To demonstrate that all static charge resides on the outside of a conductor proof plane
1. Touch a proof plane on the outside of the Van de Graaff dome and then test with an electroscope to show it has gained charge (Fig. 20.9).
2. Earth the proof plane. 3. Touch a proof plane off the inside of the Van de Graaff dome and test with an electroscope to show absence of charge. This shows that charge resides on the outside of a conductor.
NOTE
A proof plane is an insulated handle with a metal cap used for holding and transferring charge between objects.
FIG. 20.9
Van de Graaff generator
electroscope
To demonstrate that static charge tends to accumulate at a point
1. Touch a pear-shaped conductor on a charged Van de Graaff dome (Fig. 20.10a).
2. Hold the rounded end of the pear-shaped conductor near the cap of the electroscope to show the relative amount of charge present (Fig. 20.10b).
3. Now turn the pear-shaped conductor around and hold the pointed end of the conductor near the cap of the electroscope, to show the greater relative charge present (Fig. 20.10c).
Van de Graaff generator
pear-shaped conductor
electroscope b
a c FIG. 20.10
Forces between charges
As we have seen, charges can attract or repel each other. The magnitude of the force between them can be calculated using Coulomb’s law, and is proportional to:
• the magnitude of each charge • the distance between the charges • the dielectric material – i.e. the medium between the charges.
Coulomb’s law The electrostatic force between two point charges is proportional to the product of their charges and inversely proportional to the square of the distance between them.
