- #1

davidbenari

- 466

- 18

The energy density in an EM field is (Its not necessarily a plane wave we're talking about here).

##u= \frac{\epsilon E^2}{2}+\frac{B^2}{2\mu} ##

The relationship ##E=cB## is supposedly "general" so then the energy density is:

##u = \epsilon E^2 ##

the intensity (power per unit area) is then

##I = \epsilon c E^2 ##

and this is a general result as well.

Now suppose I have two plane waves coming in at different angles towards a point on which they intersect. I want to know the average intensity at that point.

I could proceed in two ways: one is to find the Poynting vector by adding the E and B fields and averaging out in time.

Or I could add the E fields and average out in time obtaining

##<I> = c \epsilon <E^2> ##

and these should be equal to one another (of course I'm taking about the magnitude of the Poynting vector).

Is the finding-the-E-field approach equally as valid as finding the Poynting vector?

Thanks.