[SOUND] So far, we've been talking about taking this unit of energy and putting all of it into the motion of one object, this ten kilogram cement block. That's not usually what happens. We're usually heating up a collection of molecules. And this is where a very important concept comes in. And that's the concept of temperature. Molecules moving much, much faster mean that they're hotter. If I put my hand over the flame, those products, the carbon dioxide and water vapor are moving very fast and they hit my hand. And they cause the molecules in my skin to vibrate faster. And of course, that tells my nerve endings, you've got your hand in a flame! Right? And it's hot. Move it [LAUGH]. If not, you're going to get burned, because those molecules in my hand will start moving so fast, they'll start changing state, charring, turning into a different chemical compound. So the motion of molecules we can think of on average as a temperature. And that, on average, is very important. Because what temperature means is it's a velocity, a speed distribution. Just about everything in nature falls into a distribution called the Maxwellian, that has a shape sort of like this. It goes up quickly and then kind of trails down. So when I say something has a certain temperature, I don't mean all of its molecules are moving at exactly the same speed. But rather, I can tell you what their average speed is. There are some moving slower, there are some moving faster. Keep that in mind, that if I now something that's colder, I will have a different distribution of speeds whose average value is smaller. Still, on that cold day, there are some molecules that happen to be moving faster than the average temperature of a warm day. But not all of them. We have to look at the distribution. The distribution of speeds. What are those speeds? Well, here's where we have yet another formula in this day of physics. How do you relate energy to thermal motion, to this concept of temperature? There's another simple equation. Energy is 3 halves k, a conversion constant, times the temperature. The temperature needs to be in the absolute kelvin scale. And we'll get to that a bit later, what that scale is. Zero absolute kelvin is absolute zero, no motion, zero energy. And you can see in that formula, if I put in a 0 for T, I would get zero energy, that's absolute zero. If I want to put in the temperature of this room, which is probably 20 degrees centigrade, quote, room temperature, which is 68 degrees Fahrenheit, which is 293 degrees Kelvin. I could now ask, what is the average speed of the molecules in this room? There's really a temperature distribution, a Maxwellian distribution. But, I should be able to tell you at least the average number. And how do I do that? Well, I take my 3 halves k T and it would equal one-half m v squared. The mass now is not the cement block, but it's the mass of a air molecule, say a nitrogen molecule. Air is 80% nitrogen approximately, 20% oxygen. I put in the mass of the nitrogen molecule and multiply by a half and I've got the speed. That's what we want to know. The other side of the equation, I've got 3 halves. I've got K, Boltzmann's constant. 1.38 times 10 to the -23 joules per degree Kelvin. And t in Kelvin, 293. Multiply the left side, divide by the other numbers on the right side, take the square root, and you have the average speed of a molecule in the air at room temperature. That speed is 510 meters per second. 510 meters per second, 11,000 miles per hour, are you kidding me? That's what the molecules in this room are moving? How come I'm not getting blown over? Well, that's because they're not all going the same direction, right? This is the average speed, but the direction is isotropic. They're moving every different way. And you know what? My skin is body temperature, maybe a little less. The inside of my body is body temperature. And that's warmer. That's higher than 20 centigrade, right? That's 100 Fahrenheit. 98.6, about 37 centigrade, body temperature. So they're moving even faster. The average speed of those molecules. So this is a balance. That's why we don't boil. We've got molecules hitting us at that speed. And I've got our molecules vibrating at comparable speeds. You've got to keep in mind that this is different than, say, the speed of the wind. A 500 meter per second wind would destroy everything in its path. When we have, say, a cold breeze, a 20 meter per second, right, 40 mile an hour wind. That's the collective motion. That's the motion of a whole group of molecules moving collectively. Individually, each molecule is moving in all sorts of random, different directions at much, much higher speeds. If I take a hair dryer and I set it on hot and I measure how fast the air is coming out from it, it might be a couple of meters per second. On that order. The individual molecules are moving much faster. If I set the hair dryer to cold, it still is blowing air out at that same meter per second or so. But the average speed of the molecules are now much less. You have to think differently of this collective speed, which is the wind, versus the actual individual speeds of molecules, which represents their thermal energy. Let's talk for a moment about temperature scales. I'm filming this here in the US, and we use the Fahrenheit scale. We're just about the only country in the world that uses the Fahrenheit scale. I'm not particularly happy with the Fahrenheit scale, but it's what you grow up with, it's what everyone talks about, it's what we got. There's always been efforts to try to change to something more, quote, sensible, centigrade. And in my scientific work, certainly I use centigrade all the time. But when someone tells me, hey, what's the temperature outside? And I say, wow, it's cold today. It's 20. I mean 20 Fahrenheit. Which for the people that use Celsius, is probably around minus 5, okay. And, if I say it's cold today and it's 20, and I'm in Europe, they're going to say, what? That's pretty decent. So it's what you get used to. But it really isn't quite as dumb as it sounds. There is a reason for the Fahrenheit scale. 100 Fahrenheit Is a approximately body temperature. At least when the scale was created, they thought 100 Fahrenheit was body temperature. We now know it's really on average 98.6, well, that's close to 100. There was a reason for it. And zero Fahrenheit is the temperature, the coldest temperature you can get a liquid salt solution to. You're saying, hey, what? Liquid salt solution? All right. It's cold outside. There's ice on the sidewalk. You throw salt on it to melt the ice. If we were below zero Fahrenheit, the ice would not melt. If you've ever made homemade ice cream, usually to cool off the temperature lower than just the temperature of the ice cubes, but you still need it to be in a liquid slurry, so how cold could you get the water? You can only get the water to 32 Fahrenheit. Zero Celsius, right. That's almost freezing water. Any colder, the water would be frozen. So you put the ice in water around the bucket, you turn it, you want to get it colder, because you want to get below that so you can make the ice cream faster. So you throw salt with it. Now the temperature of that water can go below zero centigrade. It can go below 32 Farenheit. And how low could you get it? You could get it all the way to 0 Fahrenheit. That was the creation of the scale. Body temperature to saturated salt solution before it freezes. In Fahrenheit, water freezes at 32 and boils at 212. Some odd numbers, Right. Whereas in Celsius, it's pegged to water. 100 Celsius boils water, 0 Celsius freezes water. The 100 units between water boiling and freezing are equal to 180 units in the Fahrenheit scale. The Kelvin scale has the same size unit as the Celsius scale. But zero is not where water freezes, it's where all motion ceases, where all motion stops. Where you have absolutely no molecular motion, absolute zero. Absolute zero is T equals 0 kelvin, minus 273 centigrade, and minus 459 Fahrenheit. Wherever you're sitting watching this, I bet you could find a piece of metal. I want you to touch it. How does it feel? Most of you will say, it feels cold. Now, somewhere, same room, sitting around you, is probably a piece of cloth or a piece of wood. Desk, something. Chair. Touch it. How does it feel? Well, not as cold as the metal. Do you realize those two things are exactly the same temperature, because they've been sitting in exactly the same room for as long as you've been watching this. Why do they feel different? It has to do with how quickly heat is transferred. You see, you are warmer than that. Your body temperature. The room's probably somewhere close to room temperature, it's less. So your body's warmer than those objects. When you touch the metal, the metal is more efficient at transferring that vibrational energy of the molecules in your finger to the molecules in the substance. They transfer it very quickly. The wood or the cloth is a very poor heat conductor. It does not transfer that heat very quickly. Generally, heat conductors go along with being electrical conductors. If something conducts electricity well, usually it conducts heat well. There's one famous, famous example, and that's diamonds. Diamonds are wonderful insulators for electricity, but they are fantastic conductors of heat. Diamonds have a slang term called ice. Now, I've never had the fortunate ability to have a giant pouch of diamonds and put my hand into it. But evidently, diamond dealers do. And if you do that, evidently, and I wish I had this experience at home every night, [LAUGH] but I wish I had this experience, but if you could put your hand in a pouch full of diamonds, it would feel like you're putting your hand in ice, because they conduct the heat away so well. We're talking about heat conduction. And that's when two objects are actually touching each other. And the vibrations from one are transferred to the vibrations of another. When you put a kettle of water on a stove, when you are touching a metal object or something, that's heat conduction. Another example which you could relate to is, what happens if you jump into a cold lake? Let's say you jump into something that's near zero centigrade. Right, 32 Fahrenheit. You won't last long, right. The heat conduction away from your body will be very rapid. If, on the other hand, you step outside in the air when it's 32 or around 0 centigrade, it's cold. You're not going to die instantly, right? Depends on how many clothes you have on, you might be just fine. You jump in water like that, the heat conduction away from you is so much faster because there are so many more molecules hitting you. The density of water is 10,000 times the density of air. So you have 10,000 times as many molecules transferring that vibrational energy away from your body. Conduction is not the only type of heat transfer, there’s two others. One of them is convection. Now, when we talk about salt ponds at some point, convection is the principle by which they work. Convection has to do with not just the transfer of heat, but also the motion of the material transferring it. Everyone knows that hot air rises, so the density's a little bit less than the air around it. Hot air rising is a wonderful example of convection. The actual warm air transfers heat because not only does it vibrate and touch its neighbor molecules, but the actual molecules themselves move. That's convection. And the last type of heat transfer is radiation. This is actually using the light that comes from something. The electromagnetic radiation to transfer energy. When you come up to a nice roaring fireplace, yes, the air is warmer, but you're also getting that bright firelight, that radiation, the electromagnetic light waves coming to your body and warming you up. Radiant heat, it's called. Radiant heat is very particular, the amount that can be transferred goes at the very steep function of the temperature. The fourth power, actually. So radiation usually only matters when you have something that's really, really hot, like a fire. Conduction, convection, and radiation. Three ways heat is transferred. In all these cases, it's the vibration, the motion in the molecules is what we talk about when we talk about temperature. [MUSIC]
