®
ES-9080B
012-07227G
Basic Electrostatics System
31
Demonstration 4: Charging and Discharging Capacitors
Introduction
The purpose of this demonstration is to investigate how the voltages across a capacitor and a
resistor vary as the capacitor charges and discharges, and to find the capacitive time constant.
When a capacitor is connected to a DC power supply, charge builds up in the capacitor and
the potential difference across the capacitor increases until it equals the voltage of the source.
Both the charging and the discharging of a capacitor are characterized by a quantity called the
time constant, which is the product of the capacitance,
C
and the resistance
R
. That is,
.
Use capacitor values in the recommended range, so that the internal capacitance of the
electrometer needs not be considered. You can adjust the resistance value for a convenient
RC
constant.
There are two variations of the same activity presented here. The first uses a direct DC
voltage source
and results are obtained in a voltage vs. time graph. Use higher resistances
(10-90 k
for this method. The second uses a signal generator with a square wave to charge
and discharge the capacitor. Use lower resistances (100-1000
for this method.
Equipment Setup
The signal output cable of the electrometer allows it to be connected to a
PASCO
interface as
an analog sensor.
1.
Open the software program and select the electrometer from the list of sensors.
Equipment Required:
Electrometer (ES-9078A)
Faraday Ice Pail (ES-9042A)
Power Amplifier (CI-6552A)
Signal Input cable (Test leads)
Capacitors, 200-400
F
Resistors, (10-90 k
; 10-1000
)
Computer with
PASCO
interface
DataStudio
®
software
RC
=
