The American physicist Robert Jemison Van de Graaff invented the Van de Graaff Generator in 1931. The device that bears his name has ability to produce extremely high voltages - as high as 20 million volts. Van de Graaff invented the Van de Graaff Generator to supply the high energy needed for early particle accelerators. These accelerators were known as atom smashers because they accelerated sub-atomic particles to very high speeds and then "smashed" them into the target atoms. The resulting collisions create other subatomic particles and high-energy radiation such as X-rays. The ability to create these high-energy collisions is the foundation of particle and nuclear physics.

Van de Graaff Generators are described as "constant current" electrostatic devices. When you put a load on a Van de Graaff generator, the current (amperage) remains the same. It's the voltage that varies with the load. In the case of the Van de Graaff generator, as you approach the output terminal (sphere) with a grounded object, the voltage will decrease, but as stated above, the current will remain the same. Conversely, batteries are known as "constant voltage" devices because when you put a load on them, the voltage remains the same. A good example is your car battery. A fully charged car battery will produce about 12.75 volts. If you turn on your headlights and then check your battery voltage, you will see that it remains relatively unchanged (providing your battery is healthy). At the same time, the current will vary with the load. For example, your headlights may require 10 amps, but your windshield wipers may only require 4 amps. Regardless of which one you turn on, the voltage will remain the same.

There are two types of Van de Graaff generators, one that uses a high voltage power supply for charging and one that uses belts and rollers for charging. Here we will discuss the belts and rollers type.

Van de Graaff generators are made up of a motor, two rollers, a belt, two brush assemblies, and an output terminal (usually a metal or aluminum sphere), as shown in the figure:

When the motor is turned on, the lower roller (charger) begins turning the belt. Since the belt is made of rubber and the lower roller is covered in silicon tape, the lower roller begins to build a negative charge and the belt builds a positive charge. By now, you should understand why this charge imbalance occurs, but you may want to look at the Triboelectric Series again. Silicon is more negative than rubber; therefore, the lower roller is capturing electrons from the belt as it passes over the roller. 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:

1. 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, towards the connection at the motor housing.
2. 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 electron-less brush tips while the positive atoms attract to the negatively charged roller.

You should now understand that 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 (over one million volts). 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 actual construction tips. Normally, a neutral material is used for the upper roller; thus the belt becomes neutral after the sphere sucks its excess charge away. Because I used a nylon upper roller (which is positive on the Triboelectric Series), I cause the belt to actually deliver more positive charge and actually 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.

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