unit time?

    2. Relevant equations

    S = e_0*c*E^2

    3. The attempt at a solution

    where e_0 = 8.85*10^-12
    c = 3*10^8
    E = 0.0228 V/m

    S = (8.85E-12)*(3*10^8)*(0.0228)^2 = 1.38017E-6 W/m^2


    I trying finding the Poynting vector S using the equation e_0*c*E^2 as I have all the needed values. This produces an answer of 1.380E-6 W/m^2 which was incorrect. Where did I go wrong?

  2. jcsd
  3. Apr 20, 2009 #2


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    Got it... I have to divide the peak of the E field by Sqrt(2) to get the RMS or the average value so I can find the average energy.

    Thanks for such a great resource!
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