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Van de Graaff Generator

The Van de Graaff generator is a popular tool for teaching the principles of electrostatics. You might remember it as the thing that made your hair stand on end. It’s now largely used for educational purposes, but it was invented by Robert J. Van de Graaff in 1930 to power early particle accelerators.

The Van de Graaf generator creates a buildup of static electricity around a metal sphere. Electric charge in the form of electrons builds until the voltage is so high that air molecules can be ionized and a spark discharge can take place to a nearby object. In this tutorial the object is a grounded rod.

Anything or anyone in direct contact with the Van der Graaf generator’s sphere, becomes charged. If a person holds their hand on the sphere, they become charged and their hair may stand on end since like charges repel and the hairs are pushed away from the like-charged metal sphere and each other.

Instructions

  1. Find the key elements of the generator labeled below.
  2. Notice the direction the electrons are traveling. See how they move up from the bottom roller and collect around the sphere.
  3. Watch the generator send a spark to the grounded rod when the sphere can’t hold any more electrons.
  4. Turn the generator off. Observe how the number of electrons remains fixed.

Inside the generator are two rollers connected by a belt. The motor turns the bottom roller, thus rotating the belt and upper roller. A metal brush drags across the bottom roller, and static electricity begins to build.

The rollers are always made from different materials. The bottom roller is constructed from material that holds a negative charge, such as silicon. The belt is made of a neutral material like rubber. The top roller is made from a material that holds a positive charge, like aluminum, and is surrounded by a hollow metal sphere. The difference in charge draws the electrons up the belt. When the electrons reach the upper belt, they meet another metal brush which transports them to the sphere.

The electrons gather around the surface of the sphere until it reaches a critical potential. The metal sphere must release some of the electrons. With a quick zap, the Van de Graaff generator releases a spark of electrons towards the metal of the discharge rod. As long as the generator is switched on the cycle will repeat. When the generator is turned off, and the stream of new electrons stops, the electrons around the sphere can stay put.