How can we prevent convection
Big Bang 3, textbook
18 17 How heat is transported The rising air of a flame can set a spiral in rotation, because it works like a wind turbine (A12), and it can even carry away small objects such as the almost burned-out teabag (A10). The shape of a flame (B 17.10 left) is also created by the heat flow of the rising air. There is no gravitation on board a space station and therefore no heat flow. The flame becomes round and the lower oxygen supply means that it burns bluish and cooler (A8). A candlelight dinner in space would not be a big hit. Convection can be quite practical, for example when boiling water (B 17.14 a). The heated water rises, gives off part of its heat, becomes colder, denser and sinks again. In this way, the heat is quickly distributed in the boiler. A similar effect occurs when heating in the living room, which mixes the air well (B 17.14 b). Without this air cylinder, it would be much hotter on the ceiling than on the floor. So convection is often quite cheap. But there are also annoying cases where the heat flow leads to undesired cooling. For example, warm air rises from us humans non-stop - even if we are not steam chatters (see B 17.15). Nature shows us how this can be prevented. For example, when birds are cold, they puff themselves up (B 17.16). This allows their plumage to hold on to more air, which is a lousy conductor of heat (see Tab. 17.1). The feathers themselves hardly warm, but they hold the air tight and thus prevent its heat flow. Convection helps to bring the water to the boil faster (a) and to mix the air in a living space (b). B 17.14 Simulation of the heat flow caused by a person B 17.15 That is also the reason why wool, feathers and fur insulate equally well (A9). Ultimately, it is not the material itself that causes the warming effect, but always the air that is held by it. Styrofoam also works with the same trick (A11). In general one can say: If you prevent the convection of the air, then you have a great heat insulator with it (1 info: convection prevention). Convection prevention The convection prevention is also used in building. Insulation materials always work with the help of trapped air, which is prevented from circulating. Perforated bricks (B 17.17 c) insulate much better than solid bricks. Even with an old double window (a), a lot of heat is lost through convection. In a modern double-glazed window (b), ascending and descending air obstruct each other. The convection is cut off and these windows insulate much better. With the heat flow, the heat is transported with the matter, and that can be quite practical. If you prevent the air from convection, you have a great insulator. The feathers prevent convection and the trapped air keeps the robin warm. B 17.16 a) old double window; b) modern double-glazed window; c) Perforated brick B 17.17 For testing purposes only - property of the publisher öbv
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