Model of ions in motion

Demonstration

A visual model showing that particles with one charge move in one direction, while those with opposite charge move in the reverse direction.

Apparatus and Materials

• Cylinder and metallised spheres

• Van de Graaff generator

Health & Safety and Technical Notes

Read this comprehensive safety note:

• Van de Graaff generator safety

• Read our standard health & safety guidance

The cylinder is made of clear insulating plastic (e.g. Perspex), with metal top and ends. One end is equipped to plug into the dome of the Van de Graaff generator. Inside are three or four metallized polystyrene spheres.

Procedure

1. Plug the cylinder into the top of the Van de Graaff generator. Turn on the generator. The spheres, initially lying on the metal base, will become charged and rise up the cylinder until they are suspended between the two parallel plates formed by the two caps.
2. Put your finger on the top metal cap and keep it there. The spheres will be set in motion as they carry charge back and forth. (Resting the other hand on a wooden bench to ensure you do not become charged up.)

Teaching Notes

• The balls collect charge from the top of the dome and are repelled by it. Their weight is then balanced by the upward repulsion.
• When the upper plate is earthed, the balls are attracted to it; they give up their charge, so that a tiny current flows through the teacher. The balls then fall back down, to become charged again.

This experiment was safety-tested in April 2006

• A video showing how to use a Van de Graaff generator: