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I have a few questions about principle how van de Graaff generator works. To avoid confusion I will use numbering from the wiki picture:

enter image description here

[ADDED LATER #1] See also here for nearly identical construction.

It seems to involve following key steps which I would like to think of as a cycle:

In (3) along the frictional contact line between the upper drum and the inner surface of the belt, the upper drum/roller becomes positively charged, while the inner surface of the belt becomes negatively charged due to triboelectric effect leading to static chanrge separation. Next, these negative charges concentrated on the inner surface of the belt are transported downwards to the lower drum, as depicted in step (5).

As a result ariving at (7), these negative static charges on the inner surface of the belt attract positive charges along the brush tip at the lower brush. These positive charges then stick on the outer surface of the belt and are transported upwards, see (4).

Finally, they migrate along the upper brush (2) to the hollow metal sphere (1) since inside the hollow sphere there are no charges concentrated, only at boudaries (priciple of Faraday cage: inner of hollow conductor stays neutral), and the process begins again, except that after each cycle, the hollow metal sphere becomes increasingly positively charged by these positive charges.

Concern: But it seems that besides of these effects described above there are few further several processes going on leading to undesired consequences implying that maybe I misunderstood some of the parts of the working mechanism and unfortunately the description in wikipedia not adresses these points I'm going to adress:

Problem 1: Doesn't besides the effect at (2) that positive charges pass along the upper brush to hollow sphere, simultaneously in each cycle the upper drum, which again comes in contact with the inner surface of the belt along frictional contact surface, also becomes increasingly positively charged resp. the inner surface of the belt increasingly negatively charged after each further cycle due to the triboelectric effect? ​​

At all, it seems that in (3) when the belt - with positive charges on the outer surface and negative charges at inner surface - arives at the top, 2 processes simultaneously happen if I not missing something:
1.: as described above the positive charges on the outer surface of the belt leave the belt along the upper bruch to the hollow sphere
2.: But also once the inner surface of the belt comes again in contact with upper drum und by triboelectric effect the upper drum should be forced to becomes after every cycle more positively charged, inner surface of the belt negatively.

But I guess that's nopt what we want: We only want that the hollow metal sphere alone enriches stonger and stronger itself with positive charges after each cycle, not the drum or the inside of the belt.

So what do I missing?

Problem 2: (B): The triboelectric effect also occurs between the inner belt and the lower drum at the bottom in (6). But the picture suggests that at the bottom the only process which actually happens is as described above that the coming negative static charges sitting on the inner surface of the belt (5) attract positive charges along the brush tip of the lower brush forcing then to stick on the outer surface of the belt.

Well that's true, but shouldn't - apart from this process - once again the inner surface of the belt comes in contact with lower drum in (6) simultaneously happen separation of charges again by triboelectric effect along their frictional contact?

The picture seemingly not takes this into account. Againt what do I misunderstand regarding this point?

In summary, neither the description in wikipedia nor any other description in found so far in web adress the issues I exposed in Problem 1 & 2. This leads me to conjecture that maybe I misunderstood completely the working principle of the Van de Graaff generator. Could somebody elaborate with focus on what I triad to elaborate in probelm 1 & 2?

[#2 ADDED LATER]: Thanks to comments by Jon Custer, mmesser314 and Ohm's Lawman it seems that there exist different constructions of VdG generator based on different working principles (...but of course all leading to same result: the cumulation of charges at hollow metal sphere (1)), some of those working completely without triboelectric effect, eg this one where at (7) instead of initially "neutrally charged object" (8), along to lower brush the VdG is connected with a high-voltage power supply, so no negative charged on inner surface of the belt is needed, thus in turn no triboelectric effect needed to bring negative charges to the inner surface of the belt.

On the other hand of (8) is initially a neutrally charged conductor, then to pull positive charges at (6) to outerside of the belt these needed to be attracted at (6) by negative charges on belt's inside. And so far I understand the job of triboelectric charge separation between upper drum and inner belt at (3) is to place the negative charges at belt's inside. So in that construction usage triboelectric effect is neccessary.

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  • $\begingroup$ This Veritasium video explains Kelvin's thunderstorm. It is something like a Van de Graaff generator. It may help. $\endgroup$ Commented Nov 24 at 14:43
  • $\begingroup$ Fundamentally, the picture is incorrect. In particular, the depiction of + on one side and - on the other side does not occur. At the bottom, one charge is induced on the belt and transported to the terminal where it is taken off. Some actual accelerators do then induce the opposite charge on the belt and send it down to ground, doubling the charge transfer, but that is not necessary. More modern machines use the Pelletron chain consisting of metal spheres to carry the charge connected by insulating linkages. $\endgroup$ Commented Nov 24 at 17:21
  • $\begingroup$ @Jon Custer: Maybe a stupid point, but maybe here sits the reason for my conceptional misunderstanding: If the belt as in picture above (consisting of electrical insulator material) rubs with drum (top or bottom one) consisting of other electrical insulator but which is far away from those of the belt's material with resp to triboelectric series, then by triboelectric effect we get separation of charges. Since belt is insulator, so no "moving" of charges inside it possible, the charges sticking on the belt staying $\endgroup$ Commented Nov 24 at 17:52
  • $\begingroup$ precisely on the side of the belt (inside vs outside) which was in frictional contact with the drum, right? Or does already here occure a fundamental thinking error? $\endgroup$ Commented Nov 24 at 17:52
  • $\begingroup$ The belt should not slip relative to the drum - there is no tribology going on there. $\endgroup$ Commented Nov 24 at 19:06

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I will describe the type of VdG generator which relies on triboelectric charge transfer by contact, not slipping.

enter image description here.

An important feature of such a generator is that the materials of which the two pulleys are made are on opposite ends of the triboelectric series and the belt is in between them.

The lower pulley on touching the belt acquires a positive charge and the inside of the belt acquires a negative charge (triboelectric effect).
The positive charge is concentrated on the pulleys whereas the negative charge is dispersed on the belt.
The positively charged pulley produces a strong electric field which induces negative charges on the points of the comb.
The electric field is so strong at the tips of the comb that the air becomes conducting and negative charge is sprayed on the outside of the belt.
On arrival at the top of the generator the comb arrangement again makes the air a conductor with the positive comb tips collect the negative charges on the outside of the belt.
The collected negative charges then move to the outside of the dome.

Note that my diagram differs from that shown in the original post.

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  • $\begingroup$ Thank you, essentially the model you describe would coincide with the picture from wiki if we swap everywhere negative with positive charges :) But there is still a problem adressed in questions 1 & 2 (up to swapping + by -), which our answer seemingly not adresses (or I'm overlooking): Let focus on charge separation due to triboelectric effect between lower pulley and inner surface of the belt in your picture. As you wrote since their materials differ strongly on tribular series, the pulley would along the contact surface accumulate positive charges, the inner surface of belt negative ones. $\endgroup$ Commented Nov 25 at 18:11
  • $\begingroup$ But wouldn't this lead in consequence to - presumably undesired side effect - that with time the pulley becomes more and more positively charged? And same game with inner belt side. On one hand the key goal is that the metal dome becomes extremely negatively charged with time, but I doubt that that's also desired that the lower pulley becomes extremely positively charged (...of same "level" as the metal dome); desired is only "charged enough" to attract electrons at lower brush. How the explain this potential problem? Compare with issue described in Problem 1 &2 above. $\endgroup$ Commented Nov 25 at 18:13
  • $\begingroup$ The more charge on the bottom pulley the more charge would be sprayed onto the belt. I assume that there would be leakage from the bottom pulley. $\endgroup$ Commented Nov 25 at 20:53
  • $\begingroup$ You mean leakage from bottom pulley of what? Guess: Of the positive charges. But where would they tend to flow? Surely its in contact with inner surface of the belt, but neiter the belt, nor the pulley consisist of conductor materials, so I not completely understand how such kind of leakage (current?) from "to positively charged" pulley should happen. A naive guess, but not sure if that's correct: Wouldn't to strongly charged lower pulley maybe just exert kind counterforce against the triboelectric effect, preventing - after reaching some "material dependent" treshold - futher charge $\endgroup$ Commented Nov 25 at 21:16
  • $\begingroup$ separation along contact surface between the pulley and inner belt by triboelectric effect? (Just as naive guess, not sure what is actually really going one in this part) $\endgroup$ Commented Nov 25 at 21:17

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