# External-excitation Van de Graaff polarities

• eigenmax
In summary, the conversation discusses the use of an external excitation VDG and how it differs from standard self-exciting VDGs. The external excitation VDG uses a voltage supply to influence the charge on the lower pulley, rather than relying solely on friction. The question posed is whether placing the positive electrode of the supply at the lower pulley will create an electron-deficient belt and a positive top terminal, and vice versa for the negative electrode. The function of the rest of the lower unit, including the conductive pulley and charged bar, is also discussed. The source of the conversation is a book from 1960, which provides limited information on the external excitation VDG.
eigenmax
My question regards the polarities of the excitation supply and terminal in an external-excitation VDG.
As you all probably know, external excitation VDGs use a voltage supply (usually 5,000-10,000V) to influence the charge on the lower pulley, instead of just relying on frictional contact to remove electrons from the belt.
My question is, if I place the positive of my supply at the lower pulley, will the positive electrode of the supply draw electrons to it from the belt (as a positive object is an electron deficient one), thereby making a electron-deficient belt and a positive top terminal?
If I use the negative electrode will I have a negative terminal?

There is a little diagram of the excitation supply at the top right.

#### Attachments

• 1957-05-04.gif
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The section of belt passing the 'excitation supply unit' will become charged by Induction. (the opposite charges are taken to ground via the comb) At the top, there is no path to ground so the charges on the belt will get transferred directly to the outer surface of the sphere by mutual repulsion
I'm not sure of the function of the rest of the lower unit, namely the conductive pulley and the apparent common the other side of the charged bar. Perhaps the induction is better with those extra parts. Or perhaps it's for operation without the external excitation supply.

The article I was reading was in The Amateur Scientist, a book written 1960 as a compilation of all the Amateur Scientist columns from the Scientific American throughout the 50s. It describes the operation of standard (self-exciting) VDGs, but this is all it has on the external excitation. I'm also not sure what the charged bar is for.

## 1. What is an External-excitation Van de Graaff polarity?

An external-excitation Van de Graaff polarity is a type of electrostatic generator that uses external means, such as a battery or power supply, to create a potential difference between the two terminals of the generator. This potential difference creates a high voltage, which can be used for various scientific experiments and applications.

## 2. How does an External-excitation Van de Graaff polarity work?

The external-excitation Van de Graaff polarity works by using a motor to rotate a belt made of insulating material. The belt rubs against two metal combs, one at the top and one at the bottom of the generator. This process creates a charge imbalance, with positive charges building up on the top comb and negative charges building up on the bottom comb. The charge is then collected by two metal spheres, one connected to each comb, creating a potential difference between the two spheres.

## 3. What are the advantages of using an External-excitation Van de Graaff polarity?

One advantage of using an external-excitation Van de Graaff polarity is that it can produce much higher voltages than other electrostatic generators, reaching up to several million volts. Additionally, the charge on the spheres is relatively stable, making it useful for experiments and applications that require a constant high voltage. It also has a low risk of electrical shock compared to other high voltage generators.

## 4. What are the common uses of External-excitation Van de Graaff polarities?

External-excitation Van de Graaff polarities have a wide range of applications in various fields of science. They are commonly used in particle accelerators, such as in nuclear physics research, to accelerate charged particles to high energies. They are also used in radiation therapy for cancer treatment, as well as in electrostatic precipitation for air pollution control.

## 5. How do you maintain an External-excitation Van de Graaff polarity?

Maintaining an external-excitation Van de Graaff polarity involves regular cleaning and checking for any wear and tear on the belt and combs. The belt should be replaced when it starts to show signs of wear, and the combs should be kept clean and free of dust to ensure efficient charge transfer. It is also important to regularly calibrate the voltage output and make any necessary adjustments to the external power source. Proper maintenance is crucial to ensure the safety and accuracy of experiments and applications using an external-excitation Van de Graaff polarity.

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