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Homework Help: Heat conduction?

  1. Nov 18, 2009 #1
    1. Suppose the insulating qualities of the wall of a house come mainly from a 4.0-in. layer of brick and an R-19 layer of insulation. What is the total rate of heat loss through such a wall, if its total area is 195 ft^2 and the temperature difference across it is 15 degrees F.

    2. Q/t = kA ( (T1 - T2)/L )
    R = L/k

    3. Okay so the R-19 thing just means that R = 19 (ft^2 * hour* degreeFahr)/Btu. R = 19, so L/k = 19, I can inverse that and substitute it into the first equation, but it says the temperature difference across the WHOLE wall is 15 degrees fahrenheit. My main problem is that they ask you for the rate of heat loss through the whole wall, but the wall is made of two different materials, and you don't know how thick the R-19 layer is.
  2. jcsd
  3. Nov 18, 2009 #2


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    Welcome to PF!

    Hi nightwing973! Welcome to PF! :smile:
    You don't need to know the thickness of the R-19 layer …

    eg, if was twice as thick, it would R-38. :wink:
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