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Temperature change in different containers

  1. Oct 13, 2016 #1
    1. The problem statement, all variables and given/known data
    This is a question for 10 year old but I have a problem understanding why the answer is so.
    (The student asked me this.)

    There are two similar containers, X and Y, with 500 ml of boiling water. X is an aluminium container. Y is a plastic container. They are then covered with a sheet of glass and left in a room for 30 minutes.

    The answer options are 4 graphs of temperature against time.

    2. Relevant equations


    3. The attempt at a solution
    (I have taken a photo of the question in the worksheet but can't seem to upload the image here. It would be better to show the graphs, but since I can't upload it, I'm describing them here.)

    Of the 4 options, I've eliminated 2 of them as they started X and Y at different temperatures.
    In options 3 and 4, they start at the same temperature.
    In option 3, the temperatures for X and Y gradually fall over time, with X at faster rate and ends at lower temperature than Y after a certain time.
    In option 4, X and Y are straight lines from the same temperature. However, the temperature of X falls over time (negative gradient) and the temperature of Y increases over time (positive gradient).

    My answer would be option 3 as heat would dissipate faster from the aluminium container. The final temperature of the water in both containers would be lower than the initial temperature.

    However, the answer (given by the teacher apparently) is option 4. This means that after 30 minutes, the temperature of the water in the plastic container is higher than the initial temperature.
    So the question is, why is this so? Help explain please?
     
  2. jcsd
  3. Oct 13, 2016 #2

    haruspex

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    None of the options sound right. I would expect the aluminium container to cool faster initially, but both shoukd end up at ambient temperature.
    If you have described the question and option 4 accurately, that is clearly nonsense. The initial temperature is 100C. How is it going to get hotter? Moreover, even the falling temperature should not be a straight line.
    Please have another go at uploading the question and figures.
     
  4. Oct 14, 2016 #3
    Yes, that would be my understanding too but none of the options given fit that. The closest one would be option 3, where Y ends at a higher temperature than X, assuming that after 30 minutes, Y still has some way to go in order to reach the same temperature as X.

    (Image uploaded)
    temp_qn.jpg
     
    Last edited: Oct 14, 2016
  5. Oct 14, 2016 #4

    haruspex

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    Seeing the gaphs, I think there is room for interpretation in option 1. It is not entirely clear that they start at different temperatures. You could read it as starting at the same temperature, but with X cooling very rapidly at the start. If so, that is definitely the right graph.
     
  6. Oct 14, 2016 #5
    Oh I get your explanation there, thank you! But still it doesn't point to the answer given, which is option 4.

    By the way, I asked the student again, and yes she confirmed that 4 is the answer. Told her to ask her teacher and then explain it to me.
    Or any other possible explanations or mistake somewhere?
     
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