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Homework Help: Radiation RMS

  1. Apr 13, 2010 #1
    1. The problem statement, all variables and given/known data

    At a distance of 25m from an isotropic 100 MHz radio source the intensity is 120W/m2.

    A) What is the rms electric field at 25m?

    The answer should be 213 V/m.

    B) Over a 2 hour period, what is the momentum given to a 4cm x 4cm surface (at the 25m distance) perpendicular to the source that reflects 3/4 of the energy?

    The answer should be 8.06*10^-6.

    C) What is the total power transmitted by the source?

    The answer should be 9.42*10^5.

    2. Relevant equations

    I=P/a = E^2max/2(u)(c)
    3. The attempt at a solution

    Emax = 274587.4

    I can not get any of these parts right. I tried them but can't get the same results. Can someone show me how to get these? Thanks for the help!
  2. jcsd
  3. Apr 13, 2010 #2


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    Hi huntingrdr! :smile:

    (have a mu: µ and an epsilon: ε and a square-root: √ and try using the X2 tag just above the Reply box :wink:)
    You mean I=P/a = E2max/2(µ)(c)?

    No, you're confusing electricity and magnetism :redface:

    it's I = P/A = ε0E2max/2c …

    see http://en.wikipedia.org/wiki/Energy_density#Energy_density_of_electric_and_magnetic_fields" :wink:
    Last edited by a moderator: Apr 25, 2017
  4. Apr 13, 2010 #3
    I figured out parts a &b. Can anyone help me with c?
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