Direct current (DC) is electricity flowing in a constant direction, and possessing a voltage with constant polarity. DC is the kind of electricity made by for example by a battery (with definite positive and negative terminals). In the first volume of this book series, DC electric circuits are explored.
DC is not the only "kind" of electricity in use. Certain sources of electricity (most notably, rotary electro-mechanical generators) naturally produce voltages alternating in polarity, reversing positive and negative over time. Either as a voltage switching polarity or as a current switching direction back and forth, this "kind" of electricity is known as Alternating Current (AC). In this volume of the book series, electric circuits are explored where the direction of current switches back and forth.
This third volume of the book series Lessons In Electric Circuits makes a departure from the former two in that the transition between electric circuits and electronic circuits is formally crossed. Electric circuits are connections of conductive wires and other devices whereby the uniform flow of electrons occurs. Electronic circuits add a new dimension to electric circuits in that some means of control is exerted over the flow of electrons by another electrical signal, either a voltage or a current.