Monday, August 24, 2015

Danger! High Voltage

A few weeks ago, we put up a "Fermi problem Friday" post about the odds of being struck by lightning. That post was met with some criticism in the comments section, so it's currently down while we revise it to reflect our readers' concerns. But last Friday, we made a discovery here at PhysicsCentral headquarters: your odds of getting zapped go up exponentially if your boss brings her old Van de Graaff generator to work!

His hair doesn't usually do that (but if he could get it to, he would.)

A Van de Graaff generator is a device that creates extreme amounts of static charge. It's similar in appearance to a Tesla coil, but markedly different in its construction and function. From the outside, they're strikingly similar; the most common design for both consists of a metal spheroid atop a column that separates them from their base, but that's just about where the resemblance ends. (One is pretty safe to touch during operation...the other, not so much!)

But while the Tesla coil is a precisely-tuned combination of resonant LC circuits, the Van de Graaff generator is as simple as a conveyor belt for electrons.
That's not an analogy; it's a literal belt that carries electrons from one place to another!
Image courtesy Omphalosskeptic, CC BY-SA 3.0
As the two rollers ([3] and [6] in the above diagram) turn the rubber belt that runs up the column supporting the sphere, metal brushes ([2] and [7]) scrape over the belt's surface. A voltage at the top roller forces charges off the belt and onto the brush, where they can migrate to the surface of the terminal sphere. The belt, now with an opposite charge, proceeds to the bottom roller. There, it discharges on the bottom brush and, roughly neutral once more, heads back up to dump more charge at the top. While simple, this design can be powerful enough to run a particle accelerator; the Westinghouse Atom Smasher, in Pittsburgh, contained a flurry of whirling rubber belts back in its heyday, creating charges of up to five million volts. Our device isn't anything close to that strength, but it packs enough punch to create sparks a few centimeters in length, as well as some other pretty wild effects.
Alike charges on the strands of my hair repel and try to spread out from one another, giving the impression that my hair is trying to get free from my head. 
The rule of thumb for a Van de Graaff is that it makes things spread out, if they can, as similar charges collect on an object. After filming the following video, for example, there was a strong scent of perfume in the room, as the odorant molecules left by Mathlete's conditioner flew free of her hair. But it's not just molecules that can collect enough charge to be repelled this way; it's everything from hair...
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to pie tins...

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to paper!

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Although paper isn't ordinarily a great conductor, the Van de Graaff puts enough charge on this piece to make it fly off. When I touch it, the paper sheds excess charge onto me and, once again neutral, it can be polarized by the strong electric field of the terminal and re-attracted.

At high voltage, you can create a miniature sandstorm in a dish of salt, just by holding your hand over it. When it's close enough, grains of salt will jump from the dish to your hand and take on a heavy charge before returning to the dish, where they'll discharge before repeating the process, in a smaller-scale version of the "Van de Graaff Volleyball" you see above.

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Though the video quality is admittedly less than stellar, you can hopefully see (and even hear!) the miniature salt explosions that occur when I touch my finger to the dish. A similar phenomenon can be seen with the "fruity pebbles volcano" demo, which we also felt obligated to try out.

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If you pay close attention, you can see that the cereal's flight-path traces out electric field lines around the machine! If you have a Van de Graaff machine in your classroom, this demo is also a great way to teach students about entropy, while they pick up the cereal that's been scattered across the floor. 

Having exhausted our repertoire of high voltage tricks, we got creative. Our test subject, Halfstache, donned an impromptu headdress made of plastic fringe-streamer, in hopes that its fringes would act like hair and stand on end.

It was about as effective as it was fashionable.
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We hope you enjoyed our bit of Friday fun; if you have questions, suggestions, or ideas for fun things to do with a Van de Graaff, post them in the comments below! We'll pick the best ones, try them out, and report back.

1 comment:

  1. Good Job! Nice to see how you can amuse yourself as I did when I made my first one when I was 15. It is the first time i see in 'how it works' that someone starts at the top roller instead of the bottom roller. I think it all depends on the materials used and there places in the triboelectric series. If i.e. teflon is used for the bottom roller and polyurethane foam for the upper you will have a charge affinity difference of 250 nC/J (source: http://www.trifield.com/content/tribo-electric-series). This is the best way to increase the current and therefore the speed the sphere will load. BTW: a very extensive explanation can be found on http://science.howstuffworks.com/transport/engines-equipment/vdg3.htm.

    I would suggest you put a drop of water on an insulator and near the loaded sphere from above slowly (with the drop hanging down) and look what happens. Now you made the cheapest High Voltage meter that exist!

    I would be carefull with electronic equipment in the neigberhood of the VDG Generator though. I blew up once my amplifier which was about 2 meter distance... And never put a grounded sphere above the VDG Generator sphere as I did (only once...). The shock you'll receive is getting ugly...

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