The Van der Graaf generator is a motorized device for charging objects
up to high voltages. It works a lot like scuffing shoes on a carpet on
a dry day, except in this case the rubber soles of the shoes are
replaced by a rubber belt spun by a motor, charge collectors are placed
near the belt, and the whole assembly charges up shiny metal spheres to
maximize the voltage.
The charge on the Van der Graaf generator is used to charge up the
thin aluminum plates. Each plate gets the same sign of charge and
therefore repels the other plates ("opposites attract, like-signs
repel"). The same effect happens if you touch the Van der Graaf
generator yourself. All of your hairs will repel each other, and if
they are long and clean and not tied down, they will all stand on end,
as far away from each other as possible.
I haven't seen exactly your demonstration, but I suspect that the
reason the plates fall to the ground is because the floating plate is
in an unstable field. The lower plates push up on the top plate which
makes it rise, but the top plate likes also to fall because of gravity.
It will slide to the side in order to be far away from its like-charged
partner plates and so it can fall due to gravity. Once the top plate
has slid off, the next plate no longer has the weight of the plate
above it to hold up either electrostatically or mechanically, and it
too can rise. With more volts on the Van der Graaf generator, it may be
possible to get more than one plate up in the air at a time, but one at
a time sounds like a nice experiment.
(published on 10/22/2007)