General Physics II Lab
EM2 Capacitance and Electrostatic energy
General Physics II Lab
EM2 Capacitance and Electrostatic Energy
Purpose
In this experiment, you will examine the relationship between charge, voltage and
capacitance of a parallel plate capacitor.
Equipment and components
Variable capacitor, DC power supply, electrometer with a cable and battery box, switch box,
Faraday ice pail, proof plate, charging probe, capacitor (15 pF), ruler, aluminium sphere,
grounding wristband,.cables: BNC to spades (unshielded), BNC to crocodile clips (shielded),
leads: 4 mm plug to spade, 4 mm plug to plug (for earth grounding).
Background
Electrostatic energy associated with an electric field can be stored in a capacitor. The storage
of such energy requires that one has to do work to move charges from one plate in the
capacitor to the other. The charge,
Q
, on the plates and the voltage,
V
, between the plates are
related according to the equation
QC
V
=
,
(1)
where
C
is the capacitance which depends upon the geometry and dimensions of the
capacitor. For a parallel plate capacitor with plate area
A
and separation
d
, its capacitance is
A
C
d
ε
=
,
(2)
where
ε
is the permittivity of the medium between the two plates. The permittivity of air is
approximately equal to that of vacuum,
0
≈
. The amount of the energy stored in a
capacitor is given by
22
V
U
C
==
(3)
In this experiment, you will carry out measurements on a parallel-plate capacitor to verify the
above equations.
Procedure
Measurement 1: Capacitance of the electrometer and cable
It should be noted that whenever you make measurements of charge, voltage or capacitance,
you need to consider the effect of the internal capacitance of the electrometer and that of the
cables connected to it. As shown in Fig. 1, the electrometer can be thought of as an infinite
impedance voltmeter in parallel with a capacitor. The capacitor C
1
represents the internal
capacitance of the electrometer, plus the capacitance of the leads. The capacitance of the
electrometer and cable C
1
adds to the external capacitor which is connected
in parallel
.
Therefore, it is necessary to know the capacitance of the electrometer and cable in order to
have an accurate measurement of the capacitance of an external capacitor.
Revised:
24 February 2010
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