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Homework Help: BLAH! Temp and engine problem, need help

  1. Sep 30, 2004 #1
    I was running through some problems, doing just fine, until I came across these two. I can't work out a correct solution. :cry:

    1) A 1-kg block of ice at 0 C falls into a lake whose water is also at 0 C, and 0.01 kg of the ice melts. What was the minimum altitude from which the ice fell?

    2) Three designs for an engine to operate between 450 K and 300 K are proposed. Design A is claimed to require a heat input of 800 J for each 1000 J of work output, design B a heat input of 2500 J, and design C a heat input of 3500J. Which design would you choose and why?
     
  2. jcsd
  3. Sep 30, 2004 #2

    Doc Al

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    Staff: Mentor

    Here are some hints that may help:
    (1) How much energy is needed to melt that much ice? Where does that energy come from? (Consider gravitational potential energy.)
    (2) What is the maximum possible efficiency of a heat engine operating between those two temperatures?
     
  4. Oct 1, 2004 #3
    :confused: still can't get 'em
     
  5. Oct 1, 2004 #4

    Doc Al

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    Show what you've done so far.
     
  6. Oct 1, 2004 #5
    For the second, I'm assuming the max efficiency is about .33. So I would choose design B.

    I have no clue of what to use for the first problem.
     
  7. Oct 1, 2004 #6

    Doc Al

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    Staff: Mentor

    No need to assume it. Calculate it based on the temperatures.
    What is the stated efficiency of design B?

    How much energy does it take to melt ice? Hint: [itex]Q = m L_f[/itex], where [itex]L_f[/itex] is the "latent heat of fusion" for water.
     
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