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Homework Help: Power and Intensity of Radio Waves

  1. Oct 28, 2007 #1
    1. The problem statement, all variables and given/known data

    An AM radio station broadcasts with a power of 21.0 kW at a frequency of 910 kHz. Estimate the intensity of the radio wave at a point 18.0 km from the broadcast antenna.

    2. Relevant equations

    I= p/A I1/I2=r2^2/r1^2 w=2pi f

    3. The attempt at a solution

    From the given frequency, we can find omega (w). However, I am stuck on where to proceed afterwards. How would you find A? Thanks so much for helping.

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    Last edited by a moderator: Mar 10, 2009
  2. jcsd
  3. Oct 28, 2007 #2
    I cannot quite decode the relevant equation. Maybe its a fonts thing.
    w (omega) = 2*pi*f is just expressing frequency in radians per second instead of cycles per second.

    In free space, a radio wave intensity decreases as the area of the wavefront expanding sphere increases. In Earth atmosphere, it is also frequency-dependent because of propagation and absorption losses, ground losses, and ionospheric conditions. Not that we need be concerned..because
    You have a given expression for the intensity, so we dodge all that hard stuff.

    I can see [tex]I =\frac{p}{A}[/tex] which makes sense. Not sure what the rest (I1/I2) is set equal to [tex]r_2^2/r_1^2 [/tex] :confused:

    I am unclear how [tex] \omega=2\pi f [/tex] while true, actually affects anything here.

    Sorry I don't quite get it yet. Have another try about what this intensity expression is.
  4. Mar 10, 2009 #3
    just use the the amount of power and the distance. you dont need frequency.

    I = P/(4*pi*r^2)
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