Introduction – PASCO ES-9070 COULOMB BALANCE User Manual

Page 5

012-03760E

Coulomb Balance

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

) 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