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Homework Help: (Thermal Conductivity Question) Need Help!

  1. Jan 19, 2009 #1
    1. The problem statement, all variables and given/known data

    1) The inner and outer surfaces of a glass window of area 2.5 m2 are at 22 ºC and 0 ºC respectively. What is the heat lost in 1 second if the glass is 5mm thick and has a thermal conductivity of 1.1 Wm^-1K^-1 ?

    2) The same sheet of glass as described in the [above Question] is used as a window in a room at 23 ºC when the general outside temperature is -14 ºC. If the inner and outer heat transfer coefficients are both 17.0 Wm^-2K^-1, what is the temperature of the outside surface of the glass?

    Can Anyone help me solve these two questions, Please? (with the formula used)

    :)
     
  2. jcsd
  3. Jan 20, 2009 #2

    CompuChip

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    Hi MoZeS, welcome to PF.

    You have given the thermal conductivity and note that there is a unit: W/(m K)
    So what does the thermal conductivity express?
     
  4. Jan 20, 2009 #3
    dQ/dt=kA(d theta/dx)
    given,A=2.5m^2, x=5mm, k=1.1 Wm^-1K^-1
    dQ/dt=kA(final temp-initial temp.)/x
    =(1.1)(2.5)(22)/(5x10^-3)
    =1.21x10^4W
    Q=1.21x10^4J since, Q/t=1.21x10^4W
    Q=1.21x10^4W/1s
    therefore W/s = J
    part 2, try on ur own ...
    is same formulae...
    dQ/dt=1.21x10^4W
    so, subst. in the equation and find the final temp.
     
  5. Jan 20, 2009 #4

    CompuChip

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    Welcome to PF lyming.
    Please note that we don't give out complete solutions here at PF. Moreover, I don't really see what formula you are using (what is theta?) and how you are getting Q from dQ/dt. Also, there must be some mathematical errors, because I happen to know that W = J / s while you write W/s = J.

    Finally, I think no looked up formula is needed at all and one can infer the answer by simply looking at the units of the given conductivity.
     
  6. May 14, 2010 #5
    I see this one's been dead a while, but if someone want to help me out here...
    I get the first part, but I'm struggling on the second one.
    Do I still need to use the 1 second? And how to do I change the heat flow to temperature? Or is there a delta Temp used to solve this part?
     
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