Van de Graaff Generator

The Van de Graaff Generator is a safe, low current machine which uses friction to accumulate electrons on the surface of a metal sphere. Willing volunteers can (without getting shocked!) help demonstrate how the buildup of like-charges creates electric repulsion. The Van de Graaff also aids visualization of the electrons’ tendency to travel toward areas of lower electric potential, such as the ground.

Activity/Demo Instructor Notes:

    • Materials
      • Van de Graaff generator with grounding rod
      • Insulating stool or surface to stand on (e.g. plastic or wood; test beforehand)
      • Mirror (so volunteers can see their hair stand up)
      • Pom-pom with long strands (if no volunteer available with long enough hair)
      • Aluminum foil pie plates (for optional extra demo)
    • Giving the demonstration
      • Introduce the generator as a device that creates a buildup of electrons (and briefly explain electrons if necessary for the audience)
  • Emphasize that it is safe because it can only generate a very low level of electric current, but it does deliver shocks
      • At this point you may turn the generator on and ask the audience any questions such as:
        • Where are the electrons building up? (on the metal sphere)
        • How are the electrons being supplied? (by the friction created by the moving belt)
        • How can you get the electrons to leave the sphere? (By giving them a conductive path to the ground)
    • Connect and introduce the grounding rod 
      • Depending on the audience, you may explain how electron movement is governed by electric potential difference, but it suffices to say that the electrons always “want” to get to the ground (where there is zero electric potential).
      • Bring the tip of the grounding rod close to, but not touching the sphere. You may explain that the visible electricity connecting the rod and the sphere through the insulating air indicates that there is a very high potential difference between the rod and the sphere, or more simply that the electrons want to get to the ground so much that they will jump through the air to get to the grounding rod.
      • Turn off the generator and demonstrate that touching the grounding rod to the sphere after turning it off completely discharges the sphere
    • Main Demo
      • Ask for a volunteer with the following criteria:
        • They are willing to receive an electric shock, since there is still a small risk of that occurring even when instructions are followed
        • They have sufficiently long and straight hair to demonstrate  electric repulsion from charge buildup
        • If no such volunteer is available, a pom-pom may be used as a substitute to replicate the effect of electrons building up in hair.
      • Situate the volunteer on the stool/platform/board and (ensuring them that they should not get shocked if they follow your instructions exactly) tell them to place their hand on the metal sphere while it is turned off.
      • Tell your volunteer to keep their hand on the sphere when you power on the generator, and that they will not get shocked. Ask the audience what they think will happen after it is powered on.
      • When it is powered on, show your volunteer a mirror so that they may see their hair stand up, and explain that this happens because the electrons are building up on them now, spreading out to the very ends of their hair and being repelled from each other
      • Ask your volunteer to point with the index finger of their free hand toward the audience (if they are close enough) or towards another volunteer, and ask what they feel. You can then explain how the buildup of charge at the end of the finger creates an electric field which affects the particles in the air, which can be felt.
      • Turn the generator off while your volunteer is still touching the sphere. The charge is still accumulated on them, and you may ask the audience what will happen if the volunteer steps off the insulating surface. (their hair will fall back after the charges go to the ground, and they will not get shocked)
      • Instruct your volunteer to step off, and explain that they are now completely discharged since the electrons can freely go to the ground.
      • Use the grounding rod to discharge the generator once finished.
    • Optional Follow-up Demo
      • Put the stack of pie plates on top of the metal sphere of the generator (currently powered off)
      • Ask the audience what they think will happen when you power the generator back on; they tend to have some good intuitions after seeing the main demo.
      • Power on the generator and explain that the plates fly off of the generator from top to bottom because they are building up charge of their own from touching the sphere, causing them to repel each other and fly off the stack since they are very light.
      • Power off, then discharge generator with grounding rod

Zachary Jerzyk, 2022-23 Wonders of Physics Outreach Fellow