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Homework Help: Help! Heat Loss

  1. Oct 18, 2005 #1
    Help! Heat Loss!!!

    A cup of coffee (with a lid) is enclosed in an insulated cup 0.5cm thick in the shape of a cube 13.1cm on a side. The thermal conductivity of the cup is 0.0002 cal/s*cm*DegreeC. The temperature of the coffee is 87C and the temperature of the surroundings is 14C. Find the heat loss due to conduction. Answer in units of J/s.

    I've tried P=kA(Th-Tc)/L ------> Energy transfer by conduction but somehow it's wrong?

    I see no other way!

    I know that:
    k = 0.0002cal/s*cm*DegC which = 0.0008372J/s*cm*DegC
    L=0.5cm
    A=13.1cm*13.1cm (because cube)
    A=171.61 cm^2
    Th=87C
    Tc=14C

    ANY IDEAS?!
     
    Last edited: Oct 18, 2005
  2. jcsd
  3. Oct 18, 2005 #2

    lightgrav

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    You're actually using kA(Th - Tc)/L , right?

    Are you leaking Heat thru all 6 sides?

    Did you convert from cal to J?

    Are you using 13.1 cm as inside measure, or outside?
    How do you know it's wrong?
     
  4. Oct 18, 2005 #3
    I know its wrong because my teacher said so :frown:

    Yes im using that equation.

    I think I'm leaking on all sides

    I did convert from cal to J.

    And I'm using 13.1cm as an outside measure I believe.
     
    Last edited: Oct 18, 2005
  5. Oct 18, 2005 #4

    lightgrav

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    Did you multiply the area of each side by 6 sides?
     
  6. Oct 18, 2005 #5
    Why x6 we are looking for overvall heat loss from this thermos like apparatus.
     
  7. Oct 18, 2005 #6

    lightgrav

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    you think Heat only leaks out the top?
    How many sides does a cube have?
     
  8. Oct 18, 2005 #7
    Ok i understand but how do i fit it into the equation? k(6A)(Th-Tc)/L ?
     
    Last edited: Oct 18, 2005
  9. Oct 18, 2005 #8

    lightgrav

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    yea, or A = 6 s^2
     
  10. Oct 18, 2005 #9
    Correct! Thank you very much!
     
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