I was watching an episode of Man vs Wild where steel wool was rubbed with phone battery. To my surprise, the wool caught fire. What is the reason for this?
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Steel wool is a mat of very fine steel fibres. Because of their small size, it's easy to heat them up quickly to the point of burning and their relatively large surface area to volume ratio lets them oxidize quickly. If you try it with a flame, you'll find that it's quite easy to burn steel wool. When you short the terminals a battery across steel wool, a lot of current passes producing heat (think about incandescent light bulbs), and especially if you touch the battery such that only a few fibres are touching, enough heat is produced to cause them to burn.
answered Sep 9, 2014 at 16:43
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$\begingroup$ Why is it easy to heat them because of their small size $\endgroup$– humbleSep 9, 2014 at 16:51
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$\begingroup$ Less material to heat. Think about heating a large bar of steel versus a tiny wire. While steel doesn't have spectacular thermal conductivity, it's enough that it takes a much hotter flame to heat a bar to the point of burning. A small wire doesn't have much material to transfer the heat to. The other thing is a bigger piece needs more oxygen to burn than can be readily obtained from the air by diffusion. This is why cutting torches are designed to give a blast of extra oxygen to the hot metal. $\endgroup$ Sep 9, 2014 at 17:05
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$\begingroup$ The small size means higher electrical resistance which means more heat from a current flowing through. $\endgroup$ Sep 9, 2014 at 23:00
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$\begingroup$ Which is why I indicated that only touching a few fibres at a time helps, but I suspect it's not the only factor. I'm not sure whether oxygen transport or heat capacity would be more important, but the iron fibres don't have to get that much thicker before it doesn't work with a normal flame, so it can't be just current. Also, assuming a fixed voltage, the point of maximum heat production in the wire is going to be when the external resistance matches the internal resistance of the battery. Beyond that, increasing resistance decreases joule heating. $\endgroup$ Sep 10, 2014 at 2:53
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$\begingroup$ What effect, if any, does coating the steel wool in soap have - could you start a fire with a Brillo pad or would you have to rinse out all the soap and dry it first? Also, how would rust effect the reaction - could you do this with a highly-used piece of steel wool? $\endgroup$ May 4, 2016 at 1:33