The Magnitude of the Poynting Vector

  • #1
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!?
 

Answers and Replies

  • #2
319
0
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.
 
  • #3
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
 
  • #4
319
0
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.
 
  • #5
thank you!!
 

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