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Please confirm answer to thermodynamic question

  1. Apr 26, 2010 #1
    1. The problem statement, all variables and given/known data

    The rate of heat loss to the outside through a 10mm-thick glass roof of a green house is 120kW. The length and breadth of the glass material of the roof are 20m and 2m, respectively. Calculate the temperature of the outside if the temperature inside the green-house is 25 degrees C.

    Assume thermal conductivity of the glass material is 1.0W/m/degrees C.

    2. Relevant equations

    dQ/dt = kA(dT/dx)

    3. The attempt at a solution

    dQ/dt = 120000W, k = 1W/m/degree C, A = 20mx2m=40m^2 and dT/dx = (T-25degree C)/10mm = (T-25)/0.01m

    T = 55 degree C

    I'm not sure this is right, any help is much appreciated!

    Thanks in advance!
    Last edited: Apr 26, 2010
  2. jcsd
  3. Apr 26, 2010 #2


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    Where is warmer? Inside or outside of the greenhouse?

  4. Apr 26, 2010 #3
    It doesn't state!

    The exact question is written above.

  5. Apr 26, 2010 #4


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    What do you think? What is a greenhouse for?

    It was said that the heat loss of the greenhouse is 120 kW. The greenhouse loses energy, heat flows out of it. What is the direction of heat flow? From a warmer place to a colder one or from a cold place to a warm one?

  6. Apr 26, 2010 #5
    Heat loss:

    [tex]\frac{dQ}{dt} < 0 [/tex]

    Heat gain:

    [tex]\frac{dQ}{dt} > 0 [/tex]

    You must interpret the question and understand that a greenhouse is a structure to regulate and maintain a certain temperature in the inside, usually hotter than the outside environment.

    Do your calculations again based on that.
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