|MadSci Network: Physics|
Greetings, Mehdi: You are encountering phenomena associated with "electrical resistance". Almost all substances that conduct electricity do so imperfectly. (Some of them, under very special conditions, can conduct electricity perfectly, but so far it costs too much to create those special conditions everywhere that we might want to conduct electricity.) Whenever electricity is conducted imperfectly, the amount of imperfection is usually described as an amount of "electrical resistance". Different substances have different amounts of resistance, and whenever electric current flows through an imperfect conductor, some heat will be produced. The more electrical resistance a substance has, and the more current flowing through it, the hotter it gets. Electric ovens are deliberately constructed with high-resistance conductors, to create the heat needed to cook food. Ordinary "incandescent" light bulbs are aptly named; their electrical resistance causes them to glow white-hot. Steel wool is not a good conductor of electricity. Steel itself is not an especially good conductor, and for ANY conductive substance, the finer the wires that you make from it, the poorer a conductor it becomes. Since steel wool consists entirely of rather fine strands of steel, it is easy to see that when electricity flows through it, a fair amount of heat is quite likely to be produced. Before getting to that, however, there is one more thing to describe. The "voltage" that you applied to your steel wool is the pure force that can cause an electric current to flow. There is a fairly simple equation that describes the relationship between voltage, current, and resistance: voltage (Volts) equals current (Amperes) multiplied by resistance (Ohms) What that means with respect to the two, six, or twelve Volts that you applied to the steel wool is this: Regardless of how much current flowed when two Volts was applied, we can call it a "base amount of current". Thus, when applying six Volts, three times the base amount of current flowed, and applying twelve Volts caused six times the base amount of current to flow. (I should mention a modification that frequently must be noted: The hotter an electrical conductor, the more resistance it has. Applying twelve Volts may indeed cause six times the base current to flow, but as the wool heats up, its increased resistance causes the flowing current to diminish a bit. An equilibrium will be reached between the flowing current and the heat-caused increased electrical resistance; the mathematical description of this is somewhat complicated, and we need not worry about it here. Crude events such as this can be described well enough without the fancy modifications -- but one should remember that there will be times when such modifiers are critically important.) As you observed, the more Volts, the hotter the fine wires of steel wool became, simply because more current was being forced to flow through them -- being poor electrical conductors. The next relevant fact concerns the "surface area" of steel wool. Think about a single spherical ball of steel, weighing 25 grams (a bit less than one ounce). The amount of steel exposed to the air is quite minimal; most of the metal, after all, is locked away inside the body of the ball. But if we chopped the ball in half, then suddenly two more (circular) areas of steel are exposed to the air, in addition to the original exterior sperical surface. The more we chop the ball up, the more and more total amount of air is exposed to the air.... The preceding is important because air contains oxygen, a major ingedient of most fires. As you know, steel tends to rust, as the oxygen in the air combines with the iron in the steel. THIS CAN ONLY HAPPEN TO THE PARTS OF THE STEEL THAT IS EXPOSED TO THE AIR. Only as pieces of rust flake off of a solid piece of steel, do the innards of the metal become exposed to air, and become able to rust. Well, if we have helped the oxygen in the air, by taking a solid piece of steel and making a lot of very fine wires out of it (steel wool), it follows that all that steel can rust rather quickly, simply because fine wires have a lot of surface area exposed to the air. Now note that another word for the verb "rust" is "oxidize" -- and note that the process of oxidation usually generates heat. A fire consists of material in the process of oxidizing quite rapidly; the heat that a fire generates makes it easier for the next bits of material to burn. If you put the above pieces together, you should see that by itself, the steel wool would rust/oxidize reasonably quickly. When electricity is made to flow through it (pushed by two Volts), the steel wool heats up and this makes it easier to oxidize even faster. This generates enough extra heat that it begins to smoke. When six Volts push current through the steel wool, it heats up even more, oxidizes faster still, and begins to burn outright, as you observed. And when twelve Volts is applied, oxidation of the steel wool occurs at a pretty fair clip, indeed! Steel can burn quite truly, with actual flames, when it is finely divided into wool, and heated enough.
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