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Heat question -- Ice cube in a cup of water...

  1. May 15, 2017 #1
    1. The problem statement, all variables and given/known data
    An ice cube of mass 0.01.kg at a temperature of 0 degrees Celsius is dropped into a cup containing 0.10kg of water at a temperature of 15 degrees Celsius. What is the max estimated change in temperature of the contents of the cup?
    SHC of water = 4200J per kg per kelvin
    latent heat of fusion of ice = 340000 J per kg

    2. Relevant equations and attempt at a solution

    heat lost by water = heat gained by ice
    mwaterC ΔTwater = mice C ΔTice + mΔL

    0.1 x 4200 x ΔTwater = 0.01 x 4200 ΔTice + 0.01 x 340000

    420(Tfinal-15) = 42 (Tfinal-0) + 3400
    420T - 6300 = 42T + 3400
    T= 25 degrees
    so change in temperature = 15-25 = -10 degrees celcius
    This is wrong as the actual answer is 8.7 K
     
  2. jcsd
  3. May 15, 2017 #2

    DrClaude

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    Staff: Mentor

    So the final temperature is higher than all initial temperatures? Doesn't make sense, right?

    First problem: the left-hand side here doesn't correspond to "heat loss by water."

    Second problem: you have to use absolute temperatures here.
     
  4. May 15, 2017 #3
    Thanks Dr Claude
    Re your first point, my understanding is as follows:
    As the water cools down it will loose heat equal to mc delta T, also heat will by lost to surroundings and to the cup. Some of this heat which is lost by the water is given to the ice to initially cause a change in state and finally raise its temperature. Am I on the wrong lines here?

    I've converted the values to kelvin and I still get the same answer?!?
     
  5. May 15, 2017 #4

    DrClaude

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    Your approach is correct, but you have to be careful about the signs. (This is a general comment: when you get a final temperature that is higher than the initial temperature, while it should be the other way around, the first thing to check are the signs.)
     
  6. May 15, 2017 #5

    gneill

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    That value looks a little high to me. Just melting the ice (ice@0C → water@0C) should take enough heat from the existing liquid water to drop its temperature below that value.
     
  7. May 15, 2017 #6
    Not that the 8.7K that was asked, is the temperature change and not the final temperature, wich is 6.3C.
     
  8. May 15, 2017 #7

    gneill

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    Ah. My mistake. I should really pay closer attention to these small details. Thanks :smile:
     
  9. May 16, 2017 #8

    haruspex

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    So far so good: heat gained is proportional to the final temperature minus the initial temperature.
    So for heat lost,
    how should you calculate the temperature difference? Not like this:
     
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