ELECTRIC CIRCUITS
An electric current is maintained when the terminals of a voltage source (a battery,
for example) are connected by a conducting pathway, in what’s called a circuit. If
the current always travels in the same direction through the pathway, it’s called a
direct current.
The job of the voltage source is to provide a potential difference called an
electromotive force, or emf, which drives the flow of charge. The emf isn’t really
a force; it’s the work done per unit charge, and it’s measured in volts.
To try to imagine what’s happening in a circuit in which a steady-state current is
maintained, let’s follow one of the charge carriers that’s drifting through the
pathway. (Remember, we’re pretending that the charge carriers are positive, just
like we imagine that test charges in an electric field are positive.) The charge is
introduced by the positive terminal of the battery and enters the wire, where it’s
pushed by the electric field. It encounters resistance, bumping into the relatively
stationary atoms that make up the metal’s lattice and setting them into greater
motion. So the electrical potential energy that the charge had when it left the battery
is turning into heat. By the time the charge reaches the negative terminal, all of its
original electrical potential energy is lost. To keep the current going, the voltage