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012-03760E
Coulomb Balance
1
The PASCO Model ES-9070 Coulomb Balance
(Figure 1) is a delicate torsion balance that can be used to
investigate the force between charged objects. A
conductive sphere is mounted on a rod, counterbalanced,
and suspended from a thin torsion wire. An identical
sphere is mounted on a slide assembly so it can be
positioned at various distances from the suspended sphere.
To perform the experiment, both spheres are charged, and
the sphere on the slide assembly is placed at fixed
Figure 1. Experimenting with the Coulomb Balance
slide assembly
torsion wire
charged spheres
distances from the equilibrium position of the suspended
sphere. The electrostatic force between the spheres causes
the torsion wire to twist. The experimenter then twists the
torsion wire to bring the balance back to its equilibrium
position. The angle through which the torsion wire must be
twisted to reestablish equilibrium is directly proportional
to the electrostatic force between the spheres.
All the variables of the Coulomb relationship
(F = kq
1
q
2
/R
2
) can be varied and measured using the
Coulomb Balance. You can verify the inverse square
relationship and the charge dependence using the balance
and any electrostatic charging source.
However, for best results, we recommend you charge the
spheres with a stable kilovolt power supply to ensure a
reproducible charge throughout the experiment. To
determine the Coulomb constant with reasonable
accuracy, we recommend you use an electrometer and a
Faraday ice pail to accurately measure the charge on the
spheres. For more information about accuracy, read the
section
Tips for Accurate Results
.
Introduction