The energy flow per unit time per unit area S of an electrom

The energy flow per unit time per unit area (S) of an electromagnetic wave has an average value of 217 mW/m2. The maximum value of the magnetic field in the wave is closest to: A) 4.26 × 10-8 T B) 8.53 10-8 T C) 5.89 × 10-8 T D) 2.13 x 10-8 T E) 3.01 x 10-8 T

Solution

S = dU/A.dt = e0*c*E^2

E = c.B

c =3*10^8 m/sec

S = 217 mW/m^2 = 317*10^-3 W/m^2

B = sqrt(217*10^-3/(8.85*10^-12*3^3*10^24)) = 3.01*10^-8 T