Storm in a Teacup: The Physics of Everyday Life(71)
Why didn’t they escape? Each one of those extra negatively charged electrons was being repelled by the others—any route away would be better than staying put. But my boots stopped them from leaving via the floor. There is another common escape route: moist air. Humid air contains lots of water molecules, each with a positive segment that could host an extra electron for a while. Most days, my extra flock of electrons would have escaped one by one, as they hitched a ride with floating water. But cold days after a heavy snowfall are often dry. There is very little water in the air, so the air offered no way out.
And so, every dry, snowy day, I’d walk down the path from the cottage to my car, completely unaware of the billions of negatively charged passengers, at least until their opportunity knocked. My car sat on the ground, a vast reservoir of balanced electrons and nuclei. The split second when my bare fingers first made contact with the metal of the car was like the opening of an escape tunnel. Metal is an electrical conductor, so electrons can flow through it very easily. My electron passengers surged through the skin of my finger tip, finally free when they met the car. The nerve endings in the skin jangled as the mob whooshed past, directly stimulated by the flow of electrons: an electric current. And I would curse, the magic of the snow temporarily forgotten.
These days, an electric shock is the most direct experience of electricity that most of us have. And yet we’re surrounded by the stuff. The walls of our buildings, our electronic devices, our cars and lights and clocks and fans are all buzzing with it. But electricity isn’t just about plugs and wires, circuits and fuses. Those are just the crude trophies advertising the human manipulation of this phenomenon. Our planet is humming with electricity in lots of surprising places. Even the humble bee is in on the act.
Imagine a warm, peaceful, lazy day in a very English garden, with a chaffinch pecking fussily at the edge of the lawn. Behind him, jaunty ranks of flowers are engaged in a slow but fierce battle for water, nutrients, sunlight, and the attention of pollinators. The scent of jasmine and sweet peas is drifting across the grass, advertising their wares. A honeybee buzzes along the flowerbed, inspecting what’s on offer. This may look like a relaxed scene, but for the bee, this is hard work and efficiency matters. It’s costing her an enormous effort to stay in the air. She has to flap her tiny wings two hundred times every single second, and the constant pummeling of the air is so powerful that it sends out vibrations we can hear: the buzzing. If you’re the size of a bee, air resistance is much greater than it is for us, so it’s much harder to push past and through all those air molecules. Thumping the air like this is not an elegant way to fly. But it works, and she hovers for a second next to a pink petunia before deciding that this will be her next stop. As she’s flying in, but just before she touches the flower, something very odd happens. Pollen grains that were sitting in the center of the flower suddenly hop across the air gap to the bee’s fur. And as she settles on the flower, more pollen settles on her. She hasn’t taken a single sip of nectar yet, but she’s wearing a coat of the plant’s DNA, and it’s almost as though it’s deliberately jumping on board.
It turns out that flying makes a bee very attractive, quite literally. It’s not because of her appearance or behavior. It’s because our bee is electrically charged, although only very slightly. Just like my electric shock, it’s because some electrons have shuffled themselves around. But this time, no one’s getting hurt.
The bee’s own electrons are hovering around the edges of each molecule in the bee’s wings. If something is rushing past the bee very quickly (air, for example) and anything is going to get knocked off, it’s going to be an electron. And this is what happens. It’s the same as rubbing a balloon against a wool sweater—static electricity builds up, which just means that something has more or fewer electrons than it should. As those frantic wings were shoving air molecules out of the way, electrons from the wings were being rubbed off and were floating out into the air. The flying bee was left with a slight positive charge because it no longer had enough electrons to cancel out the positive charge of all the protons in its atoms. It’s small, though, certainly not enough to give a human an electric shock.
As the bee approaches the flower, it attracts negatively charged electrons to the surface, and repels the positive charges. Just as the north pole of a magnet tugs its opposite (magnetic south poles) closer to it, so a positively charged bee tugs on negatively charged electrons. When it’s very close to but not yet touching the flower, the positive charge of the bee pulls the surface of the pollen hard enough to tug a few grains off the flower, across the gap, and onto the bee. Then the pollen sticks to the bee’s fur, just like a statically charged balloon sticking to a wall. When the bee flies on to the next flower, that pollen will travel with it. Pollination by bees would work without the static electricity, just because the bee’s fur will touch the pollen when the bee lands on the flower, and the pollen will cling to the fur because it’s sticky. But the shifting of a few loose electrons so that pollen can jump the gap definitely gives it a boost.§ Electrons are tiny and mobile, and so when an electrical charge moves, it’s usually the electrons that are providing the transport. They move around quite a lot, but we don’t usually notice. Negatively charged electrons will repel each other, so if lots of them accumulate in one place, they’ll push each other away and drift apart. A significant charge never builds up. But there are two possible situations that can stop the drifting and trap the charge: Either the electrons have nowhere to go, or they can’t move. When the bee is flying, the positive charge indeed has nowhere to go, so it builds up on the outside of the bee’s body.