Storm in a Teacup: The Physics of Everyday Life(9)

As our elephant and her family group approach a watering hole, the “still” air around them is bumping and jostling just like anywhere else, hammering against their wrinkly gray skin, against the ground, and against the water surface. The matriarch is slightly ahead of the group, swinging her trunk as she saunters into the pool and sends ripples through her reflection. She dips her trunk into the water, closes her mouth, and the huge muscles around her chest lift and expand her ribcage. As her lungs expand, the air molecules inside spread out to take up the new space. But that means that right down inside the tip of the trunk, where the cool water touches the air in her nostrils, there are fewer air molecules hitting the water. The ones that are there are going just as fast, but there aren’t as many collisions. The consequence is that the pressure inside her lungs has dropped. Now the atmosphere is winning in the shoving contest between the air molecules hitting the pool of water and the air molecules inside the matriarch. The push from inside can’t match the push from outside anymore, and the water is just the stuff in the middle of the competition. So the atmosphere pushes water up the elephant’s trunk, because what’s inside can’t push back. Once the water has taken up some of the extra space, the air molecules inside are as close together as they were at the start, and the water doesn’t move any farther.

Elephants can’t drink through their trunks—if they tried, they’d cough just as you would if you tried to drink through your nose. So once the matriarch has perhaps 8? quarts of water inside her trunk, she stops expanding her ribcage. Curling her trunk up and under, she points the tip into her mouth. Then she uses her chest muscles to squash her chest, decreasing the size of her lungs. As the air molecules inside are squashed closer together, the water surface halfway up her trunk gets hit much more often. The battle of the air inside and the air outside reverses, and the water is pushed out of the trunk into the elephant’s mouth. Our matriarch is controlling the volume of her lungs to control how hard the air inside her is pushing on the outside. If she shuts her mouth, the only place where anything can move is at her trunk, and whatever is at the tip of her trunk will get pushed in or pushed out. An elephant’s trunk and lungs together are a combined tool for manipulating air so that the air, rather than the elephant herself, does the pushing.

We do the same thing when we suck a drink up a straw.§ As we expand our lungs, the air inside is spread more thinly. There are fewer air molecules inside the straw to push on the water surface. And so the atmosphere pushing on the rest of the drink pushes the drink up the straw. We call this sucking, but we’re not pulling on the drink. The atmosphere is pushing it up the straw, doing the work for us. Even something as heavy as water can be shunted about if the hammering of the air molecules is harder on one side than the other.

However, sucking air up a trunk or a straw has limits. The bigger the pressure difference between the two ends, the harder the push will be. But the biggest difference you can possibly make when you’re sucking is the difference between the pressure of the atmosphere and zero. Even with a perfect vacuum pump instead of lungs, you couldn’t drink through a vertical straw that was longer than 33 feet, because our atmosphere can’t push water any higher than that. So to exploit to the full the ability of gas molecules to push things around, you need to get them working at higher pressures. The atmosphere can push hard, but if you force another gas to be hotter and put it under greater pressure, it can push harder. Get enough tiny gas molecules hitting something often enough and fast enough, and you can move a civilization.

A steam locomotive is a dragon made of iron, a hissing, breathing, muscular beast. Less than a century ago these dragons were everywhere, hauling the products of industry and the needs of society across whole countries and expanding the world of their passengers. They were mundane and noisy and polluting, but they were beautiful pieces of engineering. When they became obsolete, the dragons were not allowed to die because society just couldn’t let them go. They’ve been kept alive by volunteers, enthusiasts, and a deep well of affection. I grew up in the north of England, and so my childhood years were steeped in the history of the Industrial Revolution: mills, canals, factories, and, more than anything else, steam. But I live in London now, and so it’s easy to forget. A trip along the Bluebell steam railway with my sister brought it all back.

It was a chilly winter day, absolutely perfect for a journey propelled by steam with the promise of tea and scones at the other end. We didn’t spend too long at the station where we started, but when we arrived at Sheffield Park, we stepped off the train into a slow but steady hum of activity. The engines were constantly being tended by an ever-changing swarm of humans who seemed tiny beside their iron beasts. The humans involved with the engines were easy to identify: blue overalls, peaked caps, a jolly demeanor, an optional beard, and in between engine-tending duties usually to be found leaning on something. As my sister pointed out, an awful lot of them seemed to be called Dave. The beauty of a steam engine is that the principle behind it is fantastically simple, but the raw power produced needs to be goaded, tamed, and nurtured. A steam engine and its humans are a team.

Standing on the ground, looking up at one large, black engine, it was hard to comprehend that in its heart, this was basically a furnace on wheels heating a giant kettle. One of the Daves invited us into the cab. We climbed up the ladder just behind the engine and found ourselves in a grotto full of brass levers, dials, and pipes. There were also two white enamel mugs and a sandwich tucked behind one of the pipes. But the best thing about the cab was that we could see right into the belly of the beast. The giant furnace at the heart of a steam engine is filled with fiery coals burning an intense yellow. The fireman gave me a shovel and told me to feed it, and so I obediently scooped coal from the tender behind me into the glowing mouth. The engine is hungry. On one 11-mile journey, it will burn through 1,100 pounds of coal. That half-ton of solid black gold is converted into gas—carbon dioxide and water; and the burning releases enormous amounts of energy, so those gases are extremely hot. This is the start of the energy conversion that powers the train.