# Homework Help: Intensity of Electromagnetic Waves

1. Mar 25, 2012

### diethaltao

1. The problem statement, all variables and given/known data
A plane monochromatic electromagnetic wave with wavelength λ = 2.2 cm, propagates through a vacuum. Its magnetic field is described by
$\vec{B}$ = (Bx$\vec{i}$ + By$\vec{j}$)cos(kz+ωt)
where Bx = 1.8E-6 T and By = 3.8E-6 T, and i-hat and j-hat are the unit vectors in the +x and +y directions, respectively.

What is I, the intensity of this wave?

2. Relevant equations
I = P/A = (1/2)cεoE^2
k = (2π)/λ
ω = ck
Bo*c = Eo

3. The attempt at a solution
I found frequency to be c/λ = 13.63GHz.

I have tried setting t to zero in the given equation and plugging in k and ω to find B, then multiplying that by the speed of light to find E.
I then plugged that into I = (1/2)cεoE^2, but that gave me the wrong answer.

I've also tried adding to the two vectors and assumed that equaled Bo (which I don't think it does), and used Bo = (√2ZoI)/c where Zo = 377 ohms.
I've tried <B^2> = (Bo^2)/2 as well.

Is there an important equation I'm missing?