Explaining the Mundane I

I started my undergraduate career set on getting a business degree from a local college and working as a branch manager somewhere. Then, I changed paths and decided I wanted to enter medical school and become a doctor. Like most 18-year-olds, I had no idea what I really wanted. As I started taking more science courses, I soon realized I had discovered my calling.

One appealing aspect of science and engineering was how seemingly mundane occurrences could be explained by scientific concepts that apply to a wide range of phenomena. I could wonder about the tiniest details of everyday life and try to use my knowledge of physics to explain how stuff works. I learned a lot this way and enjoyed testing my physical intuition. I’ve gathered a few of my favorite mundane events that are explained by seemingly-esoteric (but ultimately intuitive) scientific concepts.

Why does it feel cold when I blow on my hand with pursed lips, but warm when I blow on my hand with mouth agape?

A friend actually asked this question, and I recalled a concept that I learned about in my first college physics courses. Our professor was explaining the wind chill factor, which is how the outside temperature feels due to the wind but has nothing to do with the actual ambient temperature. A human body is constantly trying to maintain a temperature of about 98oF. A thin layer of air surrounding the body will constantly exchange heat with the body, which makes the surrounding air feel warmer. In other words, this layer of air is insulating you from the heat. However, a strong gust of wind may snatch your insulating air bubble away, and the wind chill factor attempts to account for how cold this may feel. The windier and colder it is, the worse your body is at keeping warm air around it.

Initially, this was my first thought as to why it would feel colder when blowing with pursed lips. The temperature of your breath is the same no matter how you exhale; the body is a hard-working machine that maintains the same temperature of every component – your breath included. So, your breath is leaving your mouth faster and taking warm air away from your hand when your lips are pursed – just like with the wind chill effect! There is another effect that plays a large role in this process though. It’s referred to as entrainment.

Entrainment occurs because the air from your mouth, which consists of very small molecules, interacts with the ambient air, which consists of similar small molecules. The flowing breath bumps into the surrounding air and friction between the molecules causes the fast-travelling warm air to drag the cooler ambient air. If you exhale a thin, fast jet of air, a relatively larger amount atmospheric molecules will hitch a ride and your hand will feel a greater effect from the cooler air. If you puff out a wide plume, you’ll mostly feel the large stream of warm breath. This same concept is the basis for how Eductor jet pumps work, which are used to cool nuclear reactors among other things. If you’re not convinced, you can try an experiment. Take a piece of paper and roll it up into a tube. Cover one end of the tube with your hand and blow into the other at variable speeds. How does the temperature change with speed/volume? It shouldn’t change much, since the tube should not let very much surrounding air become entrained!

How does entropy explain why wires (like headphones), when left on their own, get tangled up?

The concept of entropy is esoteric. In high school chemistry, it’s described as randomness. Sometime it’s referred to as disorder. In statistical thermodynamics, it is thought to represent the number of states accessible by a system. In a way, this relates to randomness and disorder. If flip a coin three times, the possible outcomes (“states accessible”) could be HHH, TTT, HHT, HTH, THH, HTT, THT, or TTH. The more times you flip the coin, the more possible outcomes there are. You could also see the range of possibilities being more disordered or more random.

The configuration of a wire is much more complex than flipping a coin. You could imagine many ways a wire could be arranged: a long straight line, folded into layers, a spiral, tangled into a ball…The possibilities would take many lifetimes to describe. Consider the wire tangled into a ball. If I take some tweezers and pull ever-so-slightly on a strand of the wire, I have just created a new possible arrangement!

The tangled state has a large number of accessible outcomes, each of which is similarly probable to form as another. Let’s go back to the coin analogy. If we now flip the coin 10 times, there would be two extremely “ordered” possibilities: HHHHHHHHHH and TTTTTTTTTT. In terms of the wire, this could be thought of as an ordered, linear configuration. There are 1022 other possibilities for the coin flips, and an insurmountable number of other possibilities for the wire. By using simple probability, it is much more likely that a coin flipped 10 times will give multiple heads and tails, and a wire will more likely end up in a tangled state. Of course, there needs to be some mechanism to form tangles in the first place, but the entropy of wires has been widely studied (as mentioned in my IgNobel post). Scientists at UCSD did their own experiments and measured how ropes tangle by being shook in confined spaces.

I’ll try to make “Explaining the Mundane” a regular segment, since I have many other similarly piquing thoughts jostling in my mind.


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