|LECTURE DEMONSTRATION MANUAL | Instructional Research Lab : ucla physics|
|main page||Van de Graaff Experiments|
|table of contents|
E.1.2 Experiments with Van de Graaff Generators
Note: All electrostatic demos work better on cold, dry days.
|a. Charge flows to
the points and sprays off. In this classic demonstration, the professor
or a student volunteer stands on the insulated base and places his/her hand on the sphere
of the generator. An assistant turns the generator on, and the demonstrator's hair stands
on end. The demonstrator should have a key or other pointed object concealed on his person
to hold up and spray off the excess charge when the demonstration is over.
A similar effect can by demonstrated by placing a wig on the sphere, or by connecting the sphere to a paper plume. The electric flier shown below will spin by spraying off charge when connected to the sphere.
|b. Action of a lightning rod. Two Van de Graaffs are provided, one of which charges its sphere positive, and the other negative. When both are turned on, they will spark to each other over 8 -12" distance. However, if a small point is placed on one sphere, aimed in any direction, even at the other sphere, no sparks will jump, because the point dissipates the charge into the air preventing the potential from building up.|
|c. Charge density is Greatest at the areas of highest curvature. When the pear-shaped metal sphere is charged by touching it to the Van de Graaff, a larger charge can be removed from the narrow end than from the fat end. The amount of charge is tested by the deflection of an electroscope. To produce a noticeable effect this demonstration must be done carefully.|
|d. Gauss' Law --charge is on the outside of a conductor. Several demonstrations of these effects are described in E.1.8|
|e. Conductors and non-conductors. A string connected between an electrostatic generator and an electroscope will not conduct charge, but a metal wire will.|
|f. Smoke precipitator. Smoke blown into a tube (from a cigarette) rapidly disappears when the electrodes on the ends of the tube are connected to the generator.|
|g. "Shot from guns" A paper cup full of puffed wheat or small Styrofoam chips placed on top of the generator produces a spectacular effect. Bring your own puffed wheat.|