In the previous chapter, we looked at the movement of charges, showing that a net charge creates an electric field with differences in electric potential energy at different points in the field. When two points in a field with a potential difference are connected by a conducting material, electrons will flow spontaneously from one point to another. For instance, when the two terminals of a battery (a source of potential difference) are connected by a copper wire (a conducting material), electrons flow spontaneously from the negative terminal of the battery toward the positive terminal. This mass flow of electrons in a particular direction creates a current, which is the source of the circuits that we will examine in this chapter.

As fans of hard rock know, there are two kinds of circuits, AC and DC. AC stands for alternating current: an electromagnetic generator induces a current that alternates in direction. AC circuits can be quite complicated, so you’ll be relieved to know this is the last you’ll hear of them: they don’t appear on SAT II Physics. However, you should expect a good number of questions on DC, or direct current, circuits. These are the more familiar circuits, where a current flows steadily in a single direction.