RC
Time Constant
After
the switch closes in the Figure 12.3, the
voltage across the capacitor begins to rise.
The voltage plotted against time is represented
by an exponential curve where the exponent
is –
/ RC
as shown Figure 23. The product RC
has units of time when R
is in Ohms and C
is in farads. When
= RC,
the voltage reaches 63% of its final value.
The value of
is called time constant.
If the capacitor starts
out with a voltage v
and a switch connects a resistor R
across its terminals, the voltage will begin
to drop following an exponential waveform
also. In one time constant the voltage drops
to 63% of its initial value.
Figure 12.3 Exponential
Charging Curve of a Capacitor.
