Basic Electrostatics System
ES-9080B
32
®
2.
Set up the circuit shown in Figure 4.1, where the resistor and the capacitor are connected
in series to the voltage source, set at 30 VDC. The electrometer output goes to one of the
analog channels of the
PASCO
interface
. Use a single-pole double-throw switch.
3.
Set up your experiment display in the computer to plot voltage vs. time. (Refer to the steps
listed above for correcting the electrometer readings.)
4.
With the switch open, press start to begin collecting data. Throw the switch to position A
to begin charging the capacitor. Observe the behavior of the voltage on the screen.
5.
When the charge in the capacitor has reached the value of the source (30 VDC), flip the
switch to position B to begin discharging the capacitor. Observe the behavior of the
voltage in the screen.
6.
You may want to try the experiment with different values of R and notice the differences in
charging time.
Analysis:
When a capacitor is charged through a resistor from a DC power supply, the charge on the
capacitor and the voltage across the capacitor increase with time. The voltage,
V
, as a function
of time is given by
V
=
V
0
(1 -
e
t/RC
) where
V
0
is the charging voltage.
After a time
t = RC
(one time constant), the voltage across the capacitor has increased to 63%
its maximum value (
V
= 0.63
V
0
at
t
=
RC
).
1.
Calculate 63% of the voltage of the source. Locate the position in the graph where the
voltage has reached this value. How long a time has passed to reach 63% of the voltage of the
source? This time is
RC
. (Using the Smart Tool in DataStudio makes these measurements
easy!)
2.
Compare the measured time constant from the graph with the calculated from the known
values of
C
and
R
. Now, when a fully charged capacitor is discharged through a resistor,
Figure 4.1: Experimental Setup and Circuit
signal output
red lead
black lead
electrometer
interface
signal
double-throw
switch
