I want to show the following equation with the maxwell equations: $$\frac{\partial}{\partial t}W+\vec{\nabla} \cdot \vec{S} = 0 $$
The problem is that I'm not understanding why I can do the following step:
$$\partial_t W=\partial_t \frac{1}{8\pi}(\vec{E}^2+\vec{B}^2)=\frac{1}{4\pi}(\vec{E}\partial_t\vec{E}+\vec{B}\partial_t\vec{B})$$
Can maybe someone explain it to me?