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# Van De Graaff: [help] [how does it work?] watch

1. I know the general idea of how a Van De Graaff generator works.

I have one problem.

There is apparently an arrangement where the surface of the metal dome is negatively charged instead of positively charged. Why is this?

Please NOTE that the positive charge is still being moved upwards. So it is not the case where there is negative charge moving upwards.

Thank you.
2. (Original post by Sayonara)
There is apparently an arrangement where the surface of the metal dome is negatively charged instead of positively charged. Why is this?
Troboelectric series for the belt and brush materials will determine the excess charge type carried to the dome.

Please NOTE that the positive charge is still being moved upwards. So it is not the case where there is negative charge moving upwards.
Thank you.
Where did you get this from?

Think about the implications of what you are saying: Protons cannot be transferred from the belt because they are fixed. That means only electrons can be transferred either from the belt to the dome or vice-versa.

If the net charge on the belt as it moves towards the dome is positive, that means there is a deficit of electrons on the belt. There will be a net attraction pulling electrons off the dome thus leaving it positively charged.

There is no other mechanism for a belt with a positive charge delivery to the dome.
3. (Original post by uberteknik)
Troboelectric series for the belt and brush materials will determine the excess charge type carried to the dome.

Where did you get this from?

Think about the implications of what you are saying: Protons cannot be transferred from the belt because they are fixed. That means only electrons can be transferred either from the belt to the dome or vice-versa.

If the net charge on the belt as it moves towards the dome is positive, that means there is a deficit of electrons on the belt. There will be a net attraction pulling electrons off the dome thus leaving it positively charged.

There is no other mechanism for a belt with a positive charge delivery to the dome.
Oh I see, thanks.

I guess the explanation I got from a classmate was not right.

So, the electrons from the belt get transferred to the surface of the metal dome?

If the dome is negatively charged, what charge does the upper comb have?
4. (Original post by Sayonara)
So, the electrons from the belt get transferred to the surface of the metal dome?
Yes.

If the dome is negatively charged, what charge does the upper comb have?
The belt is in contact with the upper pulley which is a lower position on the triboelectric series. Electrons are pulled off the belt and the pulley gains a net strong negative charge.

This creates a dense negative electric field which repels the electrons on the outside of the belt forcing them off the belt and onto the upper comb.

The upper comb therefore also acquires a net negative charge.

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Updated: November 14, 2013
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