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# what can i prove with this? Answered

picking a topic for science fair has been hard. i was wondering what i could prove with a van de graaf generator. somthing i can test with it. any ideas?

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Not for free, but it's still a useful approximation. For your experiment, the input variable would be the diameter of the dome, output would be the length of the spark produced. That could be related to voltage afterwards in your evaluation.

you prove static electricity

static electricity is anything but static though :-)

That's it! You prove that static electricity is not really static, it's really high voltage DC.

Around 5-10kV, depending on air pressure, humidity and my memory. I'll google a bit.

good question. make it and see.

That is, make one VdG with interchangeable domes of varying diameters.

(you're repeating yourself.) An isolated sphere acts as a capacitor, and there's a nice formula for figuring out the capacitance. Bigger capacitors make bigger sparks, but less frequently for a given power source (belt, pulley materials, motor/etc.) I don't know if that would make a good subject for experimentation. Aside from being closely related to something you can just look up, making the sphere on the VdG is one of the harder parts of construction (Hmm. You could do a bunch of non-spherical "spheres" and see how they perform...) Another interesting idea would be to add a capacitor to your setup and see how different dielectric materials behave... (Don't get carried away. Even "small" capacitors can get quite painful when charged to VdG voltages.)

Control can be the materials the brush and belt are made from, and variable can be the voltage of the motor you use. Just a suggestion.

I built a mini van de graaf generator and won first place in the 6th grade science fair, back when they weren't so picky about your project actually being "science." (I learned stuff, but ... no hypothesis, no experiments; just a neat THING.) I like the idea of testing different belt materials and such; measuring output isn't that hard; use a fixed spark gap and time the discharge frequency. In a way, I don't like the current emphasis on "Scientific Method." It takes a lot of the fun out of it, and Engineering is important too...

Typically, a good science fair project shows what you've learned... through experimentation... Rather than - hey, look at this cool thing I built. Yes, cool things are... well, cool - and fun to play with... But a good project will go a few steps beyond the cool bits....

On the subject of a Van De Graff.... you could experiment with different brush and belt materials to see what different outputs you can get (hard part is measuring such high voltages :p).

Don't measure voltage directly, just time how long it takes to make a spark of given length with each material.

I suppose one could rig an electrostatic homemade device to demonstrate different voltages too.

Do your homework : ) It takes something like 10,000 volts to create a 1 inch spark (look it up-I may be full of it : )). If it takes 10 seconds for a rubber belt to create one spark and 15 for a neoprene one and the leather one never does, then you know which is the best material, right?

Is this a school science fair? If so, check with the moderator to see if there is an invention category. We had an annual science fair in middle school, and you could either design an experiment and carry it out, or we could create an invention of some sort. In eighth grade, I built a little device that chucked into a cordless (or corded) drill that shook a spray can with the same motion as a human arm. My hypothesis was something like "I can build a simple, reliable device that will automatically mix an aerosol can." I'd make an Instructable, but I didn't take construction pictures, and I don't need another one.

as Jtobako was saying, you can show what is generating electricity, you can demonstrate capacitance, and high voltage discharge. You can measure it (albeit not very accurately) with some home made instruments:
Here....From here

or here, the opposite of high voltage: Very low voltage

Look at how the machine works. Then see what you can do to make it work better and worse. Is a rubber belt better than PVC? Neoprene? leather? How fast does it have to go? Is faster better? How? Circular terminal, toroid, square, cylindrical, crumpled aluminum foil (how tight?), fish bowl of water...how large of a diameter? What type of brush works/is best? Pick something, or just make one and show what static electricity can do.

A simple VDG made from a 3 volt motor can produce 12,000 volt sparks.

If you use an electric motor to power it, you prove just that: you can put in (insert motor voltage here) and produce a hundred thousand volts here. And then shock someone.