where the voltage across the resistor is proportional to the current through it, and the voltage across the capacitor is

proportional to the capacitive reactance, XC.

The instant the switch is closed, in figure 1-12, the voltage across the capacitor is zero and the voltage across the

resistor is EA. After a period of time, the transient action will be complete, the capacitor voltage will be EA, and the

resistor voltage will be zero. Figure 1-13 shows the relative values of resistor voltage and capacitor voltage as time

elapses.

the value of R determines the rate of electron displacement. Therefore, the time from T0 to T1, in figure 1-13, is

determined by the value of R and C in the circuit. The time constant of the RC circuits is the length of time required

for the capacitor to charge to 63.2 percent of the applied voltage. In other words, at T1 the capacitor charge has

reached 63.2 percent of its final charge.