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Model No. ES-9080
Basic Electrostatics System
21
Demonstration 3: Capacitance and
Dielectrics
Introduction
The purpose of this series of demonstrations is to investigate the
relationship between charge, voltage and capacitance for a parallel
plate capacitor. Each one of the variables will be held constant in turn,
varying one of the others while measuring the third. The capacitance of
a parallel plate capacitor is given by
, where is the
dielectric coefficient,
A
is the plate area, and
d
is the plate separation.
Various materials can be inserted between the plates to measure the
dielectric coefficient of the materials.
NOTE:
At this point, the students should understand the theory of
capacitors connected in parallel. If not, go to Procedure D of this
demonstration.
For all experiments, the electrometer can be thought of as an infinite
impedance voltmeter in parallel with a capacitor, as shown in Figure
3.1. The capacitor C
E
represents the internal capacitance of the
electrometer, plus the capacitance of the leads.
Whenever you want quantitative measurements of charge, voltage or
capacitance, you need to consider the effect of the internal capacitance
of the electrometer, unless you are certain that the capacitor you are
using has a high enough capacitance to disregard C
E
. The capacitors in
the PASCO RC Network (ES-9053), for example, are high enough that
Equipment Required:
Electrometer (ES-9078)
Faraday Ice Pail (ES-9042A)l
Charge Producers (ES-9057B)
Proof Planes (ES-9057B)
Electrostatic Voltage Source (ES-9077)
Test leads
13 cm Conductive Spheres (2)
(ES-9059B)
Variable Capacitor (Parallel
Plates) (ES-9079)
Capacitor (about 30 pF) (ES-9043)
Sheet of dielectric material
(See Table 3.1 for options)
C
H
A
d
------
=
H
