Basic Electrostatics System
Model No. ES-9080
30
!
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
ScienceWorkshop
interface as an analog sensor.
1.
Open the software program and select the electrometer from the list of
sensors.
Equipment Required:
Electrometer (ES-9078)
Faraday Ice Pail (ES-9042A)
Power Amplifier (CI-6552A)
Test leads
Capacitors, 200-400uF
Resistors, (10-90k ; 10-1000 )
Computer with
ScienceWorkshop
®
interface
DataStudio
®
or
ScienceWorkshop
software
:
:
W
RC
=
:
:
