3
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See section 7 for further accessories as required for
particular measurements.
6.2 Operation with the "Cobra3" computer interface
In this case, the supply voltage required is supplied via the
"Cobra3" computer interface and connecting cable 12150.07.
The output signal is brought to display by the "Universal
recorder" software module 14504.61.
Acessories required for this mode of operation:
Cobra3 Basic-Unit
12150.00
Cobra3 power supply
12151.99
Data cable 2xSUB-D, 9 pin
14602.00
Cobra3 cable for sensors (9/9)
12150.07
Software Cobra3 Universal recorder
14504.61
The requirements of further accessories are dependent on
the measurement to be carried out, and are described in
section 7.
6.3 Operation with the "Cobra4" computer interface
Acessories required for this mode of operation:
Power supply 12 V AC/500 mA
11074.93
Cobra4 Wireless Manager
12600.00
Cobra4 Wireless-Link
12601-00
Cobra4 Sensor-Unit Electricity,
12644.00
current ±6 A / voltage ±30 V
Connecting cord, 32 A, 250 mm, blue
07360.04
Connecting cord, 32 A, 250 mm, red
07360.01
The requirements of further accessories are dependent on
the measurement to be carried out, and are described in
section 7.
7 MEASUREMENT PROCEDURES
7.1 Measurement of charge
Fig. 2: Experimental set-up for the measurement of a charge that is filled into
a Faraday pail by means of a hollow ball coated with graphite.
The charge to be measured is to be given to a capacitor of
known capacitance, e.g. via a Faraday vessel. The potential
that the capacitor has acquired is measured. The amount of
charge present is to be calculated using the relationship:
U
C
Q
⋅
=
; with
Q
: Charge [Cb];
C
: Capacity
V
Cb
and
U
: Voltage [V]
Schematical experimental set-up
Caution!
Appropriate earthing of the person carrying out the
experiment is necessary here to reduce additional
electrostatic effects. Further to this, the electrometer amplifier
is to be earthed via the available earth connector.
Note:
The capacitance values given on capacitors are nominal
values and may deviate from the actual values. To obtain
exact measurement results, the actual capacitance must be
experimentally determined.
Examples of experiments:
Determination of the capacitance of a spherical capacitor
Apply a voltage of 500 V to charge the hollow ball with
conductive surface. Transfer the charge to a Faraday vessel.
A potential is generated at the 1 nF capacitor. It can be
shown that the charge is transferred in "portions". The
formula for the capacitance "C" of a spherical capacitor can
so be experimentally checked:
r
C
⋅
=
0
4
πε
, with
m
V
Cb
⋅
⋅
=
−
12
0
10
85
,
8
ε
;
r :
spherical radius [m].
Measurement of charge generated by friction
Rub two rods of different materials against each other and
successively dip them into the Faraday vessel. It can be
recognized that the charges generated are opposite. When
only the front parts of the rods are used, it can be shown that
the opposite charges are of the same size.
Recommended accessories:
Faraday pail,
d
= 40 mm,
h
= 75 mm
13027.03
(small vessel)
Crocodile clips, bare, 10 pcs
07274.03
Connecting plugs, set of 2
07278.05
alternatively:
Faraday cup
06231.00
(large vessel)
Plug with 3 sockets, red, pack of 2
07206.01
Connecting plug, 4 mm/19 mm
39170.00
(for short-circuiting)