Chapter 5, Problem 5.47CU

When two systems are in contact, the entropy transfer from the warmer system is equal to the entropy transfer into the cooler one at the point of contact. That is, no entropy can be created or destroyed at the boundary since the boundary has no thickness and occupies no volume.

A 60-lb aluminum bar, initially at T₂ = 150°F, is placed in a tank together with 190 lb of liquid water, initially at Tw= 70°F, and allowed to achieve thermal equilibrium. The aluminum bar and water can be modeled as incompressible with specific heats c₂ = 0.216 Btu/lb.ºR and cw = 0.998 Btu/lb.°R, respectively. Consider the aluminum bar and water as the system and ignore heat transfer between the system and its surroundings. Determine the final temperature Tf, in °F, and the amount of entropy produced within the tank, in Btu/°R.

2. An ideal gas undergoes a process from state 1 ( T1= 300 K, P1= 100 kPa) to state 2 ( T2= 600 K, P2 500 kPa). The specific heats of the ideal gas are : c, = 1 kJ/kg-K and c, = 0.7 kJ/kg-K. The change in specific entropy of the ideal gas from state 1 to state 2 (in kJ/kg-K) is (correct to two decimal places).

= 15°C. The substance A piston-cylinder assembly contains water that is initially at p1 = 2.7 bar and T₁ undergoes a process to reach a final state at p2 = 1.2 bar and T₂ 70°C. Determine the change in specific entropy As in kJ/kg K using an incompressible model and tabulated property data. = (a) Incompressible model: As = Ex: 0.987 kJ/kg. K (b) Tabulated data: As = = Ex: 0.666 kJ/kg K

A container of 1.5 Kg of gas is at a temperature and pressure of 293 K and 1 bar respectively.   The gas is adiabatically compressed until its temperature and pressure are 450 K, 4.49 bars.  Adiabatic processes are processes with no heat transfer.  The properties of this gas are cv = 10.3 KJ/(Kg K) and R = 4.158 KJ/(Kg K).  Neglect kinetic and potential energy terms.    Use the first law to determine the work into the system. Calculate the entropy production for this process. Is this a reversible process?

When a system undergoes a Isochoric process (P = 5 bar), the change in internal internal is 5 kJ and temperature during the process is 15°C. Then its change in entropy would be approximately 1.736 kJ/kgK Select one: True False

A divider separates 1 lb mass of carbon monoxide (CO) from a thermal reservoir at 150o F. the carbon monoxide, initially at 60o F  and 150 lbf/in2, expands isothermally to a final pressure of 10 lbf/in2 while receiving heat transfer through the divider from the reservoir. The carbon monoxide can be modeled as an ideal gas. (a) For the carbon monoxide as the system, evaluate the work and heat transfer, each in Btu and the amount of entropy produced, in Btu/oR. (b) Evaluate the entropy production, in Btu/oR, for an enlarged system that includesthe carbon monoxide and the divider, assuming the state of the divider remains unchanged. Compare with the entropy production of part (a) and comment on the difference.

Entropy generation outside system boundaries can be accounted for by writing an entropy balance on an extended system that includes the system and its immediate surroundings.

No entropy accompanies work as it crosses the system boundary. But entropy may be generated within the system as work is dissipated into a less useful form of energy.