MadSci Network: Physics |
Hi Walt, The answer lies in the information you gave in the question: a stainless-steel pipe 4.5 feet long. The two important points to keep in mind are the thermal conductivity of the steel, and the fact that the steel is shaped as a long tube. As it happens, steel is a pretty poor heat conductor, as compared to (two typical examples) copper or aluminum. Just to put some numbers on this, you can surf over to "matweb.com", and look up the thermal conductivities of copper, aluminum, and various grades of steel: Metal Conductivity (W/m-K) copper 385. aluminum 210. steel (most types) 51.9 A factor-of-four or factor-of-seven difference may not sound like much, but it has profound consequences for working with these metals, especially if you're trying to do welding. Steel is dandy for welding, aluminum and copper far less so (nearly impossible in some circumstances). For your problem, the low conductivity of steel, together with the tube geometry, makes all the difference in the world. As it happens, this problem (tube with one hot end, immersed in room-temperature air) can be set up and solved, using either pencil-n-paper or a computer. But, the solution can also be described intuitively, and that might be more help to your visitors: One end of your tube is indeed very hot (2300F!!), and so the heat will try to flow down the length of the tube to your unsuspecting hand. But, the tube is surrounded by room-temperature air. As it happens, the air is pretty good at cooling things down by way of convection; and, this convective cooling gets larger as the temperature difference between the hot object and the air gets larger. So, as heat energy travels down the tube from the red-hot end, it is being convected away by the air, which is trying to keep the entire tube at room temperature. And, this convection is very effective for the tube, since the air can be cooling both the outside and the inside of the tube. Most people are familiar with the idea of a fireplace poker: a rod of iron or steel about 3 feet long, which can be held in one hand while the other end is quite hot -- glowing, even. Your blowpipes may get hotter than a fireplace poker, but you have both a bit more length and the tube geometry helping to keep your hands safe. If the Museum is willing to humor you on this, you could contact Alaskan Copper and Brass (or your favorite Puget Sound metal vendor) and order three solid rods 4.5 feet long, made of steel, aluminum and copper, each with the same outer diameter as your blowpipes. Then, try putting each of these rods, along with one of your blowpipes, into the furnace, and see how hot the "handle end" becomes. Wear protective gloves, of course, especially since the copper rod may get uncomfortably hot. What this (hopefully cheap and easy) experiment would illustrate is the effect of the thermal conductivity (with the steel, copper and aluminum rods) and the effect of the tube geometry (with the steel rod versus the steel blowpipe). I hope this helps. Maybe be able I'll get down to the Museum and see what you've got going on down there. Good luck! Aaron
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