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Homework Help: Thermodynamics and ice melting.

  1. Mar 3, 2008 #1
    [SOLVED] Thermodynamics and ice melting.

    1. The problem statement, all variables and given/known data
    An open container holds ice of mass 0.585 kg at a temperature of -13.3 *C. The mass of the container can be ignored. Heat is supplied to the container at the constant rate of 800 J/min.

    The specific heat of ice to is 2100 J/Kgand the heat of fusion for ice is 334x10^3 J/Kg.


    2. Relevant equations
    Q=cm[tex]\Delta[/tex]t
    Q=mLf where Lf = fusion for ice


    3. The attempt at a solution

    The first part asks:

    How much time t_melts passes before the ice starts to melt?
    I got the answer to this to be 20.4 minutes and I know it's correct. I used Q=cm[tex]\Delta[/tex]t= 1.63x10^4 J, where(c = 2100, m=.585, delta T = 13.3). I then used the rate of 800J/Min to get 20.4 min.

    The second part asks:

    From the time when the heating begins, how much time t_rise does it take before the temperature begins to rise above 0 *C?

    I'm absolutely stumped. It's probably very simple and right over my head. The hint for the problem had me find how much time it'd take to melt once it got to 0*C and I found that to be 244 minutes (using Q = mLf, then Q / 800J/min to get time), which is correct.

    I feel like these two are the same question, since for the first one I'm finding the time it takes for the ice to start melting, which is at 0*c. Anybody have any clue? I've tried 20.4 min and 223.6 min and Mastering Physics tells me they're both wrong. Thanks a lot!
     
    Last edited: Mar 3, 2008
  2. jcsd
  3. Mar 3, 2008 #2
    The time doesn't start to rise until all the ice has melted. So you just need the sum of both times you already calculated.
     
  4. Mar 3, 2008 #3
    So you're saying that the temperature won't start to rise over 0 until the ice has all melted? Seems like there could be other variables in play. That was the answer though, thanks a lot :). So simple it went right over my head.
     
    Last edited: Mar 3, 2008
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