All right, this video is about heat transfer and the way in which heat is transferred, which are three primary ways, and I'm just going to go ahead and list them right now they are conduction, convection and radiation. Once you understand how heat is transferred, it becomes much more simple to think about things like how an air conditioning system works and comfort and heat gains and heat losses. Those sorts of things that you have to know about in order to work effectively in the HVAC trade, whether you're a technician or a designer, no matter what the case may be. So let's talk first about conduction, so we understand conduction really well, it's the one that we learned.

First, when we are infants, it's when we take our hand and we stick it on the hot stove and it burns us, and we realize that conduction can be no fun, and that is because we are actually touching a hot surface, a surface that is higher temperature Than our hands and heat energy is transferring from molecule to molecule from one solid surface to another, solid surface through our hand, and so that is conduction conduction occurs when two molecules actually touch each other and transfer heat through contact, because temperature, if you remember, is average Molecular velocity and heat energy is a measurement of molecular velocity or kinetic energy, the moving molecules, and so something that is higher temperature. It's got molecules in it that are really bouncing around fast, and so the molecules in your hand, are bouncing more slowly and when you touch them, the energy is transferred from one to the other. And we feel that, as you know, that burning sensation that we get when we touch the stove, so we learn about conduction. First, conduction in a home is prevented through insulation, for example, so the walls and the outside of your house get higher temperature or lower temperature than the inside, and so we put insulation in the walls, whether it's air gaps, or maybe we put in some blown-in insulation Or some foam or some bad insulation in the walls and in the ceiling and that helps prevent that conduction of heat energy by means of using an insulator and so that insulator helps prevent that conduction through the wall.

And that's. But we also have something called convection and convection happens within a fluid, and both liquids and vapors are both considered fluids. When we hear fluid, we often think water, but but fluid can be liquid or vapor, and that means that the molecules are more free to move there tied together in a crystalline formation that are actually free to move around, and so in. Convection is the movement of molecules, so when you have a molecule, that's of higher energy and it moves to another area, it's transferring its heat along with it as it moves, and so an example of convection in a home would be when you open the refrigerator door And your grandpappy says: don't leave that door open, the cold will get out and the Penguins will leave.

Maybe just my grandpappy, who used to say that, and I didn't call him grandpappy when you open a refrigerator door and you open it and warm air rushes in and some of the cooler air falls out and there's a convective effect there. That occurs because the molecules are free to move, and so they transfer heat through convection and that happens within fluids, and then there's this weird one called radiation and what's funny is, is that we all experience radiation. We just don't think of it. That way, I think when we hear radiation we think of like atomic bomb or something you know, but there's different types of radiation and there's different types of electromagnetic energy.

And so we are constantly being bombarded with this electromagnetic radiant energy and that's called the light from the Sun, because light is actually an electromagnetic force. It's an electromagnetic field and it is transferred from the Sun to earth and it is transferred to us radiantly and the way we know it's radiant is because space is of essentially a vacuum. There are a few molecules bouncing around here there and the ones that are bouncing around there are actually very high temperature. That's because they're not protected by an atmosphere, and so this is actually a common thing that that we misunderstand we think of space as being very cold.

And it is because there are very few molecules. But the molecules that are there exposed to the sun's radiation and they can actually get quite hot, and so anyway, space is largely a vacuum. There's not really many molecules in between us and the Sun, but the Sun still manages to get its heat energy to earth and it does that through electromagnetic radiation, known as sunlight, you know we see it as sunlight. This is the visible spectrum, but there's also spectrums of electromagnetic radiation that we cannot see, and so, when you think of things like microwaves or you know, nuclear decay from their decay from nuclear fallout and things like that, we can't see that electromagnetic energy, whereas light we Can see because it's within the spectrum that our eyes are designed to see so we're constantly being heated through ready, but we don't think about radiation a lot in our homes, but I'll give you a perfect example.

Let's say you can't fire outside it's not really cold. Outside the air is cold, but it's heating, your body, but you notice that your face is starting to get a little too warm. So you put your hands up in front of your face in order to block your face from the light, and you notice that almost instantaneously your face cools off the temperature of your face drops and that's because that fire, the electromagnetic energy coming at you is heating. Your face, but it's not necessarily heating the air in between and there's a lot of factors that play into that.

But that's what radiation does? Is it heats surfaces, heats or cools, or transfers energy between surfaces, but radiation does not only go in one direction. So if you're in a room that has very cold walls on it, so say it doesn't not well insulate it. The air temperature might be comfortable temperature, but you still feel chilly because your body is giving up its heat via radiation to that colder wall. So it's not just associated with visible light, like we think it is.

We actually are giving off a heat signature via radiation if you've seen movies or you've seen maybe pictures from helicopters where they're using thermal imaging cameras where you can see people within a building. That sort of thing. That's the ability to see these invisible heat signatures of electromagnetic radiation that aren't in the visible sight, and so you have these special cameras. They can see that, because we're constantly giving off heat via radiation and radiation happens in between surfaces primarily of different temperatures, heat is always moving.

The net effective heat is always moving between surfaces of different temperatures, because heat moves from hot to cold or from higher temperature. To lower temperature, but the ways in which heat moves are these three conduction, which are things that are touching convection, which are molecules that are moving with a fluid and the actual motion of the molecules themselves. You know if you're heating, a bathtub and you and you kind of like move the water to try to mix it. That's a you know.

You understand that that's convection and that's how you're you know, mixing it more quickly to move heat around and then radiation, and we see that all the time with the Sun, with with a fireplace those sorts of things or even with radiators an old-school, radiator heater. That is designed to transfer heat through radiation because those old radiators they didn't have a fan or blower or anything to move the air around through convection. They just used radiation. They had a good sized thermal mass and they gave off their heat to via radiation, which is actually an effective and comfortable way of receiving heat.

It's the way that we receive heat from the Sun, so it's very natural to us and it actually feels quite comfortable if you've ever been in a room. That's heated through radiant heat, it feels kind of different and that's because our body responds slightly differently to radiant heat transfer, but it still moves from hot to cold, whether it's using conduction, convection or radiation. So that's all we have to say about that for now, we'll catch you in the next video.