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Homework Help: Need Help with Question

  1. Oct 25, 2005 #1
    Hey all,
    i got a problem im stuck with and it'll probably be easy for you but i cant figure it out for some reason... It has to do with heat transfer and latent heat...

    A tank containing 740 kg of water cools from 15 degrees to 0 degrees and completely freezes. What is the total heat given off? How many hours would a 1.2 kW eletric heater have to work to provide the equivilant heat the water gives off? (The heater provides 1.2 x 10^3 J/s of heat)


    heres my work:

    for part 1: Q=mcT
    Q= (740kg)(4200 J/ kg C)(0 degrees- 15 degrees) 9specific heat capacity for water= 4200 J/kg C
    = -46 620 000 J

    Q= LfM
    Q= (3.3x10^5 J/kg)(740 kg)
    Q= 244 200 000 J

    Total heat= -46 620 000J + 244 200 000J
    =197 580 000

    book says answer is 2.9 x 10^8 (with S.d.) I know how to do the second part but its this first part messing me up....
    Last edited: Oct 25, 2005
  2. jcsd
  3. Oct 25, 2005 #2


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    What's the minus sign for? Why doesn't the answer from the second equation have a minus?
  4. Oct 25, 2005 #3
    the minus sign is there because deltaT is equal to : final temperature-initial temperature....which in the first part is 0-15...there is no negative sign in the second part because the delta T is positive
    Last edited: Oct 25, 2005
  5. Oct 25, 2005 #4


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    In the second part (referring to Q = LfM), if the water didn't freeze, but melt, how would the equation show it (where would the plus/minus sign come from)?
  6. Oct 25, 2005 #5
    its the same regardless because the energy will be the same whether you are freezing or melting the object....
  7. Oct 25, 2005 #6


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    Umm.. No. You're now saying that freezing and melting both bind energy (as in opposite to give off).
    There's a difference between you heating water from 0 to 15 degrees and cooling from 15 to 0.

    Thus, there must be a minus sign in the equation Q = LfM, when something freezes (logically it gives off energy).
  8. Oct 25, 2005 #7
    really? that must be the problem then...my teacher never taught us about a negative sign...he just said use this equation and plug in the numbers... sigh...thanks for your help BTW
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