Here’s something that has always bothered me in my physics lectures. Suppose I want to calculate the work done by electromagnetic forces onto one charge
$$ \begin{equation} dW = \vec{F} \cdot d\vec{s} = Q \vec{E}\cdot\vec{v} dt \end{equation} $$
Now if I want to expand this idea to a continuous charge distributions $\rho$ my textbook teels me to do the following using $Q =\rho dV$ and $\vec{J}=\rho \vec{v}$.
$$ \begin{equation} dW = \rho dV \vec{E}\cdot \frac{1}{\rho} \vec{J} dt = \vec{E}\cdot\vec{J} dV dt \end{equation} $$
Now up until here I am thinking about the values $dW, dV, dt$ as finite differences. Now making the switch to Infinitesimal values is what causes problems for me. In my textbook the next line simply states $$ \begin{equation} \frac{dW}{dt} = \int_{\Omega}^{} \vec{E}\cdot\vec{J} \,dV ' \end{equation} $$
Now I have a couple of ussues here: Where do I suddenly pull out the integral? Why can i just divide by $dt$ as if it's a regular variable?