Radio transmitter broadcasts at frequency of 105 MHz and its power is 7 W. The t

Question

Radio transmitter broadcasts at frequency of 105 MHz and its power is 7 W.
The transmitters amplitude (strength) is distributed evenly over a semi-sphere . For every question the distance from the transmitter is 2 km.

a) Find the wavelength and velocity of the wave
b) Find the strength of the electric- and magnetic field of the wave.
c) Find the strength (I, intensity) of the wave.
d) Describe the waves Poynting-vector, it's size and direction.
e) Find the waves radiation pressure on a 3 m2  are which is prependicular to the wave.

Explanation / Answer

The frequency of the transmitter f = 105MHz then the wavelength                    = c/f = 3*10^8 m/s/ 105*10^6                               = 2.85 m the velocity of the wave is velocity of ligth v =c (b) The intensity of the radiation               I = P/A                 = 7W/3r^2                 = 1.85*10^-7 W/m^2 The strength of the electric field            E = 2S/0c               = 2(1.85*10^-7)/(8.85*10^-12)(3*10^8)               = 0.011 N/C The strength of the magnetic field           B = E/c = 0.011 / 3*10^8 = 3.66*10^-11 T (c) The intensity of the wave              I = P/A                 = 7W/3r^2                 = 1.85*10^-7 W/m^2                 = 7W/3r^2                 = 1.85*10^-7 W/m^2 (d) The poynting vector          S = 1/0(E*B)             = 1/0 [ (0.011)j*(3.66*10^-7)k)             = -455 i (e) The pressure radiation             P = S/c = 1.85*10^-7 W/m^2 /c                 = 6.2*10^-16 N/m^2         

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