Storm in a Teacup: The Physics of Everyday Life(72)
But it’s the other situation, the one where the electrons can’t move, that gives us our spectacular control of electricity. If the bee settles on a plastic plant pot, the positive charge can’t move into the plastic because plastic is an electrical insulator. That means that even though the plastic has lots of electrons of its own, they’re tightly bound to its molecules and they can’t move. It’s hard to add or subtract a few extra electrons to or from the mix, because they can’t sneak in among the others. This is what defines an electrical insulator—it has no capacity to take on or give up a few extra electrons. So when a bee lands on a plastic plant pot, the positive charge stays put on the bee. A metal garden fork would rob the bee of its charge immediately; metals are electrical conductors and electrons can shift about inside them very easily. The reason metal behaves like this is that all its atoms share their outer electrons in a giant surrounding mob. Since these electrons are moving about all the time and none of them belongs to any particular atom, it’s easy to add or subtract a few.
Our society can only have and control an electrical grid because we have both types of materials, conductors and insulators. That’s all you need: a mosaic of materials that is really just a maze for electrons in which some paths are easier than others, and a way of controlling some parts of that pattern. Once you’ve got those basics, you have amazing control over the world.
STATIC ELECTRICITY IS a start, but the real power comes when you start to move electrons and electrical charges more systematically. Our electrical grid, the network we use to move energy around, is an astonishing resource. By pushing electrical charge down wires, and controlling it by using tiny switches and amplifiers, we can deposit energy wherever we need it. An electrical circuit is just a way of redistributing electrical energy. The most important thing about a circuit is that it is just that—a circuit. It has to be a loop, so that electrons can keep shuffling around it without building up at the far end. Every circuit must begin and end at a power supply, something that will keep the electrons on the move, taking them in at one end, pushing them along, and putting them back into the circuit at the other end. The power supply is a bit like an elevator that carries people up to the top of a very long slide. The people can go down the slide and up to the top again, around and around all day, as long as there’s an elevator to give them enough energy to get back to the start. The rule of every circuit is that you have to lose all the extra energy from the power supply before the electrons get back to where they started.
An electron shuffling along a wire is all well and good, but what’s pushing it around the circuit? We’ve said that the first thing is to have an electrical conductor, something that provides a path down which an electron can move. But the other thing you need is a force to push it with.
A fridge magnet and a balloon charged with static electricity are both weird for the same reason: They show that it’s possible to have an invisible force field. That is, one stationary object is pushing or pulling on another one nearby, but you can’t see what’s doing the pushing. This similarity isn’t accidental, but the real link is only obvious once you start moving the electrical or magnetic fields around. First, let’s go back to that principle of a force field. It’s not just humans that can make use of them.
The stream bed is a murky brown maze of rocks, plants, and tree roots. It’s dusk and the muddy water is flowing lazily through and over the obstacle course. A yard below the surface, two small antennae are poking out from beneath a pebble, twitching as they taste the water. Something moves nearby and the antennae vanish. This freshwater shrimp is a scavenger, hungry but vulnerable. Upstream, a hunter slides into the dark water. It paddles along the surface toward the center of the stream with two webbed front feet, then shuts its eyes, closes its nose, seals its ears, and dives. The platypus is ready for dinner.
If the shrimp stays perfectly still, it will be safe. The platypus swims quickly, picking its way confidently through the maze even though it’s currently blind, deaf, and unable to smell anything. Its flat bill sweeps from side to side, scanning the mud. Another foraging shrimp feels the water move as the platypus approaches and snaps its tail, jerking backward into the gravel. The hunter swerves toward it. The signal forcing the tail muscle of the shrimp to contract was an electric one. That electric pulse created a temporary electric field centered on the shrimp. This electric disturbance flashed through the surrounding water, exerting tiny pushes and pulls on nearby electrons. It lasted for a fraction of a second, but it was enough. A platypus has an array of forty thousand electrosensors on the upper and lower surfaces of its bill. The simultaneous water movement and electric pulse were all it needed to get a direction and range. The bill hammers into the sand in exactly the right place, and the shrimp is no more.
The shrimp’s movement condemned it because the act of moving changed its electric field. Every electric charge pulls or pushes on other electric charges around it. An electric field is just a way of describing how strong that push or pull is in different places, while talking about electric signals means that an electric charge moved somewhere, and something nearby noticed the change because the push on it increased or decreased. Since all muscle movements involve moving electric charges around inside the muscles, they all generate electric fields. So electrosensing is a reliable hunting technique underwater if you’re close enough to your prey, because no amount of colorful camouflage can disguise an electric signal. Any animal has to move eventually, and the tiniest motion will generate an electric signal that can give it away.