I'm doing a problem where I have to calculate the electric and magnetic fields. I've found B=0, so S=ExB=0. Has it got any meaning? Then I calculate the energy density e=E^2 + B^2 = E^2. And finally I have to verify the conservation theorem grad(S)+de/dt=0 for points out of the particle's trayectory. I do this part but the conservation theorem is not satisfied. What is the significance of "out of the particle's trayectory"? Or maybe I've got wrong expressions for E and B... Thanks.