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
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.