Griffiths' electrodynamics discusses Poynting's theorem in the context of moving charges and currents. The movement of charges is essential as it generates currents, which are influenced by the electric and magnetic fields created by other charges. The discussion emphasizes that any charges can move for various reasons, leading to a general proof of the theorem. The current density J is derived from the motion of these charges, yet it is connected to the electric (E) and magnetic (B) fields from different sources through Ampère-Maxwell law. This highlights the interconnectedness of charge movement, current generation, and electromagnetic fields in electrodynamics.
