Grounding

The grounded sphere in Figure (1) that we used to produce the sparks, provides a good example of the way we use conductors and wires.
Beneath the Van de Graaff generator apparatus we have placed a large sheet of aluminum called a grounding plane that is attached to the metal pipes and the electrical ground in the room. (Whenever we have neglected to use this grounding plane during a demonstration we have regretted it.) We have attached a copper wire from the grounding plane to the “grounded” sphere as shown.
Thus in Figure (1), the grounding plane, the room’s metal pipes and electrical ground wires, and the grounded sphere are all attached to each other via a conductor. Now there can be no electric field inside a conductor, therefore all these objects are at the same electric potential or voltage. (If you have a voltage difference between two points, there must be an electric field between these two points to produce the voltage difference.) It is common practice in working with electricity to define the voltage of the water pipes (or a metal rod stuck deeply into the earth) as zero volts or “ground”. (The ground wires in most home wiring are attached to the water pipes.) Any object that is connected by a wire to the water pipes or electrical ground wire is said to be grounded. The use of the earth as the definition of the zero of electric voltage is much like using the floor of a room as the definition of the zero of the gravitational potential energy of an object. In Figure (1), when the grounded sphere is brought up to the Van de Graaff generator and we get a 2 inch long spark, the spark tells us that the Van de Graaff sphere had been raised to a potential of at least 200,000 volts above ground.
Van de Graaff generators are found primarily in two applications. One is in science museums and lecture demonstration to impress visitors and students. The other is in physics research. Compared to modern accelerators, the 200,000 volts or up to 100 million volts that Van de Graaff generators produce is small.
But the voltages are very stable and can be precisely controlled. As a result the Van de Graaff’s make excellent tools for studying the fine details of the structure of atomic nuclei.

Figure 1: We can discharge the Van de Graaff generator by bringing up a grounded sphere as shown. Since about 100,000 volts are required to make a spark one inch long, we can use the maximum length of sparks to estimate the voltage produced by the Van de Graaff generator.