Evaporative cooling

- [Voiceover] So if you are like most of us, your body probably sweats when it is warm, when your environment is warm, and you probably realize that it sweats in order to cool itself, in order to keep the body from overheating. But you probably have wondered, "Well, how does this work? "What is actually causing that?" And the simple answer is it's happening-- or what's allowing your body to cool-- the chemical process, I guess you could say the physical process is happening is evaporative cooling. Evaporative cooling. Which is really the notion that as that water, those beats of sweat vaporize, it's actually gonna cool your arm down. But that begs the question, how does that actually happen? And so let's just visualize it a little bit. Let's just say that is your arm, so this is your arm right over here, so let's... I don't draw my best... Draw a quick arm right over here, so okay, that's your arm, and it's got beads of sweat. Let's say it's a hot environment it's got beads of sweat right over here. And if we were to zoom in on that sweat, if we were to zoom in on that sweat, we would see the constituent water molecules and sweat is mainly H20. It is mainly water. Now when we talk about the temperature of something, we're talking about the average kinetic energy. Each of the individual molecules, they all have different kinetic energies. They're all bouncing around in different ways and transferring the momentum in all different ways. And so you can imagine a reality. Maybe this one has a fairly high kinetic energy. It's moving in that direction. This one has a lower kinetic energy. moving in this direction. Maybe this one has a medium kinetic energy, moving in this direction. Maybe this one has a really high kinetic energy moving in that direction. And so we've already talked about how hydrogen bonds in water between the partially negative end and the partially positive ends. That's what keeps the water together as these things move past and flow past each other. What gives the water its cohesion is these hydrogen bonds. But if all of a sudden-- remember, we're talking about the average kinetic energy-- but even if we're at room temperature, and the average kinetic energy isn't so hot, you might have individual particles, individual molecules that actually have quite a high kinetic energy and if they're in the right place, if they're near the surface and their kinetic energy is high enough to break the hydrogen bonds with neighboring water molecules, and to overcome the pressure in the atmosphere, so let's say that this is, these are just gas molecules in the atmosphere here. But it's enough to break free and none of these things bounce into it and force it back to form hydrogen bonds. This thing could actually break free and enter and become water vapor. And become in its gaseous state. And it'll be so far apart from other water molecules that it won't form hydrogen bonds anymore. So by vaporizing or by this process of evaporation, what's happening? Well if your highest kinetic energy particles or some of your highest kinetic energy particles are able to escape, what's going to happen to the average kinetic energy? Well as the highest kinetic energy things escape and those are the ones that are most likely to escape, well then your average kinetic energy is going to go down. So average kinetic energy is going to go down. Or another way of saying it, is that your temperature is going to go down. Your temperature is going to go down because as these molecules turn into water vapor, they're going to be the highest kinetic energy, energy is transferred to them, and then they escape. And so what's left over is going to have a lower average kinetic energy. And you're saying, "Well, how does that "actually cool down my hand?" Well, your hand is made up of molecules as well. So let's say this is the surface of your hand, those are the molecules, they have some average kinetic energy, they are kind of vibrating in place, especially if we're talking about they're... they're a solid. And so maybe I'll draw the more, you know, they're vibrating like this, they're bonded to each other in some way. I won't go into the details of what types of molecules these are, but then if you have your water molecules here, water molecules that are sitting on the surface, and I'm drawing this is kind of a cross-section. Let me draw the water molecules. I'll draw them as blue molecules. So this is an H2O right over here. H2O. This is an H2O. And this is an H2O. And they have some hydrogen bonding, so there is some hydrogen bonding going on. Well, as the high kinetic energy water molecules escape, I'll say this one right over here escapes, and so the average kinetic energy of what's left over is lower, so then the temperature has gone down, and now your body molecules, the ones that are all warmed up, and because of whatever's going on inside of your body, well, those can now bump into, they can vibrate and bump into these water molecules and increase their kinetic energy more than the ones that have the most kinetic energy. Those might escape again. And so it's a--one way of thinking about it is that all that heat is being used to allow these individual water molecules to escape in order to vaporize. And so that heat is leaving your body, so it allows you to cool down. Cooling down happens by heat actually leaving. So that's how evaporative... I wrote evaporative cool... That's how evaporative cooling... That's how evaporative cooling actually works.