A video of this demonstration is available at this link.

Turn on the Van de Graaff generator. As the dome of the generator charges, the paper strips accumulate charge and begin to repel each other. Important! Always use the grounded wand to turn the generator on and off.

Demonstration 56.45 -- Sparks, describes how a Van de Graaff generator works. Briefly, a motor-driven belt carries charge that has been deposited on it via corona discharge from a row of needles at the bottom of the generator, below the drive pulley, up to a second row of needles at the top, inside the dome, where via corona discharge, the charge is deposited on the dome. The generator can thus charge its dome to very high potentials.

Resting on top of the dome is a cap made of heavy card stock, to which are attached many narrow strips of thin paper (cut from laboratory wipes), to mimic a head of hair. When the generator is running, the dome charges to a high potential, which produces a large electric field around it. Because the paper strips are in this large electric field and close to the dome, they sit at a high potential, and they accumulate charge. This charge is of the same sign as that on the dome of the generator. Since all the paper strips accumulate charge of the same sign, they repel both the dome and each other. This repulsion is strong enough to extend the strips straight out in all directions.