The poynting vector is defined as
→S=μ−10→E×→B
Taking the divergence of the poynting vector, one arrives at
→∇⋅→S=−∂u∂t=0
after some algebraic manipulation.
Note u is the electromagnetic energy density.
The claim is that the →∇⋅→S is an energy flux density.
How do I see this is true?
Thanks in advance.
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