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Question. not Homework

  1. Oct 14, 2007 #1
    Question....... not Homework!!!

    Here is a question paper that I am not able to solve... neither my teacher can. This is not a homework! Please help me if you can.

    Q. Mercury-in-glass thermometer is used to measure the boiling point in a liquid. Suggest why the boiling point of measured liquid is not affected by the thermal energy absorbed by the thermometer bulb?

    Also can anyone tell me relative advantage of thermistor thermometer over mercury-in-glass thermometer?

    Thanks
     
  2. jcsd
  3. Oct 15, 2007 #2

    vanesch

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    Ok, think of it this way: if water is boiling, what's its temperature ? And 10 minutes later, when half of is boiled up, what's the temperature NOW ?
     
  4. Oct 15, 2007 #3

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    The boiling point is a property of the liquid, as Vanesch has pointed out. Why should that change if some heat is absorbed by the thermometer, if other conditions like pressure remain same?

    (The thermometer absorbs very little heat anyway, so at most the liquid will start to boil maybe a few microseconds later, than when the thermometer was not there.)
     
  5. Oct 15, 2007 #4
    I think it certainly would take more energy to get the water to boil with the thermometer in it than without it, but temperature isn't a measure of how much energy was put into it, in the absolute sense, or you would have units of Joules. The point is the thermometer will be the same temperature as the water, so add energy to the water until it boils. A material cannot change phase until the entire mass of the material is at that temperature in which a phase change occurs. I think the question is worded poorly and tries to add confusion where there shouldn't be any. Why would anyone think that the thermometer would affect the boiling point of an independent substance anyway? The thermometer just comes into thermal equilibrium with the substance, it doesn't change it's makeup.
     
  6. Oct 15, 2007 #5

    vanesch

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    The pedagogical high point of the question was probably that it had been treated before that putting a thermometer in contact with a body of a certain temperature can change the temperature of the body if the heat capacity of the thermometer is not neglegible wrt the heat capacity of the body, as some heat has to flow to or from the thermometer to get both bodies at the same temperature. And then now the question is: why doesn't this matter in the case of BOILING liquid...
     
  7. Oct 15, 2007 #6
    This procedure arises at the coal mine when quality samples are taken to determine the btus per ton. Now I haven't done this but I watched and think I understood the principals. The tech grinds up some coal and carefully puts a measured amount in a cup that has an electric heating element. He puts this in what he called the "bomb." This was a fancy thermos bottle designed to keep the heat generated from the burning coal in the water surrounding the small coal furnace. The temperature of the water was measured and the peak rise in temp equated to the known mass of the H2O and he figured in the electric value and some other stuff and was happy. In such a case a thermistor would give less mass to deal with and be able to have a finer resolution for a temperature rise that required measurements in the tenths of a degree.
     
  8. Oct 16, 2007 #7

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    Dear OP (chani10in),

    Now we'd like to know if you have understood the answer.
     
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