How does this specific Van de Graaff generator work?

In summary, Dave found that the outside roller on his generator attracts more electrons from the lower electrode.
  • #1
greypilgrim
538
36
Hi.

I have access to following Leybold Van de Graaff generator:
vdg.png


The instructions and data sheet can be found here, but they don't give much insight.

I have found different working principles of VdG generators. Some need the rising and falling sides of the belt to be closely together from top to bottom to act as a capacitor, others don't rely on this at all. I haven't found other descriptions of VdG generators like this one where the sides are brought together only at one point (6).

I thought it might be an improvement to the following kind of VdG generators:
371px-Van_de_Graaff_Generator.svg.png

They might have brought together both sides of the belt where they placed the lower electrode such that more positive charge gets attracted to the outside of the belt (since now there's more negative charge on the inside of the belt close to the electrode).

However, I found that the dome actually gets charged negatively. Also, all three rollers seem to be made of the same kind of plastic.
 
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  • #2
greypilgrim said:
However, I found that the dome actually gets charged negatively

how did you determine this ?

greypilgrim said:
Also, all three rollers seem to be made of the same kind of plastic.

3 rollers ? I have only seen ones with 2 rollersDave
 
  • #3
davenn said:
how did you determine this ?
I first charged an electroscope positively on the plus pole of a high voltage source (I later repeated this using the positive charge of a wool rubbed glass rod and got the same result). Then I charged a small isolated sphere on the dome of the generator and approached the top part of the electroscope, which made the spreaded leaves in the electroscope approach each other. If my understanding of electrostatic induction is correct, this means that the dome and the electroscope must be of opposite charge.

I repeated this with different parts of the generator, mainly the in- and outside of the belt and it seems to me that all charges are opposite compared to the second image I posted. So I guess they just used materials such that the belt is closer to the positive end of the triboelectric series than the rollers.

davenn said:
3 rollers ? I have only seen ones with 2 rollers
There are different working principles, some even use 4 rollers:

vdgself.jpg


But yeah, 3 rollers seem unusual. I still think the outside roller on my generator was added to attract more electrons from the lower electrode, because they now feel more positive charge from inside the belt (not only from the part going upwards but also from the part going downwards).
 

Related to How does this specific Van de Graaff generator work?

1. How does a Van de Graaff generator create static electricity?

A Van de Graaff generator creates static electricity by using a rubber belt to transfer electrons from a metal comb to a metal sphere. This creates an imbalance of positive and negative charges, resulting in a buildup of static electricity on the sphere.

2. What type of energy is produced by a Van de Graaff generator?

A Van de Graaff generator produces high-voltage electrical energy in the form of static electricity.

3. What is the purpose of the metal dome on a Van de Graaff generator?

The metal dome on a Van de Graaff generator serves as a conductor for the static electricity produced by the machine. It also increases the surface area for charge accumulation.

4. How does a Van de Graaff generator maintain a constant charge?

A Van de Graaff generator maintains a constant charge by using a motorized belt to continuously transfer electrons from the comb to the metal dome. The motor is powered by an external power source and ensures that the generator can continue to produce high-voltage static electricity.

5. What are some common uses for Van de Graaff generators?

Van de Graaff generators are commonly used in physics demonstrations, electrostatic experiments, and in research laboratories for radiation and particle acceleration. They are also used in some medical devices, such as X-ray machines, and in industrial processes for electrostatic coating and surface cleaning.

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