The example studied in the text is generalized into Faraday's law, which states that the EMF around a closed loop equals minus the time rate of change of magnetic flux through the loop. The EMF is the work per unit charge done on a charge particle moving around a closed loop by the non-conservative part of the electric force. The fact that real photons are massless gives us the dispersion relation for electromagnetic waves. The four-potential is in effect the wave function for photons. The electric and magnetic fields in an electromagnetic wave are perpendicular to the wave direction and to each other, but oscillate in phase.
Electromagnetic radiation is created when charged particles accelerate.