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Thermodynamics -- piston-cylinder device with helium is compressed...

  1. Oct 25, 2015 #1
    1. The problem statement, all variables and given/known data
    A piston-cylinder device initially contains 0.5 m^3 of helium gas at 150 kPa and 20°C.Helium is now compressed in a poytropic process(Pv^n = constant) to 400 kPa and 140°C. Determine the entropy change of helium and whether this process is reversible, irreversible, or impossible.Assume the surroundings are at 20°C
    http://postimg.org/image/s7f507n6z/


    2. Relevant equations


    3. The attempt at a solution
    I think helium in this problem is an ideal gas because T>>Tcritical(5.3 K) so I can use ideal gas rule at state 1 and 2
    At state 1 Pv=RT I can find specific volume at state 1 from v1=(2.0769*(273+20))/(150)=4.057 m^3/kg
    and At state 2 Pv=RT I can find specific volume at state 2 from v2=(2.0769*(273+140))/(400)=2.144 m^3/kg
    next I can find the mass of helium from Pv=mRT then m=Pv/RT=(150*0.5)/(2.0769*(273+20))=0.1232 kg

    From Entropy changes of an ideal gas with constant specific heat ΔS=Cv,avg*ln(T2/T1)+Rln(v2/v1)
    and from tables Cv,avg of helium=3.1156 kJ/kg⋅K and R=2.0769 kJ/kg⋅K
    then ΔS=3.1156*ln((273+140)/(273+20))+2.0769*ln(2.144/4.057)=-0.2551 KJ/Kg⋅K
    and entropy changes of Helium is -0.2551*0.1232=-0.0314 KJ/K

    Is my solution correct or not? please help me for my thermodynamics homework
    thankyou
     
  2. jcsd
  3. Oct 25, 2015 #2
    I haven't checked your arithmetic, but your methodology for finding the change in entropy of the system is correct. But, you are not done yet. This is an exercise in applying the Clausius Inequality. You next need to determine how much heat Q is added to the system.

    Chet
     
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