The Norwood Science Center. Energy Grade 3

Transcription

1 The Norwood Science Center Energy Grade 3 BACKGROUND INFORMATION: The most widely used electrostatic generator is a device that carries the name of its inventor, Robert J. Van de Graaff ( ). At one point this machine was considered to be on the cutting edge of nuclear physics. Now it is used at every level of education to demonstrate principles of static charge. Household current (120 volts) is supplied from a wall outlet and turns a small electric motor in the base of the generator. This motor, similar in size and appearance to an electric drill motor, turns a felt covered pulley in the base of the machine. Between this pulley and a pulley in the dome runs a flat rubber belt or simply a very large rubber band. As this lower pulley is driven, the rubber belt runs around the felt covered belt. Since these are different materials, electrons are given up to the belt. The electrons are carried on the belt to the upper pulley, which is plastic coated. As the charged rubber belt passes over the upper pulley, it runs very close to a metal screen that picks up the electrons and through a small length of wire which allows the electrons an opportunity to accumulate upon the aluminum dome. Van de Graaff 08 Page 1

2 This aluminum dome is insulated fro m the machine by a tall Plexiglas tube allowing the charge to accumulate. There is only one way for the charge to dissipate. A conducting grounded material can be brought close enough to the dome so the charge can overcome the insulating effects of the air and jump from the dome to the conducting material and run into the ground. The smaller aluminum sphere is attached by a wire to the case of the generator. The charge will leap to the smaller dome from the larger dome and run into the case through the wire. The case is wired into the ground through the wire that brought the electricity into the machine to drive the pulley motor. There is one fact all students should know about electricity. All electricity will flow into the ground if given the chance. On occasion the humidity in the air will be high enough to decrease the insulating properties of the air sufficiently to allow the charge to bleed off into the base or other nearby objects. The first evidence of this type of situation will be a crackling, snapping sound as the electricity discharges into the air. Van de Graaff 08 Page 2

3 The only other way for the charge to dissipate from the large sphere is if an unsuspecting person walks close enough to the generator to allow the charge to discharge through them into the ground. A large spark will jump to the unsuspecting person and flow through them into the ground. While this will not hurt, much, it does not tickle. It will be, electrically speaking, a big surprise! Van de Graaff 08 Page 3

4 TITLE: THE VAN DE GRAAFF GENERATOR PURPOSE: PROCEDURE: Demonstrate basic principles of static charge. Beakman's Hair 01. Set up the Van de Graaff generator for standard operation. 02. Stand on a milk crate. 03. Place one hand on the dome of the Van de Graaff generator. 04. Turn on the switch with the other hand and stand facing the class. 05. Static charge will start to accumulate. You will feel fuzzy. Your clothes will start snapping away or towards your body. You will have massive static cling. 06. If you are fortunate, your hair will start to stand up as the electric charge builds up. The electrons, not able to discharge, are trying to push away from each other and they are taking your hair with them. 07. Turn off the generator and stand down. As soon as you touch the ground the accumulated charge will discharge into the ground. Sometimes the snap of the spark jumping through your shoe into the ground can be heard, like the crumpling of Saran Wrap. The Statue of Liberty 01. Have the teacher assume the same position as when the hair was made to rise. 02. In the free hand hold a cup of Rice Krispies or Styrofoam packing material. As the generator runs, charge will Van de Graaff 08 Page 4

5 accumulate on the cereal and it will fly out of the cup as a result of the repulsive forces. Van de Graaff 08 Page 5