Lab 2
Static Electricity

A Van de Graaff Generator creates static electricity by rubbing two dissimilar materials together. A motor inside the generator rubs a Teflon® pulley against a rubber belt. The static charge created by this process is transported by the belt to a large aluminum dome where it is stored. The charge on the dome creates a large potential difference between the dome and the ground terminal at the base of the generator.

The goal today is to observe static electricity with a Van de Graaff generator. The operation of the Van de Graaff generator will be explored. See Figure 1.

Figure 1. A Van de Graaff Generator

Review Theory

Robert Jemison Van de Graaff conceived the idea of the generator that bears his name while working in Oxford in the late 1920’s. The purpose of the Van de Graaff generator is to collect a large amount of charge in order to generate a very high voltage, typically in excess of 150,000 Volts. The generator uses a process that is familiar to everyone. If you rub your shoes across a carpet in dry weather (typically in the winter) your body can acquire a static charge. The spark that occurs when you approach a radiator indicates that a high voltage was generated by the static charge on your body.

The Van de Graaff generator creates a high voltage by storing a static charge on a conducting dome. The charge is created when two different insulators are rubbed together. One insulator is typically made of Teflon® and acts as a pulley. The lower pulley is pressed onto the shaft of an electric motor in the generator. See Figure 2.

Figure 2. The Lower Pulley Assembly

A rubber belt that passes over the teflon pulley is the second insulator. As the shaft of the motor rotates, the surface of the pulley acquires a negative charge and the inner surface of the belt a positive charge. The outer surface of the belt acquires an equal amount of negative charge by a corona discharge at the pointed ends of the lower brush.

The charge on the inner and outer surface of the belt is transported to the upper pulley which is isolated and made of aluminum. Free electrons in the aluminum neutralize some of the positive charge on the inside of the belt. As a result, the upper pulley looses electrons and acquires a net positive charge. See Figure 3.

Figure 3. The Upper Pulley Assembly

The positive charge that remains on the inside of the belt moves to the surface of the lower pulley where it neutralizes some of the negative charge. As the belt moves, the upper pulley acquires more positive charge.

As the positive charge on the upper pulley increases, a faint blue-red glow associated with a corona discharge appears at the ends of the copper wire brush. Electrons from the generator’s dome (that rests on the conductive frame) feed the discharge and are attracted to the positively charged pulley. But before they reach the pulley they are intercepted by the outer surface of the moving belt. Since the dome loses electrons the dome acquires a net positive charge.

As the belt moves, the electrons on its outer surface are transported around the lower pulley and move toward the lower brush. As the electrons pass the lower pulley a corona discharge is created at the pointed ends of the lower brush. Positive ions in the discharge are attracted to the belt where they neutralize some of the electrons on its outer surface. As the belt turns, the process continues and the dome acquires a greater positive charge. See Figure 4.

Figure 4. The Operation of the Generator