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Van De Graaff Generator Wonders

Most people have seen a Van de Graaff generator before at a science centre or on TV. You know that it makes peoples' hair stand on end, but do you actually know how it works?

Van de Graaff experiments are all based on the fact that like charges repel.

A Van de Graaff generator pulls electrons from the Earth, moves them along a belt and stores them on the large sphere. These electrons repel each other and try to get as far away from each other as possible, spreading out on the surface of the sphere. The Earth has lots of room for electrons to spread out upon, so electrons will take any available path back to the ground.

The grounding rod is a smaller sphere, attached by a wire to the Earth. It provides a convenient path for electrons to move to the ground. If we bring the grounding rod close enough to the large sphere, the electrons rip through the air molecules in order to jump onto the grounding rod, creating a spark and crackling noise.

When a fluorescent light tube approaches the negatively charged generator, the electrons on the generator flow through the tube and the person holding it. Flowing electrons result in an electrical current, lighting up the light tube. It doesn't take very much current to light a fluorescent bulb!

Putting Styrofoam peanuts or confetti on top of the Van de Graaff generator can create a cool trick. The electrons that collect on the sphere spread out into the Styrofoam peanuts and confetti, making the little, light objects negatively charged. When the negative charges on the peanuts repel the negative charges on the generator, the peanuts push off the sphere.

When a student puts a hand on the sphere, the electrons will spread out onto that person as they repel from the other electrons. They are most obvious in a person's hair because the like charges of the electrons repel each other and cause the hairs to stand up and spread away from each other. As long as the person is standing on an insulated platform, the electrons will not be able to travel down to the ground and their hair will remain standing up.

Objectives

  • Explain how static charge causes materials to attract or repel each other.

Materials

  • Per Class or Group:
    A Van de Graaff Generator (available at Arbor Scientific)
    a plastic stool
    Styrofoam peanuts (or confetti)
    metal pie pan
    a mirror

Key Questions

  • Where are the electrons?
  • What is making your hair stand on end?
  • Why doesn’t the hair come down after the machine has been turned off?
  • What caused a shock when the volunteer touched a fellow student?
  • Why did your teacher ground the generator before allowing the volunteer to step off the stool?
  • What is the role of the plastic stool?

What To Do

Safety note: Make sure you ground the large sphere after each use by touching it with the ground wire or small sphere. Although the Van de Graaff generator produces a very low current, it may cause problems with people who have heart problems or a pacemaker. Warn students they may get small shocks which will scare them more than hurt them.

Part 1: Making Sparks

  1. Touch the small sphere (connected to the ground wire) to the dome.
  2. Turn the knob counter clockwise.
  3. Turn the generator on.
  4. Slowly turn the knob clockwise so the motor turns the belt.
  5. Take the small sphere away and let a charge accumulate on the dome. Ask a student to turn off the lights to make it easier to see the sparks.
  6. Move the small sphere around the sphere in different positions so that everyone can see the sparks.

Part 2: Sautéing Styrofoam 

  1. Ground the dome by touching the grounding rod to it.
  2. Without removing the grounding rod, place Styrofoam peanuts (or confetti) on top of the large sphere.
  3. Take the ground away, and the Styrofoam peanuts will fly off the generator.
  4. This can be repeated by placing a metal pie panplate (or three!) on top of the generator and repeating the steps above.

Part 3: Hair-Raising Experience

  1. Ask a student to step up onto the insulated stool.
  2. Ground the dome by touching the grounding rod to it.
  3. Without removing the grounding rod, ask the volunteer to put one hand on the dome, the other hand by their side and make sure they understand not to move their hands until you tell them to.
  4. Take the ground away, and their hair will start to stand up. Shaking their head will help too!
  5. Hold the mirror so that the volunteer can see their new hair-do!
  6. Ask the volunteer to move their hand from the ball to their side, and to keep it there. Immediately ground the Van de Graaffand then turn it off.
  7. The volunteer can simply step off the stool or touch elbows with a classmate to get rid of their extra electrons (note: touching elbows will result in a shock!).

Extensions

  • Place a piece of fake fur on the large sphere, the individual fur strands will stand.
  • Tape streamers to your volunteer, like an extra-long moustache!
  • Have someone hold onto the large sphere while blowing soap bubbles with a wand, the bubbles will become positively charged and will be attracted to anything that is grounded e.g. a person walking by.
  • A fly stick is a miniature, battery powered Van de Graaff generator. It charges mylar objects, which are then repelled by the stick (and by each other). You can make small objects hop up and down between the stick and your hand or levitate the more visible ones. For fun ideas, check out the Educational Innovations' teacher blog.