The Van De Graaff Generator

A Fascinating Thing from the World of Electricity: The Van De Graaff Generator

The picture shows a simplified construction of a Van de Graaff generator. By rubbing, charges can be separated, and thus objects can be charged. A roller on the lower right side rubs against a belt. As a result, the belt becomes positively charged and the roller becomes negatively charged. Next, the positive charges are transported upwards together with the belt. There, a hollow metal sphere is situated. Via a comb, the positive charges are transported to the outside of the metal sphere. Since the belt is constantly charged and new charges are transported onto the surface of the sphere all the time, very high voltage is generated there.

If you touch the sphere of the generator during this process, the electrical charges causes your hair to stand up.

Most schools have such a generator. Ask your physics teacher, maybe he/she can make your hair stand on end!

You have to be very careful with this experiment, though. If you touch the sphere during the entire process, the charges continuously flow off through your body, and not too high a voltage can be generated. If you put the generator into operation without touching it with your hand, however, more and more charges are transported to the sphere, but no charges can flow off. A high voltage is generated, which may cause an electrical shock.

In this case, you can try a different experiment: Place a grounded, uncharged metal sphere close to the heavily charged sphere of the generator, and you will be able to observe a jump of a spark. Depending on the humidity of the air, this jump may come up to 3000 V/mm. Thus, you can estimate the voltage that was generated by the Van de Graaff generator. For example, if you can still observe a jump of a spark over a distance of 10 cm, this would mean a voltage in the range of 300 kV.