Finding an electric field from a radio station

[Noone1982]

If a radio station transmits at 500 kW, how does one find the electric field 25 kM away?

 

[clive]

The energy emitted by the source in dt is Pdt
where P=500\,kW. At R=25\,km from the source this energy will be uniformly distributed in a spherical shell of radii R and R+dR. The volume of this shell equals
dV=\pi R^2 c dt
where c is the speed of light. Now you have to know that the energy density of an electromagnetic wave can be written as
w=\frac{1}{2}\epsilon_0 E^2
with E the elecric field amplitude. So
P dt=w\cdot dV
... and you find E!