Buzz Blog

Danger! High Voltage

Monday, August 24, 2015

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...