Storm in a Teacup: The Physics of Everyday Life(73)
If that’s the case, why aren’t we more aware of the electric fields that we generate ourselves? It’s partly because those fields aren’t very strong, but mostly because electric fields decay quickly in air, which doesn’t conduct electricity. Stream water (and especially salty ocean water) is a very good conductor of electricity, so electric signals can be detected from much farther away. Almost all the species that use electrosensing are aquatic (bees, echidna, and cockroaches are the known exceptions).
In an electric circuit, the electrons move because there’s an electric field inside the wire. That electric field is pushing on each electron, shoving it along. But where does the electric field come from? A good place to start is with a battery. Batteries come in all shapes and sizes, but there is one set in particular that I will never forget. They were chunky sea batteries, and I worried about them because they were floating free in a giant storm, powering my one shot at an important experiment.
To study the physics of the ocean surface in storms, we need to go and look at that surface. The ocean is such a complicated environment that theorizing from a nice warm office is of limited use unless you’re sure that what you’re working on is definitely based on reality. But even when you get “there,” on a ship miles from shore in rough seas, it’s still difficult to touch the region I’m interested in—the water just a few yards below the sea surface. Knowing what happens there will improve our understanding of how the oceans breathe, and will contribute to better weather forecasts and climate models. But to see the details, you need to be in it; and it’s a violent, messy, dangerous place to be. I can’t swim in that water, but my experiments have to. The experiments need power, an electricity supply, and they need it while they’re bobbing up and down in the waves, free of the ship. You can’t plug them in, so you have to rely on batteries. And fortunately for me, electrical circuits work just as well when they’re bobbing up and down as they do when on dry land.
THE BOSUN SCOWLED at the horizon, stuffed his hands deep into the pockets of his paint-splattered hoodie and swayed along the deck of the ship toward me. It was November in the North Atlantic, and I hadn’t seen land in four weeks. Everything was always either going up or going down as we clung to a heaving gray sea that merged with gray sky in every direction. The roll of electrical tape I’d just put down on the deck took advantage of my temporary distraction and skidded across the deck until it met the bosun’s boot. His thick, cheery Boston accent seemed comically out of place in this forbidding environment. “How long you gonna be?”
For me, the worst bit about doing experiments at sea has always been these final checks before letting the experiments float free. I was nervous, and this bit was my responsibility alone. To measure the bubbles just beneath the breaking waves, I was using a large yellow buoy with a variety of measurement devices strapped to it. The bosun was in charge of maneuvering this beast off the ship and into the rolling sea, but I had to make sure that it was ready. The storm that was coming would be a big one, and I desperately wanted good data from it. “I’m just about to plug in the batteries, and then I’m ready to go,” I said. The monstrous yellow buoy, 36 feet long, that carried my experiments was strapped to the deck, shackled securely until it was safe to release it. I started with the armored camera near the top and put my hand on the power connector, following the wire all the way down to the bottom of the buoy where the chunky batteries were and then plugging it in. Then back up to the acoustical resonators. Hand on the power cable, follow it down to the batteries, plug it in. Check that the connection is secure. Check again. Back again to the other camera. These experiments could carry out incredibly delicate and sophisticated manipulation of the physical world, but only when provided with electrical energy. And the providers were four cumbersome lead-acid sea batteries that weighed 88 pounds each, and whose basic design hadn’t really changed since they were invented in 1859. But they worked.
When it was time, we scientists huddled in our oilskins at the far end of the deck and the crew and crane took over, maneuvering the swaying monster over the side and into the dark ocean. As the last rope slipped free, there was a weird shift of perspective, and the huge yellow beast became a vulnerable bobbing piece of flotsam, tiny compared to the vast ocean and frequently hidden by the waves. A burst of chatter spread along the ship’s rail about how the buoy was sitting in the water and the speed at which it was drifting away from the ship. But I wasn’t thinking about any of that. I was thinking about electrons.
Below the waterline, the dance of the electrons had started. They were shuffling out of the battery, around the circuits carried by the buoy and then back into the other side of the battery. There were a fixed number of electrons held in the circuit, all just going around the same loop. The electrons don’t get used up—they just go around and around. The trick is that it takes energy to push them around, and they give that energy away as they travel. The source of that energy is the battery, and a battery is a very ingenious device.
The clever thing about batteries is that they join up a chain of events. Each link in the chain supplies the electrons that the next link needs; and so once a battery is connected to a circuit, everything is in place for electrons to flow around the loop. These sea batteries had two terminals sticking out to connect them to the outside world. Inside, each terminal was connected to one of two sheets of lead, but those two sheets weren’t touching. The space in between the lead sheets was full of acid, which is why they’re called lead-acid batteries. There are two ways in which the lead can react with the acid. There’s one that needs some extra electrons from somewhere, and there’s another one that gives away extra electrons. A lead-acid battery is charged when those two reactions have been pushed as far as they can go.