2. Show that $\tilde F_{\mu\nu}F^{\mu\nu}$ can be rewritten as $\partial_\mu K^\mu$.
3. What is the vector potential $A_\mu$ of a static charge at the origin in the axial gauge $A_0=V=0$ ?
4. What is the expression of the energy density and of the momentum density of an electromagnetic field as a function of $\vec E$ and $\vec B$?