To understand electronics, it is necessary to know how to apply basic electric theory. Electronics is a somewhat mathematical subject. The mathematics is easy to understand, because it has practical applications that are easy to see. The basic theory used is called Ohm’s law. Learn Ohm’s law; it applies to the basic theory of electric circuits. All the examples in this unit are dc circuits. Alternating current (ac) circuits are more complex. AC circuits are described in a companion text, Understanding AC Circuits.