Van de Graaff generator: how it works?

The Van de Graaff generator was invented in 1929, by the American engineer Robert Jemison Van de Graaff and can produce millions of volts.

The triboelectric effect

This is the principle behind the Van de Graaff generator. When two materials rub against each other, they become electrified. One with a negative charge (excess of electrons) and other with a positive charge (lack of electrons). Who never rubbed a Bic pen to attract small pieces of paper?

A table of triboelectric series. Any material will lose electrons, when it’s under friction with any other material below it on triboelectric series. Source: (Kim et al., 2017).

The operation

In the below part, an electric motor turns a pulley and a conductive brush is on friction with a belt linked to the pulley, one side of belt becomes positively charged. The brush is connected to the positive pole of a high voltage direct current (DC) power source. However, not all generators have a DC source. Inside the conductive dome, another brush receives positive charges brought by the belt and transfers them to the dome, where they are uniformly spread.

van de graaff generator — The belt and the pulleys must be made of different insulating materials and the insulating material to sustain the dome usually is made of acrylic or PVC. Source: BC Campus Open ED.

The ion source serves to create a beam of positive particles for experiments and research. The positive charges on the dome create an electric field, ionizing the air, creating the corona effect. More detailed explanation will be in another post.

van de graaff generator discharge — To generate electric discharge, the generators come with a conductor sphere linked to the ground and with an insulating cable for a person to hold it and don’t be shocked. The discharges are caused by a potential difference between the dome and the sphere. Source: clipartkey.

When somebody touches the generator dome above an insulating material, avoiding direct contact with ground, the person has the same positive charge as the generator. As equal signal charges repel each other, electrified strands of hair from the same signal repel each other and get ruffled.

Van de Graaff generator and nuclear research

The particle accelerators need high voltages to accel electric charged particles, to collide with a sample, producing nuclear reactions. Nowadays, Pelletron accelerators are used to speed up electrified particles.