illuminating science

So, supposedly, a man yesterday built up 30,000 volts of static electricity in the jacket that he was wearing while walking around town. Apparently, he left a trail of scorch marks and molten plastic behind him, as he discharged electricity into his surroundings.

Hmm.

This sounds vaguely fishy to me, but part of that I suspect will be dodgy reporting. Let’s take the article as gospel first, then see if can’t tweak it a little to make more sense.

First of all, the dielectric breakdown of air is 30,000 volts per centimetre. That means, that to get a one centimetre spark you need something charge to 30,000 volts. Alternatively, if you charge something to over 30,000 volts and put a grounding rod (which probably includes another person) within 1cm of it, you’ll get a spark as the object discharges its charge into the grounding rod.

So, could our friend get charged to 30,000 volts? In reality, it’s probably not that hard - I can make a capacitor out of a film canister and alfoil that charges to 3,000 volts just by rubbing some plastic tubing with cloth. And my Van de Graff generator can build up 300,000 volts with no effort. So, it’s conceivable that if he had a coat capable of storing charge well, he might build up a 1cm spark. (You’ve probably seen small sparks if you take of a jacked in a cold night in a dark room, or if you stroke your cat vigorously - each spark is probably of order 3,000 volts discharging.) I’m not clear though, how the coat gets charged - I guess just by him walking, or sliding in and out of his car, but it’s a little hard to swallow.

Let’s assume though that it’s possible. Could this spark melt plastic, and burn carpet? The thing is, most of these sparks only contain a relatively small number of electrons. That’s why a 3,000 volt shock from rubbing your feet on the carpet doesn’t kill you, but a 240 volt power supply (or even a 12V car battery) will - the latter two can pump out a huge number of electrons. It’s really the current that’s the killer. I’ve been shocked off the Van de Graff, and it tingles a little bit but nothing deadly. Furthermore, the most this 30,000 volts can only go 1cm - if it’s really in his jacket, it’s not going to reach very far, and if it’s in his shoes, it should just discharge into the earth and be done with it. So, I can’t see that 30,000 volts is responsible for all this.

So, the most likely option is that he was charged much higher - say, 3 million volts. That would get you a metre of spark, capable of reaching the floor. But again, this spark really can’t contain many electrons - could it really melt plastic? I’m trying to think of a back of the envelope calculation here, but I’m not quite sure. (The energy would be charge*voltage, I guess - how much energy is needed to melt plastic, and so how much charge is needed?)

Either way, this seems a slightly improbably scenario. If nothing else, once he’s discharged, it would take some time for him to recharge! He’s not going to be “giving off voltage” constantly (and note, that one typically gives off current!) Interestingly, famed science communicator Dr. Karl was contacted about this - I have to say, that’s pretty cool He seemed to think it plausible, but I remain a little skeptical. Unfortuantely, we’ll probably never hear from him again in the media (and the Powers that Be have probably already whisked away his jacket for their evil projects…) so we’ll probably have to remain in ignorance. Too bad!