I am following this online video: http://www.youtube.com/watch?v=fsT69XnjxgU but my Van De Graaff generator is not working. I am using pvc pipes for the body, nylon and electrical tape as two rollers, and rubber band as the belt. On top, I have tried two different cans, one metal sphere but none work. What could be the issue?
A grounded conductor rubs against the dielectric belt, tribocharging it. Charge is moving belt transported to inside the bell where another conductor takes it off into the inner surface of the bell. Charge then migrates to the outside convex surface of the bell. Rinse, repeat.
Are your charging and discharging electrodes on the same side of the untwisted belt? Is it humid? How well is the bell isolated from the world? How much belt area/second is being used, and how well is it being charged? Lab work in a a dirty uncooperative world is learned by doing.
Get a resistance meter and look for insulating path leaks. Help things along by snapping a few 9V batteries in series to pump electrons or holes (which way does your belt tribocharge?) onto the belt at the grounded end. 10 batteries is 90 V. Respect that.