
Passive components 13
discharged to e' =
1/2.718
28 = 0.37 V approxi-
mately.
We have seen that in a circuit with a direct
current (DC) source such as a battery, and con-
taining a capacitor, only a transient charging
current flows; this ceases entirely when the capaci-
tor is fully charged. So current cannot flow con-
tinuously in one direction through a capacitor. But
if we connect first a positive supply to the capaci-
tor, then a negative one, and so on alternately,
charging current will always be flowing one way or
the other. Thus an alternating voltage will cause an
alternating current apparently to flow through a
capacitor (see Figure 1.7) ...