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Gre Problem #75

  1. Aug 31, 2004 #1
    Hello ALL,
    I am really having a hard time with this question, any effort will be greatly appreciated!
    Window A is a pane of glass 4 mm thick, as shown above. Window B is a sandwich consisting of two extremely thin layers of glass separated by an air gap 2 mm thick. If the thermal conductivities of glass and air are 0.8 watt/meter oC and 0.025 watt/meter oC, respectively, then the ratio of the heat flow through window A to the heat flow through window B is
    (A) 2
    (B) 4
    (C) 8
    (D) 16
    (E) 32
    D is the answer, And I have no clue :blushing:

    many thanks!
  2. jcsd
  3. Aug 31, 2004 #2
    let Kair = Hair / (A * delT/delXair)
    let Kglass = Hglass / (A * delT/delXglass)

    your looking for ratio of heat flow, or Hglass to Hair.
    it should be almost trivial from here. the problem is mostly just testing whether you know the equation for thermal conductivity (in fact, i just looked it up online, i've never seen it before).
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