•
The resistance introduces the element of
time
in the changing and discharging of a
capacitor.
•
When a capacitor charge or discharge through the resistance, a certain time is
required for the capacitors to charge fully or discharge fully.
•
The voltage across the capacitor cannot change instantaneously because a finite time
is required to move charge from one point to another.
•
The time constant of the series RC circuit determines the rate at which the capacitor
charge or discharge.
The RC time constant is a fixed time interval that equals the product of the resistance
and the capacitance in the series RC circuit.

Capacitors in dc circuits
RC Constant Time

Capacitors in dc circuits
When a capacitor is charged
through a series resistor and
dc source, the charging curve
is exponential.
C
R
I
initial
t
0
(b)
Charging current
V
final
t
0
(a)
Capacitor charging voltage

Capacitors in dc circuits
When a capacitor is discharged
through a resistor, the
discharge curve is also an
exponential. (Note that the
current is negative.)
t
t
I
initial
0
(b)
Discharging current
V
initial
0
(a)
Capacitor discharging voltage
C
R

Capacitors in dc circuits
The same shape curves are
seen if a square wave is
used for the source.
V
S
V
C
V
R
C
R
V
S
What is the shape of the
current curve?
The current has the same shape as
V
R
.