Hi;
I have mightily confused myself, maybe someone can straighten me out. A
spherical wave spreads out so obviously the energy per area (intensity)
decreases as 1/r2, we expect that just from geometric considerations.
But does the amplitude also decrease? A simple y(r,t)=Asin(kr-wt) with r
being a polar coordinate says no (and a plane wave in a perfect medium
does not). My intuition is that it seems like if you are far away from a
source the amplitude can't be the same but amplitude squared is
proportional to energy. So is the intensity also decreasing because
amplitude is decreasing? But that would mean the 1/r2 dependence for
energy/area is wrong.
Several wave books (Berkeley physics waves p 372) I have show that the
Poynting vector of the radiation from an accelerating charge is
proportional to charge squared, acceleration squared and 1/r^2. But how
does that relate to amplitude of the wave?
kyle
--
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"Physics is like sex. Sure, it may give
some practical results, but that's not
why we do it."
R. Feynman (as quoted by Bill Beatty)