1. A closed, rigid tank, well-insulated contains air initially at 1 bar, 300 K.
ID: 1857346 • Letter: 1
Question
1. A closed, rigid tank, well-insulated contains air initially at 1 bar, 300 K. The volume is 2 m^3. A stirring
rod transfers energy by work until the pressure is 2 bar. The air behaves as an ideal gas. Determine
the work, in kJ, and the amount of entropy produced, in kJ/K. Comment.
2. Air is compressed in a piston-cylinder assembly from 20 lbf/in.^2, 520 R to 50 lbf/in.^2, in a polytropic process with n = 1.3. The air can be modeled as an ideal gas with constant c_p = 0.24 Btu/lb?R. For the process, determine the work and heat transfer, each in Btu per lb of air, and the change in specific entropy, in Btu/R per lb of air. Comment.
Explanation / Answer
A)
work=pdv.. since dv is zero as there is no change in volume of gas
work done=0
entropy=?S = n*Cp*ln(Pf/Pi)
for air Cp=1KJ/KgK
so?S=n*1*ln(2)=.693n
n=PV/RT=10^5*2/8.314*300=80.1
?S=.693*80.1=55.56KJ/K
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