24.12: Van de Graaff Generator
Van de Graaff generators (or Van de Graaffs) are devices used to demonstrate high voltage due to static electricity that can also be used for research. Robert Van de Graaff first built one in 1931 (based on original suggestions by Lord Kelvin) for use in nuclear physics research.
Van de Graaff uses both smooth and pointed surfaces, conductors, and insulators to generate large static charges and, hence, large voltages. A substantial excess charge can be deposited on the sphere because it moves quickly to the outer surface. The Van de Graaff generators have two metallic combs, one at the bottom and one at the top next to the metallic sphere. The charges are moved from the bottom metallic comb to the upper one via the rotating belt made up of rubber with the help of a motor and two rollers. At the top metallic comb, the charges are transferred to the metallic sphere. This process continues and more charges are transferred to the metallic sphere, increasing its electric potential. Practical limits arise because the large electric fields polarize and eventually ionize surrounding materials, creating free charges that neutralize excess charge or allow it to escape.
Nevertheless, voltages of 15 million volts are well within practical limits. Protons accelerated to such high energies can easily cause nuclear reactions with other protons and various nuclei. Smaller-sized generators are used for practical learning in laboratories and science museums.
If a person touches the Van de Graaff generator, which is carrying high voltage, the hair of a person acquires a net positive charge, and strands of the hair repel each other. The person is insulated from the ground, but since the excess charge on the generator is very small, the current, if any, passing through the person's body will cause no harm.