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Question on heat

  1. Mar 30, 2008 #1
    1. The problem statement, all variables and given/known data
    a beaker of boiling water can be picked up with bare fingers without burning if it is grasped only at the thin turned-out rim at the top , why?


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 30, 2008 #2

    neu

    User Avatar

    By beaker do you mean glass? in any case it wouldn't be a conductor.

    Glass is an example of a dielectric, a class of materials which are poor conductors. As opposed to metals which a very good conductors as, generally speaking, the valence (outer) electrons of a metallic atom are able to move freely between all the atoms of the metal allowing for thermal energy to freely diffuse throughout.

    Dielectrics don't allow this behavior as they have whats known as an energy gap between neighbouring valence electrons, hence conduction only occurs when this gap is overcome.

    Boiling water in a glass beaker shows that heat is able to be conducted by the glass in contact with the water but heat does not diffuse to the rim which is at some distance from the water. The molecular energies of the water molecules are statistically distributed and, in this case, enough have energy to overcome the band gap of glass; this is sharply attenuated in sucessive layers of the dielectric compared to metals which allow for much greater heat diffusion.
     
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