Van de Graaff generator: Nearly as simple as combing your hair
To understand a Van de Graaff generator, you must first understand static electricity. All matter is made up of atoms — tiny objects that contain positively charged particles called protons, and negatively charged particles called electrons. In most materials, the number of protons exactly equals the number of electrons, and the material is uncharged. Some materials, such as hair and nylon, lose electrons very easily. When you run a dry comb through your hair, your hair loses some of its electrons to the comb. As a result, your hairs become positively charged (too few electrons), and the comb becomes negatively charged (too many electrons). This has two effects. First, if you bring the comb close to a metal object, such as a doorknob, you will sometimes see a spark as the extra electrons jump from the comb to the metal — static electricity! Second, your hair may stand on end. Because each hair is positively charged and positive charges repel one another, your hairs repel each other! To get as far away from each other as possible, they stand up.
A Van de Graaff generator is only slightly more complicated than a comb running through hair. Van de Graaff generators have several parts: a motor, a belt, two rollers, two "combs," and a metal sphere. In the generator I built, the bottom roller was made out of nylon, and the upper out of Teflon. As the motor turns, the rubber belt first goes over the nylon roller. One comb pulls electrons from the nylon (which loses electrons easily) and transfers them to the rubber belt. The belt then travels to the Teflon roller. The second comb collects the electrons from the belt and stores them on the metal sphere. The motor turns very fast, so the sphere quickly collects a lot of electrons and becomes negatively charged. Touching a charged sphere is truly a "shocking" experience!