When an uncharged object is placed near a charged object its charges rearrange themselves. Those charges attracted to the charged object move towards the charged object and those charges repelled move away. This effect is known as polarization. Charges on a conductor tend to gather at sharp points. A Van de Graaf Generator is plugged into the wall.
Charges flow from the outlet to the bottom of generator.
Strips of metal located there are placed very, very near a rotating rubber strip. The charges from the strips of metal jump onto the rubber strip and are carried up to the top. There, more metal strips are located. The charges from the rubber jump onto the metal strips up top and are conducted to the metal dome at the top of the generator. When the demonstrator places his hand near the generator, his hand becomes polarized.