The Magnitude of the Poynting Vector

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Homework Help Overview

The discussion revolves around the relationship between the magnitude of the Poynting vector and the intensity of electromagnetic waves, exploring definitions and implications of time-averaging in these concepts.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants examine the definitions of intensity and the Poynting vector, questioning whether their magnitudes can be considered equal. There is a focus on the implications of time-averaging and the distinction between instantaneous and average values.

Discussion Status

Some participants have provided clarifications regarding the definitions and relationships between the quantities, noting that intensity is the time-averaged value of the Poynting vector. There is ongoing exploration of how these concepts relate to each other without reaching a definitive conclusion.

Contextual Notes

Participants are navigating the nuances of definitions and assumptions regarding electromagnetic wave properties, particularly the role of time-averaging in the context of the Poynting vector and intensity.

abcdmichelle
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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!?
 
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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.
 
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
 
abcdmichelle said:
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.
 
thank you!
 

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