A Brief Note on the Main Components of the Van de Graaff Generator

A Van de Graaff generator creates static electricity. The current generated by a Van De Graaff generator remains the same, while the voltage changes according to the applied load. 

There are mainly two types of Van de Graaff generators: one that uses a high-voltage power supply for charging and one that uses belts and rollers. Here, we will discuss the belts-and-rollers type. 

An elementary Van De Graaff generator consists of the following:

• Motor
• Rollers
• Insulated belt
• Brush that touches the rollers
• The metal sphere as an output terminal

The Van de Graff generator is primarily dependent on two principles: corona discharge and accumulation of charges. Charged particles are transferred from the brushes to the belt and then from the belt to the sphere employing corona discharge. After this, the distribution of charges among the sphere occurs by the accumulation of charges. 

Construction of Van de Graaff Generator

There is a prominent circular conductor with a few metres of radius. It is mounted on two pillars at a certain height from the ground. One is P1, and the other is P2. 

A belt made of protective material travels over two planks, P1 and P2. Pulley P2 is placed at the centre of the spherical conductor, and pulley P1 is located near the ground with two brushes that touch the rollers/pulleys. 

When the engine creates a spin on the belt for its primary function, two camps with pointed heads and an exhaust tube are used when ion acceleration is performed. The point from which the ions come is located at the end of the discharge tube. However, the other end of the tube is earthed. All tools are placed in a metal position. The room is full of nitrogen or methane. The pressure inside the room is kept very high.

The very first Van de Graff generator was built using a tin can, a silk ribbon and a motor. However, this was a model demonstration of a project by Robert J., which his chairman later funded. The upgraded version of his first demonstration achieved around 1.5 million volts, whose actual price was evaluated at around 90$ only. 

Working Principle

A motor is required to open the belt at a constant speed near two rollers and two brushes that touch the rollers. The bottom roller is made of a material with a robust triboelectric surface. When the engine starts to rotate the belt around the bottom, the electrons are taken from the insulated belt and rolled down. Gradually, the charge becomes more and more focused on the roller. This state of focus expels electrons from brush assembly tips. It also attracts electrons from the air atoms between the bottom roller and the brush joint. As a result, well-charged air atoms are carried away in a belt from a badly charged roller. Hence, the belt is well charged and goes to the top rollers. The charge from the belt accumulates on the large metal sphere. 

The capacitance of the metal sphere is given by:

C = 4 × π × R × ε₀

R = Radius of Metal sphere

ε₀ = permeability of space

Therefore, the charge accumulated by the metal sphere is given by:

C = Q/V

Q =4 π R ε₀ V

For air, the breakdown voltage for air can be calculated experimentally:  3×10⁶ V/m  . 

Beyond this, the air around the sphere gets ionised, and the charge leaks. The leakage can be minimised by surrounding the sphere with methane or nitrogen at high pressure.

Common Uses 

In the earlier decades of its invention, Van de Graff played a crucial role in research and science experiments. Afterwards, its role in specific applications was replaced by some other technologies, like Tesla coil. Though its requirement is reduced, it is still used for numerous purposes due to its unique features. These are as follows:

1. It is used to sterilise food and appliances. 
2. It is also used in cancer treatment.
3. It is used for educational purposes to demonstrate its working and the related concepts of electrostatics. 

Conclusion

A simple electrostatic effect allows the Van De Graaff generator to discharge very high volumes continuously. The Van de Graaff generator is used to generate a maximum of about a million voltages. Then, the potential difference is used to accelerate particles such as protons and deuterons, and it is used for many other purposes. It is a widely used tool in nuclear physics testing.