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The information below is intended to provide a description of the demonstration, an explanation for elementary students, and further explanation for high school students.

Please keep in mind that not all demonstrations are presented at each show.


VAN DE GRAAF GENERATOR

Figure I. Figure II. Figure III.


Figure IV. Figure V. Figure VI.

Description

Equipment: Van de Graaf Generator
Faraday screen -- Mesh wire screen wrapped in a coil
Long nail
Long flourescent light bulb
Diode
Plastic stool

Note: There are many different little demonstrations that can be done using the Van de Graaf Generator. This page will discuss many of these, but it is unlikely that all would ever be shown at a single show, nor is this intended to be an exhaustive list.

Also, electrostatic experiments are best performed when there is very little relative humidity. In fact, when the relative humidity exceeds 50%, this demonstration is usually not performed.

Step 1: The Van de Graaf generator is shown to the audience and how it works is discussed. It is taken apart a little to illustrate how it works. (See Figure I.)

Step 2: The generator is turned on. The demonstrator makes a fist and slowly brings it near the generator. At a distance of about 4 or 5 inches, a spark leaps from the generator to the demonstrator's fist. The sparks can be felt by the demonstrator (they hurt a little bit, but not enough to worry about). (See Figure II.)

Step 3: The demonstrator then puts his arm inside the Faraday screen and brings his arm near the generator. Again a spark leaps towards his arm, but it collides with the screen instead. Even though there are very large holes in the screen, the demonstrator never feels a thing! (See Figure III.)

Step 4: The demonstrator then holds the long nail in his fist. He makes the analogy of a house and a lightning rod. He again brings his hand near the generator, but this time holding the long nail. Whereas sparks leapt to his hand before, with the 'lightning rod' in his hand, no sparks ever appear! (See Figure IV.)

Step 5: Finally, the demonstrator brings the long flourescent light bulb near the generator. The sparks leap from the generator to the light bulb, lighting up the light bulb! (See Figure V.) When the demonstrator puts one end on the generator, the bulb lights up continually.

Step 6: A volunteer is chosen from the audience (preferrably with long, fine hair). He/She stands on a plastic stool and holds the small diode. The volunteer carefully places his/her hand on the generator and the generator is turned on. The volunteer's hair starts to stand up and after a while is standing up a lot! The small diode is continually lit. (See Figure VI.) When the demonstrator brings the long nail near the volunteer's arm, the diode goes out!

Explanation

Basic Ideas: There are two types of charges: positive and negative. Like charges repel and opposite charges attract.

In general, a material is either a conductor or an insulator. A conductor allows electric charge to travel through it easily; an insulator does not.

A person's body acts as a conductor.

When an uncharged object is placed near a charged object its charges rearrange themselves. Those charges attracted to the charged object move towards the charged object and those charges repelled move away. This effect is known as polarization.

Charges on a conductor tend to gather at sharp points.

Step 1: A Van de Graaf Generator is plugged into the wall. Charges flow from the outlet to the bottom of generator. Strips of metal located there are placed very, very near a rotating rubber strip. The charges from the strips of metal jump onto the rubber strip and are carried up to the top. There, more metal strips are located. The charges from the rubber jump onto the metal strips up top and are conducted to the metal dome at the top of the generator.

Step 2: When the demonstrator places his hand near the generator, his hand becomes polarized. The negative charges located in his hand run down his arm and away from the generator. Since the generator is negatively charged and the demonstrator's hand is positively charged, the charges on the generator are attracted to the demonstrator's hand. When his hand is sufficiently close, there is enough of a force of attraction that some charges leap from the generator to the hand. Once this has been accomplished, there is less charge on the generator, so there is a time delay before another spark occurs.

Step 3: When the demonstrator puts the metal cage on his arm, he is protected from the charges. A mesh metal cage such as the Faraday screen generates electric fields such that no charges will travel through it. The exact physics behind this are much too complicated to be discussed here.

Step 4: When the demonstrator holds the lightning rod, the charges accumulate on the tip of the rod. The electric field generated by this tip is much, much stronger than the field generated by the hand alone. The field is so strong in fact, that only a little bit of charge is needed to jump from the generator to the tip. So rather than storing up a lot of charge and releasing it very quickly in a spark like before, the generator slowly and continuously bleeds off its charge to the tip. This saves the demonstrator's hand from any sparks. (It should be noted, however, that the demonstrator is still accumulating charge.)

Step 5: When the light bulb is held near the generator, the same thing occurs as in Step 2. But when the spark jumps to the light bulb this time, the charges travel the length of the bulb to the demonstrator's hand. This current is enough to light the bulb up!

Step 6: When the volunteer is being charged up, some of the charge rushes to his/her hair. Since the hairs contain like charges, they repel. This repulsion is enough to lift some of the hairs off his/her head! Some more of the charges rush down the volunteer's arm and through the diode. This current lights the diode up. When the lightning rod is put in near the volunteer's arm, however, the circuit is shorted: the charges rush to the lightning rod instead of to the diode.

More Specifically


Related Topics

The following physics topics are discussed during this demonstration:

Sponsored by the Physics Department and the Center for Science, Mathematics, and Engineering Education -- University of Virginia