A bunch of like charges collect together on the rod and they push each other into the air. A Van der Graaf generator is an electrostatic device that uses a moving non-conductive belt to carry electric charge into a hollow metal sphere. This sphere is insulated from the ground and can accumulate charge until enormous voltages are reached. 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.