Van de Graaff Generator

A Van de Graaff generator achieved in modern Van de Graaff generators can reach 5 megavolts. Applications for these is an electrostatic machine which uses a moving belt to accumulate very high electrostatically stable voltages on a hollow metal globe. The potential differenceshigh voltage generators include driving X-ray tubes, accelerating
electrons to sterilize food and process materials, and accelerating protons for nuclear physics experiments. The Van de Graaff generator can be thought of as a constant-current source connected in parallel with a capacitor and a very large electrical resistance.

A simple Van de Graaff generator consists of a belt of silk, or a similar flexible dielectric material, running over two pulleys, one of which is surrounded by a hollow metal sphere.[1] Two electrodes, (2) and (7), in the form of comb-shaped rows of sharp metal points, are positioned respectively near to the bottom of the pulley and inside the sphere. (2) is connected to the sphere, and a high DC potential (with respect to earth) is applied to (7); a positive potential in this example.


The high voltage ionizes the air at the tip of (7), repelling (spraying) positive charges onto the belt, which then carries them up and inside the sphere. This positive charge induces a negative charge to the electrode (2) and a positive charge to the sphere (to which (2) is connected). The high potential difference ionizes the air inside the sphere, and negative charges are repelled from E2 and onto the belt, discharging it. As a result of the Faraday cage effect, positive charge on (2) migrates to the sphere regardless of the sphere's existing voltage. As the belt continues to move, a constant charging current travels via the belt, and the sphere continues to accumulate positive charge until the rate that charge is being lost (through leakage and corona discharges) equals the charging current. The larger the sphere and the farther it is from ground, the higher will be its final potential.

The other method for building Van de Graaff generators is to use the triboelectric effect. The two rollers for the belt are made of different materials, far from each other on the triboelectric series. When the belt comes into contact with one and is then separated, charge is transferred from the roller to the belt, and the roller becomes charged. When the belt comes into contact with the other roller and is then separated, charge is transferred from the belt to the roller, and that roller develops an opposite charge. The strong e-field from the rollers then induces a corona discharge at the tip of the pointed electrodes. The electrodes then "spray" a charge onto the belt which is opposite in polarity to the charge on the rollers. The remaining operation is otherwise the same as the voltage-injecting version above. This type of generator is easier to build for science fair or homemade projects, since it doesn't require a potentially dangerous high voltage source. The trade-off is that it cannot build up as high a voltage as the other type, and operation may become difficult under humid conditions (which can severely reduce triboelectric effects).

Since a Van de Graaff generator can supply the same small current at almost any level of electrical potential, it is an example of a nearly ideal current source. The maximum achievable potential is approximately equal to the sphere's radius multiplied by the e-field where corona discharges begin to form within the surrounding gas. For example, a polished spherical electrode 30 cm in diameter immersed in air at STP (which has a breakdown voltage of about 30 kV/cm) could be expected to develop a maximum voltage of about 450 kV.

The Van De Graaff generator is an energy source. It could produce up to 20 million volts in charge. It is used for various physics experiments. Examples would be particle accelerators, food sterilization, and X-rays among others.

It is the eponymous invention of an American scientist. This was in the year 1931. The Van de Graaf generator is a constant current generator.

Its current is expressed in Amperes. It is static unlike its dynamic voltage value. The different parts of the Van de Graaff generator are the metal globe, insulating belt, the lower and upper assemblies of brushes, a couple of rollers, and a motor. You can create your own Van de Graaf generator but it can only produce a lower voltage.

This is how a Van de Graaff generator works:

1. It all starts with the Roller

The roller will begin to move and roll the belt. It will then attract the belt's electrons. It will give it a negative charge.

The strong negative charge of the roller will repel the electrons on the brush assembly. The main reason for this is because similar poles repel each other.

The brush assembly is situated above the roller. The electrons of the brush then move on to the brush's tips. The charge in the lower roller repels the electrons in the air.

It will convert the air into plasma. This is purely positively charged particles.

2. Opposites Attract

The roller and lower part of the brush assembly are highly negative in charge. They can attract positive charges in the air. The protons adhere to the roller and the lower brush but the belt will block the path.

The belt will become charged positively. This positive charge is then carried to the upper part of the generator. The upper portion is the spherical metal.

3. The Sphere

The electrons in the brush are attracted when the positive charge in the belt move to the upper roller. This is situated inside the sphere. The protons in the air are also attracted to the brush. It will leave free electrons like the lower roller did. The free electrons then become attracted to the positively charged belt.

The presence of two strong charges will make anything that touches the inside of the spherical metal lose its charges.

There are also the "excess charges". They are the charges responsible for the Van de Graaf's high voltage.

A Van de Graaff generator could last forever as long as the belt moves and air exists. This is a theory affected by so many factors. Time will be the sole judge of that theory.



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