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Homework Help: Thermal Conductivity

  1. Feb 23, 2012 #1
    1. The problem statement, all variables and given/known data
    I was wondering why there is such a disparity between thermal conductivty and how they act in the real world.
    Ex. If if have 3 cups filled with hot water. The 3 cups are made of glass, plastic and metal.
    The metal will obviously have way higher thermal conductivity compared to the glass and plastic cups. But the temp. goes down slowly in all 3 cups and the rate that the temp. goes down. between the 3 cups are similar. The metal will drop in temp. faster than the other 2 but not that much faster. I was wondering why

    2. Relevant equations

    No Equations

    3. The attempt at a solution
    Said in 1.
  2. jcsd
  3. Feb 24, 2012 #2
    A cup loses heat through its walls as well as from its surface. Furthermore, in order for the sides to lose heat, the heat must be released via natural convection. Natural convection from horizontal surfaces is greater than it is from vertical surfaces (side of container). Natural convection is a relatively slow mover of heat so it tends to blunt the differences in thermal conductivities of the walls of the cup. What diffuses through the wall must be released to the surrounding air.

    The cups also radiate energy. That must be a consideration as well. Water radiates.
  4. Feb 24, 2012 #3
    Rates are similar in them but not equal. Probably your experiment and ambient is too small to judge any significant difference due to the nature of the matters. Rest is clarified by LawrenceC
  5. Feb 24, 2012 #4
    Thanks a lot
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