Hints for Using the Van de Graaff Generator

Any good physics text or website will have a good summary of how the Van de Graaff generators works. This website is excellent: http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

The motor in the Van de Graaff base plugs into an normal AC outlet. Once turned on, the motor scrapes then deposits charged particles onto the tan-colored rubber belt, which runs around a roller to the upper metal dome. Charged particles are deposited inside the dome but immediately move to the outer surface of the dome. Since these particles have the similar charges, they repel from one another as much as possible and evenly distribute themselves around the surface. Any object—finger, meter stick, elbow, hair, tongue, balloon, pith ball, etc.—that comes near enough to the dome will respond to these extra charged particles.

Preliminary cautions:
1. The Van de Graaff can produce a spark that is simply irritating or quite dangerous. A spark several centimeters long can be the equivalent of 100 000 volts or more. The size of the sparks and their voltage will change with the speed of the motor, ambient room temperature and humidity (fewer sparks on humid days, nasty ones during dry winter days). The “good thing” is that this spark is of short time duration, so it may be painful but not health-threatening. The “bad thing” is that some students and teachers are very sensitive to shocks, either for emotional reasons or actual physical reasons (what about the teacher or student with a heart condition or pacemaker??)
2. So, before we even turn on the generator, I alert students to some health hazards (good time for discussion of difference between current and voltage; what’s a “tickle” and what’s “fatal”; what is lightning; what to do during a thunderstorm or when lightning is present; etc.) I always offer the option that students who don’t want to be “tickled” should stay more than arm’s length away from colleagues, and others must respect the wishes of those who don’t want to be shocked.
3. We also post and discuss how to telephone for emergency help (nurse, administrators). We review who does and who doesn’t know CPR.
4. We also discuss how to use the baseball-size wooden bat that will be used if someone truly has come in contact with large, live current. We discuss possible medical results of electrocution.
5. The Van de Graaff should have an attached grounding wire or wand (discharge electrode). . This provides the teacher with a way to discharge the dome quickly and relatively safely. I would NOT use a generator that does not have a grounding wire or wand. There are professionally made discharge electrodes. I also have a homemade grounding rod made from a ball of aluminum foil, wooden meter stick, some electrical tape, and a 30 cm long piece of insulated wire with an alligator clip on one end and a spade connector on the other. Use the alligator clip to attach to the motor base. Pinch the spade end into the aluminum foil ball.
6. Finally, I recommend you NEVER allow students to operate the generator. I believe the teacher should maintain control of the on/off switch. If I must step away from the generator, I always unplug the motor. Nuff said.

Suggested activites: Always try these with YOUR generator and willing volunteers before you try these in class.

1. Blow up two balloons, tie closed, tie balloon to balloon with light string. Suspend the balloon assembly from a wooden meter stick (don’t use one with a metal cap end) and bring balloons close to the dome. Observe and explain.
2. Old-time pith balls may also be used.
3. Set up an electroscope near the generator. Observe and explain.
4. Use masking tape to attach a cheerleader pom pom to the top of the dome (the tape helps keep the pom pom from wiggling loose.)
5. Turn off the generator. Ground it. Ask for a volunteer with dry hair (without a lot of styling product) to stand on the wooden chair and place his/her hand on the dome. Start the motor slowly and allow charges to build up on the victim’s hair. STOP immediately if he/she complains of any pain. When you turn off the generator, have volunteer lift his/her hand and stop down and away from the chair before you ground the dome again, or he/she may receive a shock. Sometimes the volunteer carries enough charge on the way back to his/her seat that he/she can tickle friends on the way.
6. Place two or three Styrofoam plates on top of the dome (tape bottom one to dome to avoid wiggling loose.) Repeat with three or four aluminum pie plates. Compare results and explain.
7. Place some paper punch bits in a paper cup, a metal cup (like the inside cup from a calorimeter), and a Styrofoam cup. Place each cup, one at a time, onto the top of the dome (use masking tape again). Compare results and explain. [Students love the mess this one makes.]
8. Finally, for the most brave of experimenters: Put on a pair of thick gloves. Get a functional fluorescent light bulb. Hold the tube along the glass in its middle. Observe. Change the position of your gloved hand. Observe. Be very cautious here—if you are shocked, don’t drop the bulb.
9. The “chain of shocks” is a popular activity. Again, have anyone who does NOT want to be shocked step far away from the group. One volunteer stands on the wooden chair, hand on top of the dome. His/her free hand is held by the next student, and so on students hold hands down the line. Let charge build up on the group then discharge the generator. The shock will be felt down the entire line of students.

CAUTION, CAUTION, CAUTION. Be alert, be aware, be safe.

Van de Graaff generator and discharge electrode balloons on a meter stick
[pic] [pic]

supplies home-made discharge electrode
[pic] [pic]

Debra Beck
Gregg Beck
6/2009