The Van de Graaff generator is an artifact that works thanks to electrostatic phenomena, and whose function is to reproduce enormous electrical potentials, in the order of megaelectronvolts (MeV), to accelerate subatomic particles. Such potentials are concentrated in its upper parts, where characteristic hollow metallic spheres rest.
It was invented in 1929 by the American physicist Robert J. Van de Graaff, building models of different sizes and electrical capacities. One of the largest, created in 1933 and seen in the image below, is capable of reaching an electrical potential of 5MeV; five times less than what is currently achievable (25.5MeV).
The potential of the Van de Graaff generator is so high that electric discharges occur in the air surrounding its metallic spheres. These discharges are the product of the unbalance of the electric charges, since the spheres acquire very negative or very positive electric charges; all depending on the materials and their designs.
This device is quite popular in the teaching of physics and electricity. This is because the volunteers, when touching the spheres or the metal domes of the small generators, experience an involuntary lifting of their hair, which is reminiscent of an electrocution.
Van de Graaff generator parts
In the image above we have the conventional parts for a Van de Graaff generator. It has a vertical frame topped by a hollow sphere or metal dome (1). Inside, we have a band or belt (4 and 5) made of polymeric and insulating material, such as the surgical tube.
This belt constantly moves between two rollers: an upper one (3), and a lower one (6). Likewise, each roller has attached a metal brush (2 and 7) that rubs the surface of the belt. The movement of the belt is activated by an electric motor connected to the base of the generator.
As can be seen in the image, the sphere of the generator is positively charged (+). Therefore, it needs electrons to supply the electrical imbalance. It is here where the electrons (-) that leave the generator end up negatively charging a nearby metallic device (8); to finally produce an electric discharge (9) in the direction of the metal dome.
The electric shock can happen either in the direction of the dome, or in the direction of the device;Â the latter occurs when it is the dome that is negatively charged.
How does a Van de Graaff generator work?
Triboelectric series
The Van de Graaff generator can be charged positively or negatively. The symbol of the charge will depend on the triboelectric nature of the materials from which the belt and lower roll cover are made.
For example, if the lower roller is covered with nylon, but the belt is made of rubber, then the triboelectric series should be checked to know which material will receive and which will donate electrons once they are in contact.
Thus, because nylon is more positive, that is, because it is higher in the triboelectric series than rubber, then it will lose electrons while rubber will gain them. Therefore, the belt will end up displacing or mobilizing negative charges when the generator engine is started.
Meanwhile, if the lower roller is covered with silicone, the opposite will happen: the belt will lose electrons, since silicone is more negative than rubber in the triboelectric series. And consequently, the belt will displace or mobilize positive charges (as in the image already described).
Shifting loads
Triboelectricity is just one of many electrical phenomena (corona and photoelectric effects, Faraday’s ice cube, electric fields, etc.) that take place in the Van de Graaff generator. But the main point is that it can move, mobilize or “pump” electrical charges towards the metal dome.
Once the lower roller is negatively charged after the motor is activated, and the belt positively, the electrons from the roller begin to repel those from the outer face of the belt. These electrons migrate, through the air, towards the lower brush, where they will be conducted towards the earth or another device.
The positively charged belt reaches the upper roll, which has a triboelectric nature opposite to the lower roll; that is, instead of being negatively charged, it must lose electrons and therefore also become positively charged. Thus, the positive charge moves towards the upper roller and, finally, towards the upper brush in direct contact with the metal dome.
Electrons from the upper brush are transported to the roller to neutralize charges. But these electrons come from the surface of the metal dome. Therefore, the dome also acquires a positive charge.
Electric shock
The dome, according to its dimensions, will reach a maximum potential. After that, the electrical charges must be balanced. Being very positive, it will receive electrons from a very negatively charged source – the device that receives the electrons from the lower brush. Thus, an electric discharge (spark) is produced from the device (negative) towards the metal dome (positive).
The higher the electric potentials reached, proportional to the dimensions of the generator, the more intense the reproduced electric discharges will be. Note that if they were not so large, the electrons could not travel through air, a non-conductive dielectric medium.
Applications
Teachers
If the metallic sphere is positively charged, and someone touches it, their hair will also end up being positively charged. Equal charges repel each other, and therefore the hair will stand on end and separate from each other. This phenomenon is used for educational purposes in courses where electrostatics is introduced.
Thus, small-sized Van de Graaff generators are used to capture the attention of observers regarding the standing of their hair;Â or in the contemplation of electric shocks, faithful replicas of those we see in science fiction movies.
Particle accelerator
When the dome concentrates many electrical charges, a potential is generated that is capable of accelerating subatomic particles. For this purpose, the Van de Graaff generator is used to reproduce X-rays in medicinal studies and nuclear physics.
