Five kg of nitrogen (N2) undergoes a process from p1 = 5 bar, T1 = 400 K to p2 = 2 bar, T2 = 750 K. Assuming ideal gas behavior, determine the change in entropy, in kJ/K, with: (a) constant specific heats evaluated at 450 K. (b) variable specific heats.

To save energy, the air supply to a 2000 ft office has been shut off overnight and the room temperature has dropped to 40°f. in the morning, the thermostat is reset to 70°f and warm air at 120 f begins to flov in at 200ft'/min. the air is well mixed within the room, and an equal mass flow of air at room temperature (changing with time) is withdrawn through a return duct. the air pressure is nearly 1 atm everywhere. ignoring heat transfer with the surroundings and kinetic and potential energy effects, estimate how long it takes for the room temperature to reach 70°f. plot the room temperature as a function of time.