Physics under a woolen blanket

Upon learning about this column, a journalist friend commented: “- Write about what really matters: why does the blanket get hot?” It was one of the first cold nights of the year, and, when waking up in the morning on these days, the inertia of getting up seems to tend to infinity. It's hard to get out of bed, because, in addition to natural laziness, when we get up we feel cold. We immediately put on more clothes to keep warm. This trick only works because our body is a source of heat. Muscle fibers are continually contracting and relaxing, performing work (in the physical sense), while various sugars are being metabolized by the body, being broken down until they become lower energy by-products. Most of this work is immediately transformed into heat. The human body has a very sensitive thermostat, which keeps the blood temperature very close to 37ºC. This temperature is generally much higher than the ambient temperature (with the exception of some hellish days in high summer).

When the skin loses heat too quickly, we feel cold. This can happen on a winter day, in the air conditioning, or even on a pleasant day, but when we have a high fever. In these situations, the body responds with greater muscular activity (for example, with chills and tremors) and with the contraction of capillaries close to the skin. People who live in very cold places can adapt by adjusting their diet and the rate at which blood flows to the skin. Eskimos, for example, have a diet richer in protein than most people who live in lower latitudes to maintain a higher basal metabolism, ideal for combating the cold. On the contrary, when we feel hot or play sports, we sweat to increase the rate of heat transfer from the body to the environment.

The rate at which we lose heat depends on the amount of skin exposed to air currents, and it certainly increases when we are exposed to strong winds. And the colder the air, the faster the heat loss through this mechanism known as convection. But if there is no wind, or if it is very weak, most of our heat is lost through thermal radiation. Any object with a temperature above absolute zero radiates heat, and the hotter it is, the more it radiates. But this object can also absorb heat from the environment, at a rate that depends on the temperature of the environment. When going outside on a cold day, for example, radiation absorption decreases, leading to a net loss of heat through radiation. In addition to convection and radiation losses, we can also lose heat through conduction, as occurs when we step with our bare feet on a cold floor.

By wearing clothing or hibernating under a cozy blanket, we reduce air currents close to the skin, and thus reduce convective heat loss. Furthermore, cold weather blankets and coats have fibers that are intimately folded and facilitate the formation of stationary air bubbles inside them. The air that remains close to our skin for a few moments is heated, which causes the temperature variation between the air and the body to be smaller, also reducing the rate of heat loss through radiation. To top it off, it is also known that air has a very low thermal conductivity, which makes it even more difficult for the body to exchange heat with the environment (this principle is also used in windows in colder countries, as it keeps the environment more isolated thermally). This also explains why our mothers always recommend dressing like “onions” when going out on a cold day, wearing a certain number of layers of cotton and/or wool (or some other synthetic fiber) clothing. This way we increase the amount of air bubbles, and we stay warmer.

There are certainly other, more interesting ways to warm the body on a cold night. But in these cases, Physics alone is not enough to explain all the phenomena involved...

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