3.9.1
Charge Cycle
A voltage divider containing resistance and capacitance is connected in a circuit by
means of a switch, as shown at the top of figure 3-9. Such a series arrangement is called
an RC series circuit.
In explaining the charge and discharge cycles of an RC series circuit, the time interval
from time t
0
(time zero, when the switch is first closed) to time t
1
(time one, when the
capacitor reaches full charge or discharge potential) will be used. (Note that switches S1
and S2 move at the same time and can never both be closed at the same time.)

NEETS MODULE 2-Alternating Current and Transformers
UNCLASSIFIED
3-17
UNCLASSIFIED
Figure 3-9 Charge of an RC series circuit

NEETS MODULE 2-Alternating Current and Transformers
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3-18
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When switch S1 of the circuit in figure 3-9 is closed at t
0
, the source voltage (E
S
) is
instantly felt across the entire circuit. Graph (A) of the figure shows an instantaneous rise
at time t
0
from zero to source voltage (E
S
= 6 volts). The total voltage can be measured
across the circuit between points 1 and 2. Now look at graph (B) which represents the
charging current in the capacitor (i
c
). At time t
0
, charging current is MAXIMUM. As
time elapses toward time t1, there is a continuous decrease in current flowing into the
capacitor. The decreasing flow is caused by the voltage buildup across the capacitor. At
time t
1
, current flowing in the capacitor stops. At this time, the capacitor has reached full
charge and has stored maximum energy in its electrostatic field. Graph (C) represents the
voltage drop (e) across the resistor (R). The value of e
r
is determined by the amount of
current flowing through the resistor on its way to the capacitor. At time t
0
the current
flowing to the capacitor is maximum. Thus, the voltage drop across the resistor is
maximum (E = IR). As time progresses toward time t
1
, the current flowing to the
capacitor steadily decreases and causes the voltage developed across the resistor (R) to
steadily decrease. When time t
1
is reached, current flowing to the capacitor is stopped
and the voltage developed across the resistor has decreased to zero.
You should remember that capacitance opposes a change in voltage. This is shown by
comparing graph (A) to graph (D). In graph (A) the voltage changed instantly from 0
volts to 6 volts across the circuit while the voltage developed across the capacitor in
graph (D) took the entire time interval from time t
0
to time t
1
to reach 6 volts. The reason
for this is that in the first instant at time t
0
, maximum current flows through R and the
entire circuit voltage is dropped across the resistor. The voltage impressed across the
capacitor at t
0
is zero volts. As time progresses toward t
1
, the decreasing current causes
progressively less voltage to be dropped across the resistor (R), and more voltage builds
up across the capacitor (C). At time t
1
, the voltage felt across the capacitor is equal to the
source voltage (6 volts), and the voltage dropped across the resistor (R) is equal to zero.