Alternating current (ac) electronics is somewhat more complex than direct current (dc) electronics. AC circuits, like dc circuits, have a source of energy and a load in which power conversion takes place. Most of the electric energy produced in the United States is alternating current; therefore, ac systems are used for many applications. In terms of electronic circuits, three important characteristics are present in ac circuits. These characteristics are resistance, inductance, and capacitance. There are two types of ac voltage: single-phase and three-phase. These are discussed in the units that follow.