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Homework Help: EM Wave questions

  1. Apr 28, 2010 #1
    1. The problem statement, all variables and given/known data

    A radio transmission tower radiates electromagnetic waves uniformly in all directions
    with an average total power Pav = 70, 000 W at a frequency f = 98 MHz. A radio
    receiver uses the induced emf in a single circular wire loop of radius r = 5.0 cm to
    detect a radio signal.
    If the maximum induced emf must exceed 0.0050 V to be detectable, find the maximum
    distance d the receiver can be from the transmission tower and detect the wave.
    (The distance d is much greater than the wavelength  of the wave.)

    2. Relevant equations

    S = EB/uo
    I = P/4(pi)r^2
    I = E(rms)^2/c*uo
    E or B are functions given by E(x,t)=Esin(kx-wt) or B(x,t)=Bsin(kx-wt)
    c=f(wavelength) and k = 2(pi)/(wavelength)

    3. The attempt at a solution

    I started with the end and went for V = -dflux/dt. I found the flux of the B field through the loop and then differentiated B(x,t) wrt time. I have an expression for this but I'm not sure what the B coefficient in the B(x,t) equation (max value of B) should be or what to do about the cos(kx - wt) term. Any help on where to go for this problem?
  2. jcsd
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