The Van de Graaff Generator
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American physicist Robert J. Van de Graaff began developing the high-voltage electrostatic generator that bears his name around 1930. They started out relatively small and got much bigger; one made in 1933 measured 40 feet high and could generate 5 million volts! (That generator now lives at the Museum of Science in Boston, Massachusetts.) Van de Graaff wanted to provide scientists a way of accelerating particles for atomic research. But his device has become familiar to a much wider audience as a means of demonstrating many of the principles of electrostatics. Generations of students have watched their hair stand on end when putting one hand on the generator. Another memorable display of the generatorís electrostatic action involves creating a large spark between the machine and a nearby object. This tutorial illustrates how a Van de Graaff electrostatic generator operates and how such a spark is produced.
Operating the Tutorial
Click the Turn On button to activate the motor and power the lower of two rollers. The rotating roller causes a belt to circulate between it and a second roller positioned above it. The lower roller and the motor are in a metal box, as is a metal brush located close to the lower roller and connected electrically to the box. A second brush positioned with its teeth facing the top of the upper roller, and enclosed with that roller in a hollow metal sphere, is similarly linked to its encasement. You can hit the Turn Off button at any point in the tutorial to turn off the generator's motor and the corresponding action of the rollers and belt. Notice that the charge on the metal sphere of the generator will remain constant when the motor is off, but if it is turned back on will continue building up electrons until the discharge point is reached (or the motor is halted again).
How the Generator Works
What makes a Van de Graaff generator work is the fact that the two rollers and the belt that circulates between them are made of different materials. This means they are not equally likely to develop a particular charge when in contact with another material (to get fancy about it, you can say they occupy different positions in the triboelectric series). In this example, the lower roller is covered in a material that tends to lose electrons when it comes into contact with another material, while the belt is made of an insulating material and the upper roller is a neutral metal. As the lower roller comes into contact with, and then is pulled apart from, the rotating belt, a charge imbalance arises as electrons from the roller are captured by the belt. The roller develops a positive charge and the belt develops a negative charge.
The beltís charge is increased by the lower brush assembly. The electrons in the metal teeth of the brush are attracted to the strong positive charge of the lower roller, so those electrons concentrate in the tips of the teeth, which are closer to the roller. The electric field at these tips becomes so intense that the electrons in nearby air molecules are torn apart from the positive nuclei with which they normally associate by repulsive and attractive forces. This results in a conductive form of matter known as corona discharge or plasma. Some of the freed electrons in the plasma may then become linked to neutral molecules of air, making the molecules negative, and positive air molecules can capture electrons from the metal teeth.
Together these processes result in a net negative charge of air (ion wind) that emanates from the tips of the teeth. The conductive capacity of the plasma allows the charge to pass through insulating air toward the strongly positively charged lower roller to which it is attracted. Instead of reaching the roller, however, the ion wind comes into contact with the belt, greatly increasing the belt's negative charge.
The negatively charged belt then cycles to the top roller, as shown in the tutorial, and comes close to the second metal brush. Here the events that unfold are opposite of those that occur near the lower brush. The electrons in the metal brush are not affected by the neutral roller but are repelled by the strong negative charge of the belt; positive nuclei concentrate in the tips of the brush teeth; and electrons freed in the plasma that forms are attracted to the tips. The connection between the brush and the inside of the large metallic sphere allows these electrons to be pulled from the tips of the teeth to the surface of the sphere (a phenomenon often called the ice pail effect).
Due to the continuous cycling of the belt between the rollers and brush assemblies, the negative charge along the surface of the sphere is able to increase until the voltage of the generator becomes so high that the sphere attempts to discharge some of its electrons to the ground via a nearby object, such as the grounded discharge rod shown in the tutorial. The leaping of electrons from the first sphere to the grounded rod can be seen as a large spark.
Not all Van de Graaff generators operate in precisely the same way, although the same fundamental principles are at work. For instance, the upper spheres of some machines become positively, instead of negatively, charged, and some operate via a hand-crank mechanism rather than a motor.
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