Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Van der Graaf generator and pie plates

Learn more physics!

Browse our 6756 answers by or search term

Q:
Hi,in my last science class we were using a Van de graaf generator.One of the experiments we did was,we stacked some aluminum pie plates on top of one an other and turned on the Van De Graaf generator.The pie plates all roase into the air one at a time and fell to the ground.Why would this happen??
- Emily (age 14)
kincardine,ontario,canada
A:
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.

Tom

(republished on 08/02/06)

Follow-up on this answer.