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Homework Help: Finding an electric field from a radio station

  1. May 14, 2006 #1
    If a radio station transmits at 500 kW, how does one find the electric field 25 kM away?
  2. jcsd
  3. May 14, 2006 #2
    The energy emitted by the source in [tex]dt[/tex] is [tex]Pdt[/tex]
    where [tex]P=500\,kW[/tex]. At [tex]R=25\,km[/tex] from the source this energy will be uniformly distributed in a spherical shell of radii [tex]R[/tex] and [tex]R+dR[/tex]. The volume of this shell equals
    [tex]dV=\pi R^2 c dt[/tex]
    where [tex]c[/tex] is the speed of light. Now you have to know that the energy density of an electromagnetic wave can be written as
    [tex]w=\frac{1}{2}\epsilon_0 E^2[/tex]
    with [tex]E[/tex] the elecric field amplitude. So
    [tex]P dt=w\cdot dV[/tex]
    ..... and you find E!
    Last edited: May 14, 2006
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