The poynting vector is defined as
$\vec{S}=\mu_{0}^{-1}\vec{E}\times \vec{B}$
Taking the divergence of the poynting vector, one arrives at
$\vec{\nabla} \cdot \vec{S}=-\frac{\partial u}{\partial t}=0$
after some algebraic manipulation.
Note $u$ is the electromagnetic energy density.
The claim is that the $\vec{\nabla} \cdot \vec{S}$ is an energy flux density.
How do I see this is true?
Thanks in advance.
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