Van Der Graaff Generator

Van Der Graaff Generator 

 

Within the Van der Graaff generator a bottom roller is in contact with a belt(6); as the roller spins this causes the belt to spin. Because the belt and roller are in contact this causes the two materials to become charged (this is called the 'triboelectric effect', where certain materials can become charged through contact with another material). For example the roller could be made of rubber and the belt of teflon. When the roller and belt are made of these materials electrons from the inner surface of the belt move to the roller, leaving the inner surface of the belt with a positive charge and the roller with a negative charge.
 

In the Van der Graaff generator a brush is situated near (not touching) the bottom roller(7). The large negative charge on the roller repels electrons on the sharp points of the brush, leaving the sharp points on the brush with a positive charge. This then creates a large area of attraction between the areas of positive charge on the brush points and the electrons on the roller. Due to the strong electric field atoms in the air gap (between the roller and brush points) can lose electrons and become charged ions . As the ions move in the air gap this can be observed as a 'corona discharge' and seen as a purple hue. The positive ions are attracted strongly to the roller, however the belt is in the way; this means an area of the belt gains positive charge as the positive ions collect on the surface. The belt then moves to the top roller with a net positive charge(4) and repelled electrons within the brush point are then earthed by the power source.
 

At the top of the dome of the Van der Graaff generator is another roller in contact with the belt(3). This roller is made from a different material compared to the first one: for example glass or metal. At this point an opposite triboelectric effect compared to the bottom roller is seen, the roller gains positive charge and the inner surface of the belt gains a negative charge. The negatively charged roller then moves downward(5). The two oppositely charged rollers reinforce the others charging.
 

At the top roller there is another brush point, here the large positive charge of the top roller repels protons at the brush, negative ions are then attracted to roller and move through the air gap(2). However, like with the bottom roller these ions first hit the outside of the belt where the negative charge is then carried down.

In this example negative ions move from the top brush to the belt, leaving the dome with a deficit of negative charge and is therefore positive(1).