Why does static electricity give you a shock?

On a winter morning you reach for the front doorknob and—zap. You snatch your hand back. Pulling off a sweater, you may hear crackling, and in a dark room you might even see tiny sparks.

Static electricity barely bothers us in summer, yet in winter it suddenly seems to be everywhere. The culprit is electric charge that quietly builds up on our own bodies.

Rubbing moves charge around

Everything around us holds equal amounts of positive and negative charge, so normally they cancel out. But when two things rub together, or touch and then separate, some of the negatively charged electrons move from one to the other. The side that gains electrons becomes negatively charged, and the side that loses them becomes positive. This is called triboelectric charging.

Which side turns positive and which turns negative depends on the pair of materials. Human hair and wool, for example, are known to become positively charged easily, while polyester and vinyl tend to become negatively charged.

Every time we walk across a carpet, get up from a chair or rub layers of clothing together, a little more charge builds up on our bodies. If the soles of our shoes and the carpet don’t conduct electricity well, the charge has nowhere to go and stays on us.

A tiny lightning bolt from your fingertip

What happens when you reach toward a metal doorknob while carrying that charge? Metal conducts electricity well, so the stored charge rushes toward it all at once. Across the tiny gap between fingertip and metal, the electricity jumps through the air. That is the zap, and it works the same way as lightning: an electrical discharge.

The voltage involved is astonishingly high. Walking across a carpet on a dry day can charge your body to as much as 35,000 volts. Household outlets in Japan supply 100 volts, so that is hundreds of times more. People are said to start feeling the sting at around 3,000 volts.

It isn’t dangerous because the amount of electricity that flows is tiny, and it is all over in less than a millionth of a second. Think of it as a single drop of water falling from a great height: it hits with force, but there’s very little of it.

Dry air is why winter is worse

Even if you walk the same way, the voltage that builds up on your body varies greatly with humidity. In the carpet example, the voltage is said to reach 35,000 volts when the humidity is a low 10 to 20%, but only about 1,500 volts when it is a high 65 to 90%.

In humid air, a very thin film of water forms on surfaces, and charge gradually leaks away through it. In dry winter air that escape route disappears, so charge keeps piling up. Wearing thick layers that rub against one another also adds to winter static.

Known remedies include humidifying the room, touching a wall or a wooden desk to let the charge drain away before touching metal, and holding a key or other metal object so that it touches the doorknob first. Self-service gas stations in Japan have pads for discharging static electricity for the same reason: to keep a spark from igniting gasoline vapor.

Electricity began with amber

Static electricity was also the first electrical phenomenon humans noticed. In ancient Greece, people knew that rubbing amber with cloth made it attract feathers and bits of straw. The philosopher Thales, around 600 BCE, is said to have recorded this.

The Greek word for amber is ēlektron. In 1600, the English physician William Gilbert named the amber-like power of attracting things after rubbing using a word drawn from it. That is where the English words “electricity” and “electron” come from. The winter zap is the most everyday doorway into a history of electricity that stretches back more than 2,500 years.