We’ve focused thus far on situations in which charges are in static equilibrium. Now it’s time to explore the controlled motion of charges—currents. Let’s begin with a simple question: How does a capacitor get discharged? FIGURE 27.1a shows a charged capacitor. If, as in FIGURE 27.1b, we connect the two capacitor plates with a metal wire, a conductor, the plates quickly become neutral; that is, the capacitor has been discharged. Charge has somehow moved from one plate to the other.

FIGURE 27.1 A capacitor is discharged by a metal wire.

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In Chapter 22, we defined current as the motion of charges. It would seem that the capacitor is discharged by a current in the connecting wire. Let’s see what else we can observe. FIGURE 27.1c shows that the connecting wire gets warm. If the wire is very thin in places, such as the thin filament in a lightbulb, the wire gets hot enough to glow. The current-carrying wire also deflects a compass needle, an observation we’ll explore further in Chapter 29. For now, we will use “makes the wire warm” and “deflects a compass needle” as indicators that a current is present in a wire.