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Old Jan 25th 2017, 10:39 AM   #1
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Some fun problems to solve

Hi, the teacher gave me this problem to solve. So i hope you can help me with it. Here is the problem:
John poured 150ml ob boiling water in 2 cups made out of
polystyrene (no heat goes out). In 1 cup he add 5 mm thick layer of hot oil. He measured starting temperatures and then he measured temperatures again after 10 min. He measured the mass of water and water+oil before and after 10 min. Here are the resaults:
-cup 1 (only water)
starting temp.: 96C
finnish temp.: 64C
starting mass: 149.4g
finnish mass: 142.42g

-cup 2(with oil on top)
starting temp.: 96C
finnish temp.:91C
starting mass: 164.72g
finnish mass: 164.70g
The surface of the both cups is 0.003 m2

Questions:
1.Why did the water covered with oil cooled down slower that the one with no oil? Show with calculations.
2.Why did John use oil with temperature around 100C and not oil with room temperature? How would the resaults change if he used room temeprature oil?
3. Which other physical values, which are relevant, can you calculate from this informations?

Thank you for your help

Last edited by XFACTORMAN; Jan 25th 2017 at 11:11 AM.
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Old Jan 25th 2017, 11:49 AM   #2
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I would think 1. at least would be obvious. If the Styrofoam cup insulates the water, what causes it to cool?

For 2. why John does one thing rather than another depends on John' purpose in doing this experiment.

For 2., again, what causes the water to cool?
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Old Jan 25th 2017, 11:55 AM   #3
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cups are open on the top so air cools the liquid down. Air temperature is 20C. i forgot to include that..

Last edited by XFACTORMAN; Jan 25th 2017 at 12:19 PM.
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Old Jan 25th 2017, 02:06 PM   #4
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I note that the cup with only water ends up 7 grams lighter,
I would assume that that would be due to evaporation...
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