# EM wave intensity!

Howdy everyone.

1. Ok, so we have and electromagnetic wave propagating through a vacuum. We know wavelength, and the magnitudes of B in the x and y directions. Also, B=[Bx(i)+By(j)]cos(kz+ωt). That's all the prereq stuff.

2. So Intensity is: <P>/A & (1/2)Bmax^2/Zo

I'm def feeling the second equation is the most useful here! But this equation yields an incorrect answer. Are these the only two equations for Intensity?

3. I've tried taking the magnitude of the B and plugging into the second eq. This didn't work.

Thanks,
Robert

## Answers and Replies

Simon Bridge
Science Advisor
Homework Helper
You seem to be trying to use equations without understanding the physics behind them.
i.e. the "magnitude of B" will change with position and time ... so which "magnitude" did you pick? Perhaps you mean you used the amplitude of the wave?

Did I read that equation correctly:
$$\vec{B} = [B_x\hat{\imath} + B_y\hat{\jmath}]\cos(kz+\omega t)$$... so what is the amplitude of the B field?

Why would you have two equations for the same thing that give different values for that thing?
Go back to the physics: what is "intensity"?
http://physics.bu.edu/~duffy/PY106/EMWaves.html