Van de Graaff Sparks

The Classic!

A Van de Graaff Generator is used to make sparks!

Watch Video: 

• Static charge
• Lightning

 Theory:

The scene is a preschool room: You're playing with the colored blocks and (unbeknownst to you) your buddy is walking up behind you, dragging his sneakers along the carpet. He reaches out and touches your skin, delivering a painful shock. Every since that traumatic day, you've known that friction between two surfaces can cause electric charge to transfer from one to the other.

A similar thing occurs inside a Van de Graaff (VDG) generator. A motor turns a rubber belt, dragging it across a piece of metal. The friction pulls charge onto the belt, which then deposits the charge onto the metal dome. After awhile, the dome accumulates a large amount of positive charge on its surface.

If a grounded conducting rod is brought near the dome, the charge on the bell induces a charge in the rod. The charge on the dome and the charge on the rod are attracted to one another. This attraction causes negatively charged electrons on the rod to move from the rod, through the air, to the dome. As electricity moves through the air, it exites air molecules, causing them to give off light. This is seen as a spark travelling between the rod and the dome.

Figure 1: Spark caused by the generator

Where does the cracking sound of the spark come from? You may remember that air expands when it's heated. The spark in the air makes theair become very hot very fast. The rapidly expanding air creates a shock wave, which is the sound you hear. This same effect occurs during a storm when lighting strikes, heating the air rapidly and producing the thunder that you hear.

Apparatus:

• Van de Graaff generator
• Grounding rod

Procedure:

• Set up the apparatus as shown in the picture to the right.
• Turn on the Van de Graaff with the grounding rod and step back.
• Bring the rod close to the dome of the Van de Graff. Holding it off to one side give the best results.
• Slowly move the rod around to see the little lightening bolts.
• Once you have finished with the demonstration, turn off the Van de Graaff with the grounding rod, and touch the rod to the machine afterwards in order to extract any charge remaining there.

Tips:

In the videos below, some big sparks were created by putting the plastic tube and dome from a smaller Van de Graaff on top of the larger one. It is also interesting to put an electroscope inside the tube.

SAFETY WARNINGS:

• A Van de Graaff can produce a whole whack of charge! Only touch the Van de Graaff with the grounding rod.
• The demo works best on days when the air is dry, and if there is insulation between the machine and the ground.