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I Why does stretching a rubber band increase its temperature?

  1. Apr 28, 2017 #1
    I was reading the Feynman lectures and in one of the chapters on thermodynamics it is stated that stretching a rubber band makes its temperature rise. I feel like I understand the thermodynamics of the rubber band and yet this statement is puzzling me. I believe that when you stretch a rubber band, it will release heat. This is in agreement with the hot sensation that you feel if you were to put the rubber band to your upper lip (this is a common experiment). Wouldn't this release of heat be consistent with a decrease in temperature, not an increase?
     
  2. jcsd
  3. Apr 28, 2017 #2
    You shouldn't think of it like "releasing heat", as if heat detached itself from the object. Rather, think of it as a process of equalization: the rubber band heats up because of the work you put into it, and now the band is out of thermal equilibrium with its environment. Your tongue gets to it, and the tongue, being of lower temperature, exchanges thermal energy with the rubber band, until they equalize somewhere in the middle. That raise in temperature of your tongue is what you perceive.
     
  4. Apr 30, 2017 #3
    If you understand the thermodynamics, you know that there are three terms in the first law:
    ## \Delta U = Q + W ##
    In the process described, assuming that initially the band has the same temperature as the environment and the process is fast, the heat exchanged is negligible. The work is done and the effect is an increase in internal energy. There is no heat released or exchanged or otherwise involved in the process.
    The increase in internal energy translates into increased temperature and now you will have heat exchanged between the band and medium (part of which may be your lip).
     
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