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Homework Help: Ideal monatomic gas engine problem

  1. Feb 3, 2010 #1
    1. The problem statement, all variables and given/known data
    An ideal monatomic gas is confined to a cylindrical container having a movable, yet "snuggly-fitting," lid that slides up and down to accommodate changes in pressure and volume. The diagram shown below, outlines the steps in a heat cycle starting at position A and moving through positions B, C, D and then back to A.
    2a7246d7-f2d0-4e94-bdb2-45bb48bd08a9.gif
    *ignore the axes labels*
    PV diagram2
    Given:
    PA = PB = 12.0 atm
    PC = PD = 6.0 atm
    VA = VD = 5.0 m3
    VB = VC = 30.0 m3
    temperature of point C is 900 K
    moles of gas are present 2437.3376

    How much total heat is added during the cycle?

    2. Relevant equations
    Q=nCp delta T
    Q=nCv delta T
    PV=nRT


    3. The attempt at a solution
    I tried:
    Q=Q+Q=nCp delta T+nCv delta T=2437.3376*20.78*150+2437.3376*12.47*1500
    =53187581 J but this is wrong.
    Why? and what is the actual answer?
     

    Attached Files:

  2. jcsd
  3. Feb 3, 2010 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Re: Engine

    You have to determine where heat is being added. It is being added(flowing into the gas) when the gas pressure increases at constant volume and when the gas expands at constant pressure. Heat flows out when it loses pressure at constant volume and when it is being compressed at constant pressure. You have to calculate only the heat flow INTO the gas.

    AM
     
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