A Van de Graaff can make the flame of the candle flicker. It does this by generating an "electric wind."

A Van de Graaff is a metal ball set on top of a rotating loop of belt. At the bottom of the loop is a comb that is kept positively charged. As the belt moves past the comb, any negative charges leap off the belt onto the comb. The belt continues upwards with a slight positive charge. As it passes the metal ball, any negative charges in the ball are attracted to the belt, leaving the ball positively charged. A few loops of the belt leave an extremely positively charged ball. Now what?

The generator can be used for any number of things, and one of them is the creation of an "electric wind." Put a needle, or any small metallic stick ending in a point, on the side of the ball. This serves to concentrate the charge on a point. The charge can get so great that it actually ionizes the air nearby. Electrons rip off the gas molecules near the needle, and zip towards it. What's left are positively charged gas molecules near a positively charged sphere. Like charges repel each other, and so the gas molecules zip away from the needle, creating wind. The charge itself causes a motion of air.


The most famous demonstration of this electric wind involves placing a lit candle near the needle. The motion of the electric wind will cause the flame to flicker and sometimes go out.

[Via University of Kentucky, The World of Physics, Physics World]


Image via Wesleyan University