The Aha! Moment: A Scientist's Take on Creativity

6 Heat and Gravity

100 6 Heat and Gravity T he mind of a creative person is never idle. Consider James Prescott Joule on his Swiss honeymoon in 1847. His young bride did not distract him from scientific thought. Indeed, he took a sensitive thermometer with them. And whenever the couple came across a river waterfall, he measured the temperature of the water at the top and at the bottom. The mechanical equivalent of heat was brewing in his mind, and waterfalls could be part of the evidence. On reasonable assumptions about the flow, the water should get hotter by 0.00239 degrees Celsius for every meter it fell. The rivers seemed to support his theory. He ultimately determined that equivalent, took the idea further, and the unit of energy is named after him. Whatever her private misgivings, his new wife went along with her scientific husband. I know the feeling Joule must have experienced—a thinker is never off duty. In this chapter I recount some of my own encounters with heat and gravity. Must Heat Rise? Heat rises. This unavoidable truth governs all fluid flow. It drives meteorology and oceanography and bedevils the reaching a comfortable temperature in our own homes—as hot air goes up and mainly heats the ceilings. For like almost everything, hot gas expands on heating and rises. Is there a way out? I recall from my schooldays the strange reversible reaction between the gases N2 O4 and NO2 : N2 O4 ⫽ 2NO2 Heat and Gravity 101 This is an equilibrium. It can go either way. In this case, heat drives the reaction to the right, as a chemist would expect. A big gaseous molecule breaks up into two small ones. On heating, gaseous N2 O4 gets lighter than cold gas even faster than usual. It is even more convective than air. Might there be a reaction that went the other way? If so, you might make air nonconvective, and stop heat rising! My RIG liked the idea of nonconvective air so much that Daedalus claimed (falsely, I fear) that you can make it nonconvective by putting a little methyl formate in it.1 Despite this absurdity, you can indeed muck about with density to stop heat rising. One such trick is the “solar pond.” Sunlight shines into a big shallow lake. Light goes through clear water with very little loss, so the brownish bottom of the lake absorbs most of the sunlight. Then you throw salt in. The salt sinks to the bottom and dissolves there. The resulting solution has a concentration-gradient: strong at the bottom (where the solution has a high density) and weak at the top. With good design, this stratified liquid does not convect. If you don’t stir the pond, the gradient can last for years. So the energy of the sunlight absorbed by the bottom just stays there. Static salt water is a very bad conductor of heat, so a high bottom temperature, approaching 100°C, can build up and pipework can extract the heat. Daedalus once adopted the solar-pond notion to work with heavy gases. He planned to use it to generate solar power.2 And I sometimes play with gas-densities high and low, quite without asking what use the results will be. Thus while playing with amateur balloons, I discovered that the air in a room is highly sensitive to very small changes of temperature. It slowly circulates thermally, rising above a heater and sinking next to a cold wall. My balloons were plastic bags and condoms, filled with domestic gas (which is methane, and buoyant in air). I tied each one with a string and weighted it to be about neutrally buoyant. Each balloon then floated stably in the air. In a room with a fire, my balloons followed the slow thermal circulation of the air. Near the fire, they rose slowly toward the ceiling but just failed to touch it. They drifted across the ceiling and down the cold opposite wall but just failed to ground on the floor. Instead, they followed the slow room-air circulation back to the source of heat. Yorkshire Television Ltd. filmed a polished version of my domestic experiment. We didn’t use bags or condoms; nor did we fill our balloons with methane 102 The Aha! Moment gas. We used Mylar balloons, weighted with a sticky flexible plastic called Blu-Tack and filled with air and helium. So...