Electrodynamics of Free- and Bound-Charge Electricity Generators Using Impressed Sources

Abstract

The conversion of external energy into electricity is the foundation of power station and energy harvesting operation. The external source supplies an impressed force per unit charge to free or bound charge to produces AC electricity. We analyze the electrodynamics of ideal electricity generators through a time dependent permanent polarization without any applied electric field, which modifies the constitutive relations and is essential to oscillate charge in a lossless way. For both cases, we show that Maxwell's equations, and in particular Faraday's law are modified, along with the required boundary conditions through the addition of an effective impressed magnetic current boundary source. For the free charge case, we highlight the example of an electromagnetic generator based on Lorentz force, where the impressed force per unit charge that polarizes the conductor comes from mechanical motion of free charge with an impressed velocity of a conductor relative to a stationary DC magnetic field. The bound charge generator is an idealized permanently polarized bar electret with a time dependent permanent polarization, the underlying principle behind piezoelectric nano-generators. In the open circuit state, both electricity generators are equivalent to idealized Hertzian dipoles, with the open circuit voltage equal to the induced emf. Analyzing the short circuit responses, we show that the bound charge electricity generator has a capacitive source impedance. In contrast, we show for the ideal free charge AC electricity generator, the back emf from the inductance of the loop that defines the short circuit, directly cancels the source emf, so the voltage across the inductor is solely determined by the magnetic current boundary source. Thus, we determine the magnetic current boundary source is the topological invariant that best describes the output voltage of an AC generator.