The world can be a minefield in the winter, when just about anything metal can zap you
with a painful electric shock.
Shocks happen for the same reason balloons stick to the ceiling after you rub them on
your hair, which is also what causes lightning: the triboelectric effect.
People have experimented with it for thousands of years, and we’ve come up with some pretty
good ways of mitigating the shocks so you don’t have to be afraid of doorknobs all
the time.
But despite millennia of experiments, there are still some things about this everyday
phenomenon that we don’t quite understand.
The triboelectric effect happens when rubbing two electrically neutral things together builds
a static charge on them, so one becomes positively charged and the other becomes negatively charged.
Our oldest records of it come from Ancient Greek philosophers, who knew that after you
rubbed amber against fur, it would start attracting feathers and hair.
People have been playing with the triboelectric effect ever since, making huge tables of which
materials build charge the best.
Mostly, they’re electrical insulators, because that lets the charges build up in one place.
In a conductor, they’d spread out.
The moving charges themselves can be negatively charged electrons, or they can be ions — whole
atoms with an electric charge.
It just depends on what kinds of materials are involved and how they’re rubbed together.
Even though I’ll be talking in terms of electrons moving for the rest of the video,
everything works pretty much exactly the same if it’s ions moving around instead.
So here’s how you get zapped:
Some things, like polyester and rubber, are good at holding onto extra electrons.
Others, like wool, are good at giving electrons away.
When you shuffle your feet on the carpet, you rub an electron-receiver against an electron-giver,
so you get negatively charged shoes and a positively charged carpet.
Those extra electrons move up from your shoes onto your skin, since humans are good electrical conductors.
But metals are even better conductors, so the electrons jump toward a doorknob when
you go to grab it.
When that happens, a tiny lightning bolt goes between your hand and the doorknob.
The moving electrons heat the air between your hand and the knob, and you feel that
heat as the pain of an electric shock.
Shocks tend to be more of a problem in the winter than the summer, because the cold air
can’t hold as much water as warm air.
When it’s warm, the extra water in the air makes the air more conductive.
So on a humid day, any charge you build up leaks off into the wet air long before you
touch metal.
But even in the winter, there are things you can do to avoid getting shocked.
Wearing clothes that build up more charge leads to worse sparks, and so does lots of
rubbing or shuffling as you walk.
Switch to full, wide steps and wear lots of cotton, which hardly builds charge at all,
and the shocks won’t be as bad
You might get some weird looks waddling around like that,
but hey, at least it’ll hurt less.
You can also try touching things with something metal first, like a key, so electrons jump
from there, and the key gets hit with the burst of hot air instead of your hand.
But even though we mostly know how to stop the triboelectric effect from zapping you,
there’s a lot we still don’t know about why it happens in the first place.
For one thing, electrons repel each other, so any extras on one surface should make it
harder to pile more on there.
But the electrons build up anyway.
This problem took scientists decades to solve, and they’re still working out some of the
finer details.
It turns out that the electrons don’t just hop between surfaces as they rub together
— they jump because of friction.
A lot of the time, friction comes from chemical bonds that quickly form and break as two surfaces
slide past each other.
And chemical bonds often involve unequally shared electrons, where electrons spend more
time on one side of the bond than the other.
Those sides are more likely to have an extra electron or two when the bond breaks, so they
become the electron-receivers.
But since the electrons tend to stay with their molecules, they don’t really keep
other electrons from getting caught by nearby molecules, and the charge builds up.
We also don’t fully understand how the triboelectric effect works in thunderstorms.
We know the bottoms of thunderclouds tend to get negatively charged as ice crystals
and dust rub against each other, but we’re not sure why.
It’s possible that denser ice crystals tend to get more negatively charged, so the negative
charge follows them down as they sink to the base of the cloud.
Another option is that crystals charge differently depending on the temperature, and the temperatures
in different parts of a cloud lead to negative charge on the bottom.
Then there are the convection currents in the clouds, which might affect the process
as they mix the different kinds of crystals and other particles together.
All of these ideas probably play a role, but we don’t really know if one is more important
than the others.
Plus, sometimes positive charge accumulates on the bottom of clouds, which is even harder
to explain.
And it’s tough to study any of this directly, since flying planes in storm clouds is … not
very safe.
We’re stuck with mostly small-scale experiments that involve crashing ice crystals and water
droplets together in labs
So, we know that you get static shocks because of the triboelectric effect, and we know that
static shocks are just miniature lightning bolts.
But somewhere between shuffling feet on the carpet and rubbing ice crystals in clouds,
the physics gets a little more mysterious.
Thanks for watching this episode of SciShow!
If you’re interested in learning more about how we figured out electricity, you can check
out our episode about Benjamin Franklin, Founding Nerd — including what really happened with
that kite and key.