Question
Air of mass 1 kg, initially at 300 K and 10 bar, is allowed to expand isothermally till it reaches a pressure of 1 bar. Assuming air as an ideal gas with gas constant of 0.287 kJ/kgK, the change in entropy of air (in kJ/kgK, round off to two decimal places) is ________.
Answer :
0.66
m = 1 kg; P1 = 10 bar = 1000 kPa; T1 = 300K
T2 = 300K, P2 = 1 bar
S2 – S1 = mcpln(T2/T1 )-mRln(P2/P1)
S2 – S1 = -mRln(P2/P1)= -1 x 0.287ln(1/10)
S2 – S1 = 0.66 kJ/kgK
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