Wrap a piece of paper around a thick metal bar and place it in a flame. The paper will not catch fire. How can i explain this in terms of the conductivity of the metal paper? (the paper will not ignite until its temperature reaches about 230 degrees C)
There used to be an old bar trick where you wrap a dollar on your wrist and hold a match to it and the dollar will not catch fire-we were always told it was because the oxygen could not get to it and it needed oxygen to ignite. But I found this site and I guess it was the heat transfer instead! It would burn your wrist but not the dollar. www.calstatela.edu/dept/chem/chem. Paper, Metal and Flame - Tightly wrap a piece of paper around a cylinder of brass or steel so that you can hold it by the paper. Then place the part of the paper touching the metal into a candle flame. It will not burn because of the heat transfer from the paper to the metal. Then remove the paper from the metal and place it back in the candle flame. It will burn immediately.
Metal is a good thermal conductor and the heat rapidly moves along the metal bar from the warm end to the colder end. Thus there won't be enough heat left for the paper to catch fire. It will only ignite once the bar has warmed up appreciably.
The paper will not catch fire until it reaches 230 deg C. To the extent that the heat of the flame can be transfered through the paper to the metal bar fast enough to keep the paper below 230 deg C, the paper will not catch fire until the metal bar heats up to the point where the paper can no longer transfer the flame's heat fast enough. The equation for heat transfer is Q Cm(T2- T1) where Q is the quantity of heat transfered, C is the specific heat for the material, m is the mass and T2-T1 is the temperature difference. Thus, the quatity of heat transfered is directly proportional to the temperature difference, the mass and the specific heat of the material. So, as the bar heats up, the temperature difference becomes smaller and it is less able to take more heat. However, it's mass and specific heat are very important as well. A massive bar with a high specific heat (like copper) placed in a small flame might be able to transfer enough heat to keep the paper below 230 deg C indefinitely assuming the paper can transfer the heat to the bar effectively.
