The van de Graaf generator is used to produce high voltages using static electricity. It consists of a metallic hollow sphere of radius R placed on an insulating material, a belt that is rotated with the help of a motor, and two metallic combs. The bottom metallic comb is connected to the voltage supply of 10 kilovolts, ionizing the air between the comb and belt. Negative charges are grounded via the same comb, and positive charges are moved up on the belt. At the top of the belt, positive charges attract negative charges from the sphere via the metallic comb, inducing a net excess positive charge on the sphere. This process continues, and the metallic sphere acquires a higher positive charge until it causes an electrical breakdown of the air surrounding it. For example, if the radius is 15 centimeters and the voltage produced is 100 kilovolts, then the excess charge on the generator can be calculated using the electric potential expression. Simplifying and substituting the known quantities gives an excess charge of 1.67 micro-coulombs.