My colleague and friend Glenn Geher from the New Paltz Psychology department was visiting our new building, and so I showed him around our teaching lab space.    He was especially excited about the Van de Graaff generator, so I gave him a little demo:

 

What’s happening? As I explained at the beginning, the device uses an electric motor to drive a rubber belt which carries electric charges (electrons) off of the metal sphere, leaving behind a net positive electric charge.   Here’s a good diagram from Wikipedia:

Figure 1: Schematic image showing the operation of a Van de Graaff generator.

Eventually, the electric charge on the sphere becomes so great that the electric field it creates is strong enough to break through the nearby air (which is called “di-electric break-down”) and a spark of charge jumps from the small electrode to the sphere.  This is a lot like what happens when a bolt of lightning jumps from the ground to a cloud, or vice-versa.  That’s the clicking sound you hear, even if you can’t see every spark.

Then I put the metal pie plates on top of the metal sphere.   The electric charge now builds up on the pie plates as well.   It’s a fundamental property of electric charges that like charges repel, while opposites attract.   Each pie plate is then, in turn, repelled by the metal sphere and the plates below it, and goes sailing up into the air.

In the video I say that the belt carries electric charges up and onto the sphere, which sounds backwards from what I described above.   But it’s really the same thing.   The belt actually takes electrons from the metal sphere, and electrons have negative electric charge, so this leaves behind extra positive electric charge.   You can just as well imagine that the belt brings extra positive charges up to the metal sphere;  the effect is the same either way.   In fact, being able to tell which kind of charge is actually moving  is a bit of a challenge.   Benjamin Franklin originally came up with the names “positive” and “negative” charge, based on his own idea of which kind was flowing where, and as it turns out he got it backwards.