PFE019: Static Electricity

I was shocked at least 740 times in the last 24 hours. Seriously, what's up with this?

There are two things that have to happen to get shocked. This first is that you have to build up a charge on your body. Anyone who has ever a) lived anywhere with a real (or even moderate) winter or b) played with a balloon knows, you can build up a charge on your body by rubbing things together. Shuffling your feet on the ground is a great example of this. How much charge you can build up depends on each material being rubbed together. (This sounds like a perfect do-it-at-home: find out what works the best.) Either way, a charge builds up on your body and since water (remember we're mostly water) conducts electricity somewhat better than air, all of this extra charge builds up on the surface of your skin.

But here's the deal, all of these like charges next to each other want to repel each other so they'd love to jump off your skin onto something else, hopefully something metal, which will conduct them away in no time. Air is working very hard to stop it.

As we all know, of course, we're more likely to be shocked sometimes than others right? Not only is friction important to build up that charge, a low humidity is important to keep it. That is, water will suck away a charge since it is alright to be slightly charged or not. So on humid days it is much harder to hold a charge, but when it's really dry out [like today apparently] charges will build up no problem all the time.

As a side note, as air increases in temperature the total amount of H₂O that the air can hold onto also increases. That's why the air is typically dryer in the winter than the summer.

But just having a charge doesn't really do anything. We feel a "shock" when all of this charge finally is released into a metal. When this happens, a lot of energy is released at once. So why aren't huge arcs flying out of me each time I drag my feet on a dry day (that never happens, dry or humid)?

The "electrical breakdown" in air happens at about 3,000,000 volts per meter. Three million volts? If you've ever licked a 9 volt battery (bad idea kids) you know that you can get a sizable tingle just from that. But three million is way more than nine. So how come we don't all fry ourselves every time we touch a door knob? What electrical breakdown really means is that to get a meter long arc we need 3,000,000 volts of separation between us and the door knob. God help us if we ever shuffle that much. Typically breakdown occurs at about one millimeter sending somewhere around three thousand volts.

Still, 3000 looks like a lot of volts. I mean, it's a lot of batteries lined up. And it is. That's why, on a large enough shock, if you're not paying attention, you'll jump at that instant. It's a lot of energy released all at once. But there's the key. It's all at once. There is no steady flowing of charge running through you at three thousand volts, so nothing really heats up from the spark.

Shocking yourself on a door knob is, of course, the same principle as lightning strikes.

 We all know lightning looks like this.

A charge separation is built up between the clouds and the ground, and once it's enough, lightning strikes and a huge amount of energy is released. Of course, this is enough to fry you so don't get hit by lightning.

I imagine you all shuffling around on the carpet touching door knobs. (Interestingly I can get a spark at all in my room while I couldn't touch anything metal without one today. I think it was my shoes.)

Thanks Josh for the topic.

That's static electricity.