The Concentration of Charge
It is important to realize that the charge on the roller is much more concentrated than the charge on the belt. Because of this concentration of charge, the roller's electric field is much stronger than the belt's at the location of the roller and lower brush assembly. The strong negative charge from the roller now begins to do two things:
- It repels the electrons near the tips of the lower brush assembly. Metals are good conductors because they are basically positive atoms surrounded by easily movable electrons. The brush assembly now has wire tips that are positively charged because the electrons have moved away from the tips, toward the connection at the motor housing.
- It begins to strip nearby air molecules of their electrons. When an atom is stripped of its electrons, it is said to be plasma, the fourth state of matter. So we have free electrons and positively charged atoms of air existing between the roller and the brush. The electrons repel from the roller and attract to the electronless brush tips while the positive atoms attract to the negatively charged roller.
The positively charged atomic nuclei from the air molecules try to move toward the negatively charged roller, but the belt is in the way. So now the belt gets "coated" with the positive charge, which it then carries away from the roller.
As long as there is air between the lower roller and brush assembly, the Van de Graaff generator will continue to charge the belt. Theoretically, the Van de Graaff generator can continue to charge forever. Unfortunately, dirt and other impurities in the surroundings will limit the actual charge that develops on the sphere.
Let's return to the belt. The belt, as we left it, is positively charged and rolling toward the upper roller and upper brush assembly. Since I used nylon for my upper roller, it wants to repel the charge on the belt. The upper brush assembly is connected to the inside of the sphere and hangs near the upper roller and belt location. The electrons in the brush move to the tips of the wires because they are attracted to the positively charged belt. Once the air breaks down as before, the positive atomic nuclei of air are attracted to the brush. At the same time, the free electrons in the air move to the belt. When a charged object touches the inside of a metal container, the container will take all of the charge, leaving the object neutral. The excess charge then shows up on the outside surface of the container. Here, our container is the sphere. It is through this effect that the Van de Graaff generator is able to achieve its huge voltages. For the Van de Graaff generator, the belt is the charged object, delivering a continuous positive charge to the sphere.
One last note before going on to building your own Van de Graaff generator. Normally, a neutral material is used for the upper roller, so the belt becomes neutral after the sphere sucks its excess charge away. Because I've used a nylon upper roller (which is positive on the triboelectric series), I cause the belt to actually deliver more positive charge and become negative. This is a technique used for doubling your current. The belt is positive on one side as it approaches the upper roller and negative on the other side as it approaches the lower roller.