We have investigated students' qualitative understanding of dc circuits containing resistors and a capacitor. We found that a year after traditional lecture instruction as part of an introductory physics course, most students were unable to predict the behavior of a series circuit consisting of a battery, a bulb, and a capacitor. Among the difficulties identified we found that almost half of the students implicitly abandoned the idea that a complete circuit is necessary for a bulb to light when a capacitor is introduced into the circuit. We have developed curriculum that enables students to construct a phenomenological model in which they liken the behavior of a capacitor to that of a wire, a switch, and a battery; this allows them to qualitatively describe circuits with batteries, bulbs, and capacitors. We have also developed curriculum on the determination of RC times. Post-test results show a significant increase in understanding of capacitive circuits.