An 800-kHz radio signal is detected at a point 8.5 km distant from a transmitter tower. The electric field amplitude of the sign
al at that point is 0.90 V/m. Assume that the signal power is radiated uniformly in all directions and that radio waves incident upon the ground are completely absorbed. What is the average electromagnetic energy density at that point
To find the average electromagnetic energy of the wave you use the fact that the wave can be taken as a spherical electromagnetic wave. IN this case, the amplitude of the wave change in space according to:
for a distance of 8.5km = 5.5*10^3m you have:
Next, to find the average energy at this point you use the following formula:
εo : dielectric permittivity of vacuum = 8.85*10^-12C^2/Nm^2
hence, the average energy of the wave is 4.96*10^-20 J