The Magnitude of the Poynting Vector

[abcdmichelle]

General question: Is the magnitude of the poynting vector equal to the intensity of an electromagnetic wave? I know that I= average S which makes me think that I cannot simply assume that that their magnitudes are equal!?

 

[Steely Dan]

The Poynting vector gives the instantaneous quantity representing power/area (which does have the same units as intensity) whereas, indeed, intensity (as it is typically defined) is the time averaged value of the magnitude of the Poynting vector.

 

[abcdmichelle]

That makes sense, I guess the thing that is throwing me off is the "time-average".

Intensity = time average of S
I = <S>
I= (1/2)*c*epsilon not*Emax^2

Can I say then that
magnitude of I = magnitude of S

or would I have to say

magnitude of I = (1/2)*c*epsilon not*Emax^2 = (1/2) * S

 

[Steely Dan]

Can I say then that
magnitude of I = magnitude of S

or would I have to say

magnitude of I = (1/2)*c*epsilon not*Emax^2 = (1/2) * S

You could say that the magnitude of I is equal to 1/2 of the amplitude of the Poynting vector, but that's it. One is a time-dependent quantity and one is not.

 

[abcdmichelle]

thank you!