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Homework Help: Thermodynamics Problem

  1. May 6, 2014 #1
    1. The problem statement, all variables and given/known data

    You have a sealed piston with a VERY LIGHT and moveable top that can easily slide up and down without friction.
    It starts at room temperature with 1 kg of water in the bottom.
    You heat it, boil the water, and further heat the water until it is at 150°C.
    Determine the final volume of the water molecules inside the system. Remember to give the answer in m3.

    Work done by a system at constant pressure is given by:
    ΔW = pΔV
    How much work (in Joules) did the piston do?

    Did the system lose energy of gain energy by doing work??
    A. The system gained energy by doing work
    B. The system gained energy because we did work on it.
    C. Doing work always causes a loss of energy to the system

    True or False:
    Although the system lost energy by doing work, it actually gained more energy because the final temperature was higher than the initial temperature. The energy it gained was from heat transfer of energy from the heat source to the water molecules. Thus, it gained more heat energy than it lost in work energy and the final state is higher energy than the initial state.

    2. Relevant equations

    3. The attempt at a solution

    Part 1:
    V= mass of water/density -> .001 m3
    T1= 25+273.15= 298.15 K
    T2= 150+273.15= 423.15 K
    n= 1000/18= 55.5556 mols
    R= 8.314
    P=100,000 Pa
    V=nRT2/P -> 1.95 m3
    Part 2:
    100,000 (1.95-.001) ->194900
    Part 3:
    Part 4:
    Not really sure, false?

    Overall, not totally sure of my calculations, just want someone to look them over and see if anything's incorrect.
  2. jcsd
  3. May 6, 2014 #2
    I'm having trouble understanding the problem description. What is a "sealed piston?" Can you please describe the initial system more precisely (a diagram, if available)?

  4. May 6, 2014 #3
    All correct except part 4 which is true.
  5. May 6, 2014 #4
    A sealed piston is one that doesn't leak.
  6. May 6, 2014 #5

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Chet, I see the initial system as 1 kg of water in a cylinder with the piston resting on the water's surface so there is no water vapor to start with. So it looks to me like the OP's computations are correct.

    Seems like none of A, B or C is correct, but the follow-up statement is.
  7. May 6, 2014 #6
    Thanks Rude Man. It looks like all the OPs questions have now been answered.

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