RC Circuits

We will consider a Resistor and Capacitor connected in series with each other and with a switch.

What is the voltage across the resistor (VR = VAB) or the voltage across the capacitor (VC = VBC) as a function of time, after the switch is closed?

How does charge on the capacitor q affect VC?

How does VC affect the current I?

At time t = 0,

As time continues, charge builds on the capacitor, the current decreases, and finally,

This is the "time constant" or "characteristic time" or "relaxation time" of the circuit.

Or, we might begin with a charged capacitor and close the switch at t = 0. Then,

Internal Resistance


Return to Ch 19, DC Circuits

(c) Doug Davis, 2002; all rights reserved