Basic Electrostatics System
ES-9080B
22
®
Procedure 3A: Measuring the Electrometer’s Capacitance
Use this procedure to measure a precise value of the capacitance provided by the electrometer
and all cables connected to it. If you are interested in qualitative, rather than quantitative
experiments, this procedure is not necessary.
When a capacitor of known capacitance C is charged by a known voltage V, the charge in it is
given by
Q=CV
.
If the known charged capacitor is connected across the leads of the electrometer, it is
connected in parallel with the internal capacitance of the electrometer,
C
E
. The total
capacitance becomes
C
+
C
E
.
The known capacitor will discharge across the electrometer and a voltage,
V
E
, will be read.
Since the total charge in the system is still just the charge of the known capacitor, we know
that
CV=
(
C
+
C
E
)
V
E
.
1.
Obtain a low leakage (polypropylene, or air dielectric) capacitor of known value, C, around
30 picofarads (pF).
2.
Charge the capacitor with a known voltage
V
, not higher than 100 V (the limit of the
electrometer).
3.
Remove the charged capacitor from the power supply used to charge it, being careful not
to ground it in any way, to avoid removing the charge.
4.
Connect the charged capacitor across the electrometer input leads. Note the voltage V
E
indicated by the electrometer.
5.
Calculate the internal capacitance of the electrometer.
Procedure 3B: Measuring C, V and Q for a Parallel Plate Capacitor
The purpose of the experiments listed in this part is to qualitatively study the relationship
between C, V, and Q for the parallel plate capacitor. Values read by the electrometer are to be
used as relative, comparative measurements. The electrometer can be connected to a
computer and used with a
PASCO
interface to obtain a graphical display of information.
3B.1: V Measured, Q Variable, C Constant
1.
Figure 3.2 below shows the equipment set up. The Basic Variable Capacitor is connected to
the electrometer. The electrometer is grounded to earth (such as the COM port on the
electrostatic voltage source). One of the spheres is connected to the +2000 VDC port on the
C
E
V
V
E
–
V
----------------------
C
=
