LeBron Asks: Why does sweating cool you down? LeBron asks Sal why sweating helps cool the body down.
LeBron Asks: Why does sweating cool you down?
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- Why does sweating cool you down?
- That is an excellent question LeBron,
- and to answer it, let's zoom in on a little droplet of sweat.
- And sweat is mostly water, so when we zoom in, and we've really zoomed in,
- even more than I've drawn over here.
- When we really zoom in, we'll start seeing mainly these water molecules.
- And the water molecules, just to be a little more accurate,
- I've drawn the oxygen in blue, and then the hydrogens that are bonded with that oxygen I've done in white.
- We all know that water is sometimes refered to as H2 -- that's for the two hydrogens -- H20.
- So each of these are a molecule of H20, or a molecule of water.
- What I've drawn down here, and this is an oversimplification of the molecules of your skin,
- but just for simplification, these are molecules of your skin.
- really, the parts of the skin cells. These aren't even the skin cells themselves,
- these are the molecules that make up the skin cells.
- And right over here, these are molecules of sweat, or it's really just molecules of water.
- So the question of why does sweat cool you down could really be restated as:
- Why does having water on the surface of your skin actually cool you down?
- And to answer that, or to think about that question,
- we have to think about what it means to have temperature, or what temperature even really means.
- Temperature, what we perceive as temperature, is really just the motion of the molecules of something.
- So higher temperature means that they're moving around more.
- So high temperature they're moving around more, and low temperature they're moving around less.
- And they can move around in different ways,
- they can have translational motion, which is they're actually moving around.
- They could be vibrating. They could be rotating in some way.
- And in general, on average, the more of this motional energy, often referred to as kinetic energy,
- that these molecules have on average, the higher the perceived temperature would be.
- Now, how does having this water here cool down the skin?
- And well, first of all, why is the skin warming up?
- Because the muscles are doing all of this work. They're releasing heat.
- That heat is being transferred to the skin.
- But then how does having this water here help?
- Well, the skin has a certain temperature, a certain kinetic energy, or motional energy.
- But when we say that it doesn't mean that all of these molecules have the exact same motion.
- The temperature is the average motion. Some of these are bumping around at a faster speed.
- Or vibrating at a faster speed, or rotating at a faster speed
- Some are doing it at a slower speed.
- But as these bump around, they're going to bump into these water molecules and get them moving around.
- They would probably be moving around a little bit to begin with,
- but then the warmer this is, the more energy here, they'll bump into these molecules.
- So let's say this guy bumps into that, then he'll bump over there ,
- so that energy, this bumping energy, or this kinetic energy,
- well, some of it will be transferred, or you could even say some of that temperature,
- some of that heat will be transferred to these water molecules.
- But the important thing to remember is this is a really kind of crazy thing, they're all bumping into eachother
- and rotating in all sorts of crazy ways.
- They will have an average kinetic energy, which we perceive as temperature,
- but this one might be going really, really, really fast in that direction,
- while this one might be going really, really, really slow,
- this one might be going really really really fast in that direction
- this one might be going really slow in this direction
- So the thing to think about is, given that you have all of this variation in the energy of each of these articles,
- which of these are most likely to escape, to actually evaporate?
- And to think about evaporation, you just have to think about that most water molecules
- or the water molecules that are in that droplet
- they do have an attraction to eachother, we call those hydrogen bonds.
- They do have an attraction to eachother,
- that's why a droplet kind of sticks together.
- But if one of these molecules is moving fast enough and if it's moving in the right direction
- it has a higher probability of being able to escape,
- being able to actually escape that droplet.
- And the process of these molecules actually escaping, that's what we refer to as evaporation.
- If a molecule has enough energy it will escape this, escape the bonds of the other water molecules,
- and just evaporate into the air.
- But we still haven't fully answered our question.
- So let's say that this is one that has evaporated, it has fully escaped.
- Why would that actually cool down this entire system?
- Why would it cool down the droplet and essentially give it more capability to accept more energy from the skin?
- Well, we just said the ones that have the highest energy are the ones that are most likely to escape,
- the ones that have the highest kinetic energy.
- So if you have a bunch of stuff, some are fast, some are slow, some are vibrating a lot, vibrating less
- but the ones that have a high kinetic energy are the most likely to escape, what happens when they escape?
- Well then the average kinetic energy will go down.
- Or you could say the temperature would go down,
- which is really just the average amount of motion or kinetic energy that's in this droplet.
- If the really fast ones, the ones with a lot of energy, are leaving, then the ones are left over, on average,
- are going to have a lower kinetic energy, or a lower temperature.
- And so that is what is cooling you down at a molecular level.
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