Where does charge go in this van de Graaff generator?

  • Thread starter uby
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  • #1
uby
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Sorry, I had a long post that just got eaten by my computer so I am having to re-type my question. It will need to be brief:

Here is an example of a van de Graaff generator: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/vandeg.html

It notes that the bottom brush is "given a positive voltage to draw electrons off the belt". I have no idea what this means. How is it given a voltage? A positive voltage relative to what? If it is drawing off electrons, where do these electrons go? Electroneutrality would require that all those electrons drawn ultimately from the metal sphere need to go somewhere. Where is this mythical negatively charged object?

This arrangement ultimately seems like a capacitor to me. Whether by pulleys or by an external power supply, work is inputted into the system to re-arrange electrons. The metal sphere that is positively charged and the mythical negatively charged object create an air-gap capacitor with the energy of the electric field between them maintaining charge separation. This is why electrostatic generators are high voltage: they have such low capacitances! But why then, can charge move all over the metal sphere uniformly, when it should be concentrated in the region of the electric field?

thanks!
 

Answers and Replies

  • #2
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On the second part of your question . There is No electric field inside the sphere created from the charge on the sphere. There might be a small E-field from the free charge that is getting thrown onto the sphere by the belt. The free charge on the metal sphere moves around to cancel the E field inside the conductor.
 
  • #3
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Sorry, I had a long post that just got eaten by my computer so I am having to re-type my question. It will need to be brief:

Here is an example of a van de Graaff generator: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/vandeg.html

It notes that the bottom brush is "given a positive voltage to draw electrons off the belt". I have no idea what this means. How is it given a voltage? A positive voltage relative to what? If it is drawing off electrons, where do these electrons go? Electroneutrality would require that all those electrons drawn ultimately from the metal sphere need to go somewhere. Where is this mythical negatively charged object?

This arrangement ultimately seems like a capacitor to me. Whether by pulleys or by an external power supply, work is inputted into the system to re-arrange electrons. The metal sphere that is positively charged and the mythical negatively charged object create an air-gap capacitor with the energy of the electric field between them maintaining charge separation. This is why electrostatic generators are high voltage: they have such low capacitances! But why then, can charge move all over the metal sphere uniformly, when it should be concentrated in the region of the electric field?

thanks!
The old Van Der Graff works on one very interesting principle, that is charge - electrons - will concentrate
on geometrically sharp parts of a conductor. You're right the thing does act as a capacitor, its the brushes
that allow this. You see because they are pointed charge builds up on them and acts as a kind of barrier.
When the brushes pick up a few electrons from the belt, by mechanical action, when they get past this
barrier they are trapped on the metal sphere behind it and can build up as a big reservoir of charge.

That make sense to you nga?
 
  • #4
uby
176
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On the second part of your question . There is No electric field inside the sphere created from the charge on the sphere. There might be a small E-field from the free charge that is getting thrown onto the sphere by the belt. The free charge on the metal sphere moves around to cancel the E field inside the conductor.
Pursuing the capacitor analogy, I guess if the electrons are going to ground then the other hypothetical negative charged surface can be taken to be symmetric with infinite radius? That is the only way to obtain a symmetric charge distribution in the presence of an electric field.
 
  • #5
uby
176
0
The old Van Der Graff works on one very interesting principle, that is charge - electrons - will concentrate
on geometrically sharp parts of a conductor. You're right the thing does act as a capacitor, its the brushes
that allow this. You see because they are pointed charge builds up on them and acts as a kind of barrier.
When the brushes pick up a few electrons from the belt, by mechanical action, when they get past this
barrier they are trapped on the metal sphere behind it and can build up as a big reservoir of charge.

That make sense to you nga?
I'm ok with the mechanism for how charge transfers onto the sphere. My problem is with the 'circuit'. In this example, the electrons aren't going to ground, they are going to some unseen external circuit. If, for example, there is no ground as an electron sink (for example, using an internal junction for ground instead), I suspect that this device would not operate.
 

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