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Heat transfer

Understanding conductive, convective, and radiative heat transfer using a thermal camera. Created by MIT+K12.
Video transcript
[MUSIC PLAYING] Hello. My name is James White. And today we'll be investigating heat transfer. Heat transfer affects our everyday life. For example, it allows our computers to run without burning up. It helps us to design houses that use less energy. It makes cooking food possible. And very importantly, it helps keep us warm or cool, like this radiator. We will look at three different types of heat transfer-- conduction, convection, and radiation, and find examples where they occur around us. To do this we'll use a thermal camera. A thermal camera can see the radiation given off by all objects and use it to measure their temperatures. For example, cold objects normally give off less radiation than hot objects, which give off more radiation. The camera records this radiation and creates a picture. The colors in the picture represent temperatures. Hotter temperatures are lighter colors, and colder temperatures are darker colors. In this picture, my hand is warm. But the copper wire below it is cold. However, the cameras isn't always correct. Let's look at a video of a lighter flame. Although the flame appears to be the same temperature as my hand, it's actually about 2000 degrees Celsius. This is too hot for the video to show us the real temperature. Using the lighter and copper wire, we will observe the first type of heat transfer-- conduction. I'll show you our simple experiment. Conductive heat transfer occurs when there is a temperature difference between two materials in contact with each other. In this case, the right side of the wire is heated by the lighter. At this point, temperature in the material is very high in one area, and cold in other areas. Heat transfers from the high temperature side to the low temperature side. Let's watch as this happens. Over time, the temperature of the material becomes more and more uniform as heat transfer moves energy from hot to cold areas of the wire. When engineering a building, we often care about keeping warm or cold air inside. How well a wall can retain heat in a building depends on two things-- it's thickness and its thermal conductivity. Materials that are thicker will let less heat through. And materials with higher thermal conductivity will let more heat through. How do you think this metal door compares to this foam panel? Which lets through more heat? What about traditional materials like these adobe blocks? When the temperature of a wall or door is different than the surrounding air, a different type of heat transfer begins to occur, named convection. A lighter, a copper wire, and a cardboard backdrop make up our simple experiment. When a material is hotter or cooler than a fluid surrounding it, it causes the fluid to rise or sink. In the case of a hot wire, heated air rises as cold air fills its place. This cools down the wire faster than conduction. If we push the wire towards the cardboard, we see that the hot air rising off the wire heats the cardboard. This hot stream of air is called a convection current, and is part of natural convection. Sometimes natural convection isn't fast enough, so engineers will use forced convection. Your computer uses forced convection to keep itself from overheating. That's what your fans do. Using the thermal camera, we can see that this laptop has several hot components-- here, here, here, and here. When these components get too hot, fans in the laptop will start to pull cool air across the parts. This results in forced convection. The hot air is then blown out the side or back of the computer. Earlier I mentioned that all objects give off radiation. When radiation from a hot object hits another object, it warms it up. This process is called thermal radiation. Two bright lights, a piece of aluminum foil, and a cardboard backdrop make our simple experiment. When the lights are turned on, let's see what happens. We can see that radiation from the lights is absorbed by the cardboard. However, the radiation is reflected by the aluminum foil. When the lights are turned off, we can see that the cardboard under the aluminum foil was not heated because radiation never reached it. The sun is an extremely hot object, and its thermal radiation warms our entire planet. This means that when designing a building, window placement is a very important part of the thermal engineering. When the sun's radiation comes through windows, it will heat up the exposed cooler indoor materials. Conduction and convection then transfer this heat around the building. By considering thermal radiation in design, a building will be warmer during the winter and cooler during the summer. Radiators are a common way to keep buildings in cold environments. Although named a radiator, this device actually uses radiation and convection to transfer its heat to the room. Thermal radiation is sent out in all directions from the radiator and makes you very warm when you stand next to it. Natural convection then circulates the hot air from the radiator around the building. There are so many examples of heat transfer around us every day, and as engineers, we have to think about using them to make the world a better place. [MUSIC PLAYING]