Small Van der Graaf generators are exciting novelties while large ones are used in research and industry. As teh belt passes through teh sphere, it s touched by a metal brush. Electrons leave the belt and flow through a wire to the surrounding sphere. There are lots of like charges on the belt pushing each other away and onto the sphere. The sphere has a build up of negative charge which are repelling the incoming negative charges.
More work is required to place additional negative charges on the sphere.
The amount of charge that can build up on the sphere depends on the sphere s surface. If there are sharp points, charge will build up there and escape into the air as we saw before. Explain why your hair stands up. It stands up because it is negatively charged and all the hairs are pushing themselves away from each other, just like before.