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Homework Help: Lava conductive cooling

  1. Mar 24, 2010 #1
    1. The problem statement, all variables and given/known data

    A lava beds cool very slowly, remaining warm for years. Calculate the rate of heat conduction per square metre through a 30 m thickness of rhyolite, if the surface is 400 degrees celcius and its interior side is at 3100 degrees celcius. The thermal conductivity of rhyolite =0.84 Wm^-1 degrees celcius^-1.

    Answer must be 75.6 W/m^2

    2. Relevant equations

    [tex]P=kA \left| \frac{dT}{dx}\right|[/tex]

    (The rate is P, k is thermal conductivity, dT/dx is temprature gradient)

    3. The attempt at a solution

    dT/dx = 3100-400 = 2700

    [tex]0.84 \times 30 \times(2700) = 68040[/tex]

    Can anyone show me the problem? ...because the right answer is 75.6!
  2. jcsd
  3. Mar 24, 2010 #2


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    Homework Helper

    Why are you multiplying by 30 instead of dividing? dT/dx is 2700/30.
  4. Mar 24, 2010 #3
    Yes, but I need a number for area A!
  5. Mar 24, 2010 #4
    Oh, never mind I get it. THANKS!
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