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Negative heat loss

  1. Nov 14, 2014 #1
    << Moderator note: This thread is missing the homework template due to originally being posted in another forum. >>

    hello everyone, this is my first post and im looking for some help.
    in my lab we did an experiment to calculate specific heat values for various metals using a calorimeter.
    the first thing we did was calculate heat lost by adding hot water to some cold water then plugging numbers in to formulas and calculating Qlost = Qintial - Qfinal
    apparently our Qlost should've been a positive value and we calculated a negative value.
    i dont understand why this is wrong. I would think that Qfinal should be larger than Qintial because we arent we adding energy to the system? It doesnt make sense to me why Qfinal should be smaller than Qinitial.
    any input would be much appreciated!!
     
  2. jcsd
  3. Nov 15, 2014 #2
    How do you calculate Qinital and Qfinal?
     
  4. Nov 15, 2014 #3
    Your measurement is in hot water , in cold water, or in the metal ? And why you adding hot water to cold water and not directly to the metal ?
     
  5. Nov 15, 2014 #4
    Maybe I should've left out the part about the metal. That was the end goal but first thing to do was calculate heat loss. we had some cold water in the calorimeter then added the the hot water. so we were calculating heat lost with only the cold and the hot water.
    These are the two formulas we're using. Letters without subscripts are for hot water, w subscripts is for cold water, c subscript is for the cup.
    m = mass, c = specific heat, T = temp.
    948B483B-233D-4186-9A56-302E16529E37.png
     
  6. Nov 15, 2014 #5

    CWatters

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    The equations look fine. Show us your working.

    If everything was perfectly insulated Qfinal would equal Qinitial. That's because all you are doing is mixing two lots of water together. No energy can escape the system if the insulation is perfect.

    However the calorimeter doesn't have perfect insulation so some energy is lost during the mixing process. This means that Qfinal is smaller than Qinitial.
     
    Last edited: Nov 15, 2014
  7. Nov 15, 2014 #6
    here is my work.
    F6C28DAE-B2C9-467C-86E9-8E0131863F6A.jpg
     
  8. Nov 15, 2014 #7

    CWatters

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    Earlier you wrote..

    Which means:

    M = mass of the hot water
    Mw = mass of cold water

    Your equation for Qinitial has the term MCT1 so T1 must be the temperature of the hot water.

    So why do you have both T1 and TW = 21.8C in the second line of your working? Was the hot and cold water at the same temperature initially?

    I believe you should also be working in Kelvin.

    error.jpg
     
    Last edited: Nov 15, 2014
  9. Nov 15, 2014 #8
    ah good catch. that still leaves me with -107 cal though. but atleast its not -4k haha. i think it should be in kelvin is my heat scale is in joules but im using calories so i dont have to convert masses to kg and temps to kelvin.
     
  10. Nov 15, 2014 #9
    ill convert everything to joules though if you think itll make a difference. i figured though because all of units matched up then the temp scale didnt matter.
     
  11. Nov 15, 2014 #10

    CWatters

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    It's fine to use calories and grams because you have the specific heat capacity of the materials (eg the water) in those units.

    However if you use Celsius the absolute amount of heat in something at a temperature of -10C would be negative. That's not possible.
     
  12. Nov 15, 2014 #11
    so i should convert everything to kelvin then? ill do that in a bit then ill get to you.
     
  13. Nov 15, 2014 #12

    CWatters

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    I haven't checked if these are the only errors.

    What was the hot water temperature?
     
  14. Nov 15, 2014 #13
    80 Celsius
     
  15. Nov 15, 2014 #14
    so converting temps over to kelvin still leaves a negative heat lost. which now that i think about it is what shouldve been expected all i was doing was making the numbers bigger lol.

    so we obviously messed up somewhere. but i dont understand what it means to have a negative heat lost. why should the value be positive?
     
    Last edited: Nov 15, 2014
  16. Nov 16, 2014 #15

    CWatters

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    I also got a negative answer.

    Do you understand why the answer should be zero if the calorimeter has perfect (ideal) insulation?

    A positive answer would imply that the system had gained energy from somewhere (Qfinal > QInitial). That's not possible because the first equation adds up all the energy before they are mixed. Mixing them in the calorimeter cannot add energy (unless the calorimeter has a heater that was accidentally switched on?).

    One possibility is that the mixture wasn't stirred properly? So the 45C final temperature was biased towards the hot water temperature ?
     
  17. Nov 16, 2014 #16

    CWatters

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    Might be constructive to work out what the final temperature should be if Qfinal = QInitial.
     
  18. Nov 16, 2014 #17
    ahhh now that makes sense. i figured my math was right but i was having trouble understanding the concept. thank you making it clear. out of 4 groups, all 4 groups got negative numbers. the instructor was going to try the experiment this weekend and see what she got. our lab equipment is not the most up-to-date lol
     
  19. Nov 16, 2014 #18

    CWatters

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    Will be interesting to see what she gets. Certainly sounds like there was a procedural error somewhere.
     
  20. Nov 16, 2014 #19
    thanks again for all your help. i have a great instructor but ive been in her office all semester, thought id bug the internet for a change lol.
     
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