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Some heat problems

  1. Aug 2, 2011 #1
    Hi, I have 2 questions and an exam tomorrow, and I have no idea how my teacher answered these 2 problems, if someone can guide me through how to solve these problems i would really appreciate it. I am not looking for an answer just a step by step how to solve it and im not really sure how to start.
    For this one, i think my teacher would convert the ice to a liquid and then to a solid, and then taking taking some equation at each part, lol im not really sure

    thanks in advance :)
     
  2. jcsd
  3. Aug 2, 2011 #2

    PeterO

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    Homework Helper

    For the roof, I think you need to know the thermal resistance or thermal conductivity of the material of the roof. With that you can calculate the heat flow per square meter [Joules per sec per square meter] then scale it up for the room size and the time period.

    For the ice to steam.

    Heat the ice - what is the specific heat for ice? [it is 1 for water]
    melt the ice - what is the latent heat for melting ice?
    Heat the water
    evaportate the water - what is the latent heat of evaportation?
    heat the steam - what is the specific heat for steam?

    NOTE: Suppose the latent heat for melting ice was 50 times the specific heat for water -it's NOT - the getting from ice at -10 to water at +10 would be the same as heating water from +5 to +75, ie 70 degrees.

    You could convert each section to a "heating water" equivalent and do the final calculation in one go.

    ***Saying latent heat is 50 times specific heat I mean suppose you need X Joules to heat a mass of water by 1 degree but need 50X Joules to melt the same mass of ice.
     
  4. Aug 2, 2011 #3

    sorry, i didnt really learn this stuff. How would I go about calculating the heat flow?

    and for the ice to steam, what do you mean by convert each section to a heating water equivalent?
     
  5. Aug 2, 2011 #4

    PeterO

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    If you don't immediately understand the conversions, ignore them and work it out the "standard" way.

    There is a conductance [or its inverse resistance] which tells you the number of joules per second per square metre per degree difference each side. You need to cram that stuff now.

    [You may have seen ceiling insulation bats. They can be R2, R3, R4 etc bats - that is referring to their thermal resistance.
     
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