1. Theoretical Background

This chapter is the introduction of the book. Induction and direct resistance heating are presented in an unified theory that, starting from the solution of Maxwell’s equations, gives the basic quantities that characterizes thermal processes based on internal heating sources. The penetration depth of electromagnetic waves is a parameter that characterizes all the relevant phenomena in these heating applications. A qualitative description is provided about the several effects that affect distribution of current density and, as a consequence, of heating sources: proximity effect (that occurs between two conductors that carry electrical current), ring effect (that occur in bended conductors), slot effect (that occurs in conductor placed in the slot of a magnetic yoke), end and edge effects (that occur due to the finite length of inductor and load and their relative position). Active and reactive power in the workpiece are computed by applying Poynting’s theorem for both induction and direct resistance heating.