Demos: 5A-07 Electrostatic Generators
Two electrostatic generators are available for producing large amounts of electric charge.
(a) The smaller, table-model Wimshurst Machine, consisting of counter rotating plates and wire brushes, is capable of producing sparks across a gap up to about 8 cm .
(b) A Van de Graaff generator that may be operated by an electric motor or by hand cranking.
Directions: The smaller Wimshurst Machine can be somewhat erratic, depending on conditions, but in general a good spark can be obtained by making sure that nothing is near the knob arms that might cause a premature sparking to ground. Begin with the knobs a few centimeters apart and start the rotation with the hand crank. As the sparks develop, increase the spacing to achieve a longer spark. Also, show the difference in sparks with and without the Leyden Jars connected. In the latter case, the sparks generated are “denser”, indicating a greater current density and hence more stored energy.
For the Van de Graaff generator, the spark is produced from the dome, which is spherical shaped. The dome should be clean and away from grounding objects. Either by hand or by electric motor, start the belt moving until a good electric field is established. You can draw sparks to yourself without pain by closing your hand into a fist and bring the knuckle toward the generator. The knuckles of the hand contain very few pain sensors and the sensation is only mildly uncomfortable. Or you can use another (grounded) conductor placed nearby to draw the spark.
Suggestions for Presentation: For maximum effectiveness, turn down the room lights so that the sparks are more readily seen. Most of us do not like to get “zapped” by electrostatic sparks, but if you use the “knuckle” approach described above, it really isn’t painful and students will love it (and be impressed). There are many activities associated with the electrostatic generators and these are described separately on the Lecture Demonstration webpages marked 5A “Static Electricity.”