E.3(1) - VAN DE GRAAFF GENERATOR WITH DISCHARGE

It is a source of high-voltage static electricity, quickly producing up to 250,000 V. The large (about 25 cm in diameter) charge sphere is mounted in a Lucite support column, which allows the students to see the belt-driven charging system in action. The belt runs between two pulleys, the lower one connected to a driven motor. Two metallic combs, one in each end of the belt, complete the charging mechanism. The pulleys are of different materials: the lower one induces negative charges on the belt and the upper one induces positive charges. The negative charges travel up on the belt until the upper metallic comb where the negative charges are transferred to the comb and then to the metallic sphere -- the air between the belt and the comb is ionized. WHen the belt leaves the upper pulley, it carries positivie charges that will be transferred to the lower comb and to the ground.

The electric potential of the large sphere increases until the air's point of breakdown is reached, then it discharges through the air to the other aluminum sphere (20 cm in diameter) mounted on a Lucite rod and grounded through the generator's metal base. Under favorable conditions, it produces up to 250,000 V.

E.3(2) - BELL CHIMES

The static electricity generated by the van de Graaff generator can also be exhibited with the following procedures:

• Arrange a wig over the generator's sphere or ask a volunteer to stand on an insulated platform and touch the generator's sphere -- the person's hair or wig's will stand up into the air
• Place an insulated beaker filled of little styrofoam balls on top of the generator's sphere -- the balls will flow out and up into the air
• A piece of wire is bent in the shape of a ring with a small tip to the side. Place it on top of the generator's sphere and turn on the generator. It will not spark regardless of how close the discharging sphere is! The piece of wire that protudes dissipates the charge before it accumulates on the sphere
• A small cylinder (8 cm tall and 8 cm in diameter) with clear plastic walls and metallic top has a small amount of tiny pieces of light-weight material. When connected to the van de Graaff generator, the material inside it will bounce around rapidly