Basic Electrostatics System
Model No. ES-9080
22
!
C
E
need not be considered. This is not true, however, when using the
Basic Variable Capacitor (ES-9079).
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 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.
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!
"
!
#$%
Figure 3.1: Ideal Schematic of the Electrometer
Capacitance of
object connects
to the
Electrometer
Internal capacitance = 25 pF, without cable