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Van de Graaf Generator Question

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scott712

Sun Jul 27 2008, 03:02AM

Registered Member #1607 Joined: Sun Jul 27 2008, 01:19AM
Location: Spokane, Washington State, USA
Posts: 5

I think I am missing something. I hope someone can enlighten me. I think understand that the "excess" electrons on a charged sphere repel each other with the result that they gather on the outside of the sphere while leaving the inside neutrally charged.

This is what is puzzling me: Why does the Van de Graaf Generator need a mechanical means to physically carry electrons up inside it. I see why this works, but I don't see why I can't simply connect the positive terminal of a battery to Earth-ground and the negative terminal to the inside of the sphere.

Perhaps the problem is that I don't have a metal sphere. I have been trying to do this with a suspended coffee can that has the top and bottom removed. This is on the theory that the same effect might be seen on a hollow cylinder for the same reasons... Is my cylindrical approach completely unworkable or is it simply less effective than a mostly enclosed sphere? Or is the whole battery concept flawed?

Apart from not having a hollow metal sphere, I think that a cylinder might be advantageous. A possible answer to my question might be this, that the charge on the outside of a sphere might tend to repel electrons that are trying to flow up the wire and into the sphere. If this is the case, then perhaps a cylinder is a better idea, since the wire can enter along the axis of the cylinder.

It seems to me that there should not be much radial opposition to axially-moving incoming electrons. Then, once they are inside the cylinder they can move to the inside wall just as in a standard Van de Graaf Generator.

One more thought! Is part of the trick the fact that needle point combs are used to impart the charges to the inside of the VDG Sphere? Does this actually have the effect of raising the voltage level? Might this make my batter work?

Thank you for your time!

Wm. Scott Smith

rp181

Sun Jul 27 2008, 03:20AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

i might be wrong but here it goes:

Think of electricty as a pneumatic system. Voltage is pressure, the sphere is a airtank, and a battery a pump. If the battery is only 9v, its not gonna be able to charge the sphere to more then 9 volts. Thats like trying to get 1000 PSI from a 120psi pump.

...

Sun Jul 27 2008, 03:22AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

What exactly are you trying to accomplish with your setup? If you are trying to measure that a charge only accumulates on the outside of a shpere you rig isn't going to work.

Also, using a battery in place of the moving belt in a VGD isn't going to work at all either, because a battery can only put out a few volts which makes your generator somewhat useless...

scott712

Sun Jul 27 2008, 03:51AM

Registered Member #1607 Joined: Sun Jul 27 2008, 01:19AM
Location: Spokane, Washington State, USA
Posts: 5

So far I am being asked why I expect thousands of volts from a nine-volt battery.

My perhaps fallacious reasoning is this: As the battery moves more electrons up from ground potential to the inside of the cylinder, they keep disappearing from the inside of the cylinder as they move to the outer surface of the cylinder. Nine volts should be adequate to move electrons up from ground potential to the neutral enterior of the cylinder. Once they are on the inside surface of the cylinder why does it matter whether a battery raised them up from ground potential or a VDG belt or fluid charge exchange system? Won't they still migrate to the outside of the sphere and accumulate in exactly the same way???

I haven't been able to get it to work (yet!) So I am very interested in uncovering any errors in my approach.

Dr. SSTC

Sun Jul 27 2008, 04:01AM

Registered Member #1407 Joined: Fri Mar 21 2008, 07:09AM
Location:
Posts: 222

:-|
mate i dont think youll ever get this to work

Arjan EMM

Sun Jul 27 2008, 08:51AM

Registered Member #149 Joined: Sun Feb 12 2006, 09:11AM
Location: Netherlands
Posts: 12

You're mixing up up electric potential en electric field strength.
The inside is not neutrally charged, but all of the same potential. including the air in theory.
Every point on or inside the sphere has the same voltage. So the electric field strengt in V/m is zero and you will have no sparks breaking out inside the sphere.
The envirement around the the sphere has a very low potential, ideally 0V/ground. So there is a high electric field strength between the charged sphere and the area around it, so you will get sparks breaking out.

Steve Conner

Sun Jul 27 2008, 12:40PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The difference is that the belt in a VDG is an insulator. That means that the electrons sprayed onto it at the bottom are trapped where they land and can't move. So you can use mechanical force from a motor to drag them up through the electric field into the top terminal. Theoretically you could get infinite voltage, if your motor could deliver enough force. If we used a water analogy, it's like carrying water uphill by scooping it into buckets attached to a rope that gets winched uphill.

A wire is not an insulator, and if it's connected between two points of different voltage, electrons will flow freely through it until the two points are at the same voltage. That is after all the whole point of wires. So trying to generate voltage with a wire like the OP suggested is like carrying water uphill by laying a pipe up the hill and expecting it to flow up of its own accord.

scott712

Sun Jul 27 2008, 07:00PM

Registered Member #1607 Joined: Sun Jul 27 2008, 01:19AM
Location: Spokane, Washington State, USA
Posts: 5

The second part of my question was this: Will a cylinder work reasonably well as a substitute for a metal sphere?

At this point I am asking in context with using a mechanical or hydraulic charge carrier since my wire is generally thought to be unworkable; (however, I am also still interested in further commentary on the wire proposal.)

Thank you for taking the time to think this through with me. I really want to see if I can at least get the cylinder idea to work since perfect cylinders are easier to approximate than perfect spheres.

coillah

Mon Jul 28 2008, 06:35AM

Registered Member #1517 Joined: Wed Jun 04 2008, 06:55AM
Location: Chico CA
Posts: 304

Dr. Conner wrote ...

A wire is not an insulator, and if it's connected between two points of different voltage, electrons will flow freely through it until the two points are at the same voltage. That is after all the whole point of wires. So trying to generate voltage with a wire like the OP suggested is like carrying water uphill by laying a pipe up the hill and expecting it to flow up of its own accord.

hits the nail on the head.

I imagine that any geometry other than a sphere would simply generate more uneven fields and what not... as long as it is conductive it should work for what a VDG is usually used for.

but then again, its been a year since I took E&M soo...

Steve Conner

Mon Jul 28 2008, 10:58AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Anything will work as long as it's got smooth, rounded edges. That's why a sphere is best, its edges are so smooth and rounded that they aren't even there. But I'm sure I've seen a VDG with a beer can as the topload.

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