Van de Graaff Generator

Van de Graaff (With “Hair”)

Van de Graaff generator (without hair)

 

 

 

 

 

 

The Van de Graff Generator produces high static voltage of up to 200,000 V. To demonstrate the static charge developed on the outer conducting shell of the generator, five variations of this demo are offered.

Materials:

The five demos offered include:

Flying Ball (Figure 1): Lightweight metallic pith ball, thread, insulating or plastic wand

Silver Snake (Figure 2): Metallic ribbon, Insulating or plastic wand

Puffed Rice Fireworks (Figure 3): Puffed rice, pie tin, broom and dustpan recommended for cleanup

Hairs (Figure 4): Pom-pom

Plane for Charge Transfer (Not Shown)
Circular proof plane with plastic handle
Electroscope

Demo:

To operate the Van de Graff generator, turn the knob on the base to adjust the belt speed. Set the belt speed to low before turning on the generator, then adjust to the desired speed to create static charge on the outer shell of the conducting dome.

  • Flying Ball
    Charge the Van de Graff generator and turn off after charging for about 5 seconds. Use the plastic wand with the pitch ball hanging by a thread to touch the pitch ball to the dome. The pitch ball will collect some charge and repel from the charged dome. Discharge the charged dome using the grounded discharge wand.
  • Silver Snake
    Charge the Van de Graff generator again. Use the metallic ribbon wand around the dome or touching the dome; it will attract or violently repel away from the dome. Compare its geometry with the geometry of the ball.
  • Puffed Rice Fireworks
    Make sure that the Van de Graff generator is discharged and off. Place a pie tin face down on the dome, and add a small amount of puffed rice on the pie tin. Charge the Van de Graff generator. The cereal will fly off the top of the dome as they collect negative charges and are repelled by the negatively charged dome.
  • “Hairs”
    Make sure that the Van de Graff generator is discharged and off. After placing the plastic pom-pom on the dome, charge the Van de Graff generator. The”Pom-pom” plastic strands spread away from the dome (“hairs standing on end”) when charge is built up.
  • Proof Plane for Charge Transfer (not pictured)
    A circular proof plane with a plastic handle can be used to transfer charges from the Van de Graaff to an electroscope.

Explanation:

 

Figure 5 – The Van de Graff Generator

 

As the rubber belt moves, it rubs against a conducting brush, building up static charges. The conducting brush is connected to the conducting metal dome, and the charges travel to the the most energetically favorable position.

The lowest energy position for the electrons will be to be spread as far apart from one another as possible. The surface area of the outer shell is greater than the surface area of the inner shell, so the charges will build up on the outer shell.

Troubleshooting 

Pom-pom “hair”: Occasionally the strands of the pom-pom are attracted to the conducting shell of the Van de Graaff generator instead of being repelled by it. This is directly contrary to what is supposed to happen in the demonstration. In theory, the charge that is deposited on the conducting shell should spread out and move to the strands of the pom-pom, so they repel  each other and the outer shell as everything becomes similarly charged. When the pom-pom sticks to the conducting shell rather than being repelled, it suggests that the charge of the strands are different from the charge of the conducting shell. The general consensus of the demonstration room staff is that on especially hot or humid days, additional water in the air acts to suck charge off of the strands faster than can be deposited by the rubber belt. This effect would be seen on hot days because hot air can hold more moisture, even if it doesn’t feel humid. This would mean that while the conducting shell remains charged, the strands become neutrally charged and the two attract. Please note this may not be a correct or complete answer to the issue, so if you have any insights please email us!

 

Notes:

  • Avoid any contact with the large dome before discharging it with the small grounded sphere.
  • There are two Van de Graafs – one has a blue base and an attached grounding sphere (use that one). The other is larger and has a black base and needs a separate grounding sphere to be attached (harder to use, kept in Thimann 130)