We’re fast approaching what are usually the coldest, driest months of the year (at least here in the northern hemisphere), and with that comes the annoying tendency of doorknobs to shock and startle us whenever they’re touched. It happens to some extent almost everywhere, but it wasn’t until I spent a week at a conference in Montana—and found myself flinching every time I had to press the elevator button—that I really gave some thought to this usually-minor annoyance.
Static electric charge accumulates on certain non-conductive surfaces when they’re brought into contact and then separated again, if electrons stick preferentially to one surface or the other. When this happens, it leaves one of the objects with a net positive charge—a deficit of electrons—while the other accumulates an excess. If either one of those objects is your body (more likely your shoes or clothes), it can result in a painful and unexpected zap the next time you touch something conductive and electrons rush to restore the balance.
Unfortunately, there’s no easy rule to tell whether or not a substance is susceptible to static charging or which kind of charge it will pick up. Silk will “steal” electrons from glass when the two are rubbed together, but will give them up to polyester, which is among the most electron-hungry of materials. Something tells me that the ski-boot-proof carpets at the hotel in Montana were polyester.
So how does the winter make this worse? There are a number of factors at play, but the big one is humidity.
Ordinarily, you’ve got a roughly equal number of protons and electrons in your body, leading to a total charge of zero, as represented by this happy little guy:
But when you stroll across a polyester carpet, a lot of those negative charges get taken away, and you end up with a net positive charge. Being confined to the heavy nuclei of atoms, protons don’t move around very readily, but since our bodies are conductive, the “holes” left by the stolen electrons are mobile. As a result, the positive charges—wanting to spread out and be as far from other positive charges as possible—migrate to the surface of our skin, as shown below:
The upshot of all this is that these water molecules “bleed off” static charge pretty quickly on a humid day. A similar effect is demonstrated spectacularly in our experiments with a Van de Graaff generator, where fruity pebbles take the place of water molecules in bleeding off some excess charge.
In the winter, and especially at high altitudes where the atmosphere is thinner, the dryness of the air keeps this bleed-off from happening, turning the air into an insulator and causing that excess charge to stay stuck to you until you give it a path to ground in the form of a conductive object.
|Image Credit: Daniel Grohmann (CC BY-SA 3.0)|