basic physics - van de graaf generator

Hi


If you are touching the generator and holding a pin why does the pin become positively charged? The diagram i am looking at suggests it does? I thought the generator was a source of electrons which flowed to you when you touch it so they would flow into the pin too making it -ve charge too

Why should you hold a pin when you approach a van de graaff generator?

Thanks
andiejeAsked:
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ozoCommented:
the generator can be a sink of electrons which flow from you when you touch it  making it + charge
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d-glitchCommented:
The spherical electrode on the top of the van de graph allows for a fairly uniform charge distribution
which makes a radial electric field.  Round shapes (spheres, rings, cylinders) are used in high voltage
systems to reduce arcing.

     http://www.rossengineeringcorp.com/toroids_spheres_corona_nuts.htm


A sharp point (the pin in this case) concentrates the electric field to initiate the breakdown at the
pin (rather than your face for example).
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andiejeAuthor Commented:
how do you know whether it is a source or sink of electrons or does it behave differently in different contexts.
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d-glitchCommented:
The pin does not have an inherent polarity.

The sphere of the van de graf is negatively charged.

If the pin is at ground potential, it will sink electrons.
If the pin is at the potential of the sphere, it will source electrons.

The pin (a conductor in a static field) concentrates electric field lines which facilitate arc formation.
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sgvillCommented:
According to http://en.wikipedia.org/wiki/Van_de_Graaff_generator, the sphere of the van de graf is positively charged.   If you are in contact with it, It seems to me that it will draw electrons from you (and your pin), leaving the pin positively charged.   You didn't mention where the pin was held, but I also would think that if the pin is held close to the generator, it should give up its electrons easier bythe same principle as a how a lightning rod works.
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d-glitchCommented:
>> the sphere of the van de graf is positively charged.

That's embarrassing.  Wikipedia is almost certainly right on this one.

>>same principle as a how a lightning rod works.

sgvill is also correct about the lightning rod.  They both work by concentrating field lines and enhancing breakdown.
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andiejeAuthor Commented:
I think it depends on the generator as I have read many different places say the generator is negatively charged and others say it is positively charged?
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d-glitchCommented:
You appear to be correct.  Amazing what you can learn if you pay attention.

   http://www.eskimo.com/~billb/emotor/belt.html
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