Posted by jay on .
The electromagnetic wave that delivers a cellular phone call to a car has a magnetic field with an rms value of 1.90 10-10 T. The wave passes perpendicularly through an open window, the area of which is 0.20 m2. How much energy does this wave carry through the window during a 35-s phone call?
Multiply the Poynting flux vector S by the window area and the 35 s time interval.
You will need an electromagnetism textbook to relate the Poynting (energy) flux to the rms B field. S is proportional to the square of the B field.
S (W/m^2) = 2*[10^7 c/(8 pi)]*Brms(T)^2
If B = 1.9*10^-10 T, the electromegnetic flux is
S = 2.38*10^14*Brms^2
= 8.6*10^-6 W/m^2
Energy = 6.02*10^-5 W