A piston/cylinder contains 1 kg water at 20°C, 150 kPa. The pressure is linear in volume. Heat is added from 600°C source until the water is at 1 MPa, 500°C. Find the total change in entropy.
Answer: C
v1 = 0.001002 m^3/kg; u1 = 83.94 kJ/kg; s1 = 0.2966 kJ/kg K
v2 = 0.35411 m^3/kg; u2 = 3124.3 kJ/kg; s2 = 7.7621 kJ/kg K
1W2 = ½ (1000 + 150) 1 (0.35411 – 0.001002) = 203 kJ
1Q2 = 1(3124.3 – 83.94) + 203 = 3243.4 kJ
m(s2 – s1) = 1(7.7621 – 0.2968) = 7.4655 kJ/K; 1Q2/T(source) = 3.7146 kJ/K
1S2 gen = m(s2 – s1) − 1Q2/T(SOURCE) = ∆Stotal
= ∆S(H2O) + ∆S(source) = 7.4655 – 3.7146 = 3.751 kJ/K.