Chapter 26
RC Circuits
By connecting a resistor and capacitor together in series, we create anRC circuit. In an RC circuit, energy
is stored in the electric field of the capacitor, and the resistor controls the rate at which charge reaches the
capacitor. The characteristic time scale required to charge the capacitor is called thetime constant , and is
given by
DRC: (26.1)If the resistanceRis in ohms and the capacitanceCis in farads, then the time constant will have units of
seconds.
There are two basic types of RC circuits:charginganddischarging.
26.1 Charging RC Circuit
Figure 26.1 shows a charging RC circuit. The circuit includes a battery, so that when the switchSis closed,
current flows through the resistor and charges the capacitor. As charge builds up on the plates of the capacitor,
it becomes more difficult for the battery to add even more charge to the capacitor, so the current begins to
drop. Once an amount of time has gone by that is large compared to the time constant DRC, the capacitor
will be essentially full charged, and the current will be negligible.
RCSVFigure 26.1: A charging RC circuit. The capacitorCbegins charging once switchSis closed.