So far what we have from Chapter 8 is the continuity equation:

\[\nabla \cdot \vec{J} = -\frac{\partial \rho} {\partial t}\]

The basic idea is that if you’re standing there throwing out charges, then the charge density is decreasing where you are. That’s the continuity equation.

(After the 2nd exam we’ll do Poynting’s theorem, but you’re not responsible for that yet.)