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Homework Help: Conductance of heat through concrete

  1. Aug 20, 2014 #1
    1. The problem statement, all variables and given/known data

    In an electrically heated home, the temperature of the ground in contact with a concrete basement wall is 10.7 oC. The temperature at the inside surface of the wall is 21.4 oC. The wall is 0.17 m thick and has an area of 6.4 m2. Assume that one kilowatt hour of electrical energy costs $0.10. How many hours are required for one dollar's worth of energy to be conducted through the wall?

    2. Relevant equations

    heat = (thermal conductivity constant x change in temp x area x time)/length

    3. The attempt at a solution
    heat energy to be conducted = 3.6 x10^6J x 10
    (1.1 x (21.4-10.1) x 6.4t)/0.17 = 443 x time
    so equating these gives 3.6 x 10^7 = 443t
    however this t value is much too large, where have I gone wrong?
  2. jcsd
  3. Aug 20, 2014 #2


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    Staff: Mentor

    So, you calculated how many hours, exactly?
  4. Aug 22, 2014 #3

    rude man

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    Homework Helper
    Gold Member

    right so far.

    so what is the thermal conductivity constant?
    where did the numbers "3.6e6J" and "1.1" come from? Is "10" the number of kwh? kw-h is not an SI unit.

    use the thermal conductivity number and change total energy from kw-h to J.
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