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Intensity From a Point Source

  1. Nov 11, 2007 #1
    1. The problem statement, all variables and given/known data

    A point-like source of electromagnetic waves radiates power uniformly in all directions at a single frequency.

    At a distance of 5.6 km from the source, a detector measures the intensity (energy flux density) of the wave to be 35.1 μW/m2 (microwatts per square meter).

    a)Calculate the amplitude of the magnetic field oscillations of the wave at the detector.

    b)What is the total power radiated by the source?

    Hint: The intensity (power/area) is c ε0 E0^2/2 where E0 is the amplitude of oscillation of the electric field in the wave. Then the magnetic field is B0=E0/c.

    2. Relevant equations

    The relevant equations are given in the "Hint" of the problem. And I used 8.85E-12 for epsilon.

    3. The attempt at a solution

    I got 0.163 T for the amplitude, but it is wrong. I got 1.425E-12 W for the power, but it is also wrong. Help please.
  2. jcsd
  3. Nov 12, 2007 #2


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    The power is easy, what is the surface area of a sphere wih radius = 5600m,
    If the intensity is 35.1E-6w/m^2 what is the total power for this area, hint the units should come out as watts!
  4. Nov 12, 2007 #3
    Alright, I got a). I cannot figure out b) though.
  5. Nov 12, 2007 #4
    Thanks mgb. I got both parts
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