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Explan this phenomena with a hot steel rod

  1. Jun 20, 2008 #1
    A friend of mine told me this; you take a metal pole or rod and hold on to one end while warming up the other until it's red hot. Then you stick the warm end into cold water. One would expect it to cool down, and it does, but what happens is the end you're holding on to very quickly gets so hot you can't hold on, as if the heat runs up the rod in order to escape the cold. I've never tried this so I have to take his word for it, but can anyone explain what happens physically?
  2. jcsd
  3. Jun 20, 2008 #2
    This is commonly seen in forges. All you have done is create a gradient so that the middle of the rod is fairly hot (perhaps 400C) while the fire end is very hot and the hand end is fairly cool (perhaps 50C). Now, when you quench the fire end, the middle becomes the hottest part of the rod. There is a heat flow from that middle to both ends and it may be enough to burn your hand (many people use tongs or gloves to hold the rod for this part). You would get the same effect (only slightly faster) if you just took the rod out of the fire and held it in the air.

    You can do a simple experiment to see this effect. Take a silver spoon (has to be silver). Hold one end and put the other end in boiling water. As soon as the end you are holding becomes warm, remove the spoon from the hot water. It will burn you quickly. Make sure you are able to drop the spoon as soon as it starts to burn - this is capable of blistering your fingers. I suppose you could even try dunking the hot end in ice water to see the full effect, but again this may blister your hand, so be careful.
  4. Jun 20, 2008 #3
    The middle part of the rod would be just as hot before I quench the fire end, so why doesn't the heat flow to my hand just as fast before quench it?
  5. Jun 20, 2008 #4
    Don't confuse heat flow with temperature. The heat was indeed flowing, but not enough time had lapsed for the temperature to rise significantly.
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