Last day of doing this daily! After today I’ll settle down to weekly like I planned.
Friction between pieces of sandpaper has an obvious source: Sandpaper is lumpy. But lumps and bumps, whether at centimeter or atomic scales, are the source of any friction between two surfaces that are rubbed together. When bumps of one encounter lumps of another, they catch. (Everything is also made of electric charges that fleetingly attract the charges of other surfaces. On really big scales, this sort of process helps make lightning. But that’s a story for another time.)
Overcoming friction means shoving atoms out of the way, jiggling nearby atoms in the process. So friction turns useful energy into useless energy—what physicists call entropy. Those two sentences say the same thing, by the way. One just focuses on atoms while the other is bigger-picture. But I think it’s easiest to understand friction if you switch zoom level mid-sentence: Friction turns kinetic energy (energy of large-scale movement) into heat (energy of atomic movement). The more heat you get, the less kinetic energy you keep—and kinetic energy is the energy we use to power things like engines.
Energy is constantly changing forms. That’s all it can do, dark energy aside. It can’t appear or disappear. But as we’ve already seen, some energy is useful and some isn’t. The second law of thermodynamics says that over time, we’ll run low on the former while accumulating the latter.
The universe, then, is like a cashier who only gives change in pennies. Sure, you might pay for something with exact change (you might find some process that turns 100% of useful energy into other useful energy), but honestly that’s pretty rare and never really happens if you have a complex bill. The rest of the time, you put in more energy (more money) than you need, and the change you get back is completely useless.
Admittedly, we should probably try to pay in exact change during a coin shortage. I’m sure there’s a metaphor for renewable energy in there somewhere.
Okay, so there’s the obvious route: Hot tubs are like bathtubs, where you’d usually use shampoo. But there’s not much science along that path. Let’s see if we can forge another one.
Hot tubs are so relaxing because heat loosens our muscles, which is a big-picture way of saying that blood vessels dilate when they’re warm—letting our bodies transport bad stuff out of muscles and good stuff into them. (That itself is a big-picture way of avoiding biology.) There’s probably some biochemistry that goes into dilating warm vessels (there definitely is), but there’s also some pretty straightforward physics. Something’s temperature is a measure of the internal kinetic energy of its atoms and molecules: The hotter the object, the more its molecules wiggle and jiggle. Cold things tend to be rigid because their molecules don’t have the energy they’d need to adjust their positions much. Warmer things loosen up, and they also tend to expand: their molecules can spread out now that they have the energy to push their surroundings out of the way. The same must be true for the walls of blood vessels.
This is fundamentally why we clean with hot water, too. It gives grimy molecules the energy they need to break free from whatever they’re affixed to. An awful lot of kinds of molecules dissolve in water, so anything loosened by the heat tends to be carried away by the water. Washing with soap and hot water is even better—at least as far as dirt is concerned. Soap molecules either bond with or otherwise break into bits of oil or dirt, and it’s easier of molecules to react and rearrange when they have a lot of energy.
Ridding any surface of oils and dirt is the central job of any soap. Soaps get specialized because we might not care very much if a soap is designed to mount an all-out attack on a plate (ceramic can handle it), but we’d probably care more about a soap that does such a good job cleaning our hair that it destroys the hair in the process. That’s why we use gentler stuff on our hair: Shampoo might not be quite as good at breaking bonds as dish soap, but the trade-off is worth it for something made to be gentle. We also want stuff in our hair that washes away easily, and shampoo is designed to be just that.