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jittapon
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Homework Statement
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
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Homework Equations
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