6. Entropy and the Second Law of Thermodynamics

Which statement best describes the equivalence of heat and work according to the principles of thermodynamics?

How is the thermal efficiency of a heat engine defined?

A heat engine produces 50 kW of net work while rejecting 75 kW of heat to a low-temperature sink. What is its thermal efficiency?

What is the primary purpose of a refrigerator, and how is its performance measured?

What does the Kelvin-Planck statement of the second law of thermodynamics imply about heat engines?

A proposed refrigerator is claimed to remove 4 kW of heat from a cold space while consuming no work (W_in = 0 kW) and rejecting 9 kW of heat to the surroundings. Which law of thermodynamics does this device violate?

Which of the following is NOT an example of an irreversibility?

A Carnot heat engine operates between a high-temperature source at 300 degrees Celsius and a low-temperature sink at 20 degrees Celsius. What is its maximum possible thermal efficiency?

What does the inequality of Clausius state for any thermodynamic cycle?

How is the property of entropy, S, defined based on the inequality of Clausius?

Which of the following statements about entropy generation (S_gen) is correct?

For a closed system, what are the two mechanisms that contribute to the change in the system's entropy?

A rigid tank containing 3 kg of R134a is cooled, causing its total entropy to decrease by 0.8049 kJ/K. Is the change in entropy (delta S) equal to the entropy generation (S_gen) for this process?

For an ideal gas undergoing a process, the change in specific entropy can be calculated using the relation s2 - s1 = Cp * ln(T2/T1) - R * ln(P2/P1). Air is compressed from an initial state of 100 kPa and 27 degrees Celsius (300.15 K) to a final state of 600 kPa and 67 degrees Celsius (340.15 K). For air, Cp = 1.005 kJ/kgK and R = 0.287 kJ/kgK. Calculate the change in specific entropy.

What is an isentropic process?

What are the two mechanisms of entropy transfer for an open system (control volume)?

For a steady-state, steady-flow process in a single-inlet, single-outlet turbine that is assumed to be adiabatic and reversible, which property remains constant between the inlet and outlet?

Steam enters a well-insulated heat exchanger at 100 degrees Celsius as saturated vapor and leaves as saturated liquid at the same temperature. The specific entropy of the saturated vapor is 7.3541 kJ/kgK and for the saturated liquid is 1.3072 kJ/kgK. Another fluid, air, flows through the exchanger and absorbs the heat. What is the rate of entropy generation in the overall well-insulated heat exchanger system per kg of steam?

How does the coefficient of performance of a real heat pump (COP_HP) change if the outdoor (heat sink) temperature decreases, while the indoor (heat source) temperature remains constant?

What is the relationship between the coefficient of performance of a heat pump (COP_HP) and a refrigerator (COP_R) operating between the same two thermal reservoirs?

The Clausius statement of the second law says it is impossible to construct a device that operates in a cycle and produces no effect other than transferring heat from a lower-temperature body to a higher-temperature body. What would be the coefficient of performance of such an impossible device?

Which of the following describes the four processes of a Carnot heat engine cycle in the correct order?

A Carnot refrigerator operates between a cold space at -10 degrees Celsius and surroundings at 25 degrees Celsius. What is the maximum possible coefficient of performance (COP_R)?

In a T-S (Temperature-Entropy) diagram for a reversible process, what does the area under the process curve represent?

Why is the entropy of the universe always increasing?

A two-stage steam engine has a first stage with thermal efficiency of 30 percent and a second stage with a thermal efficiency of 40 percent. 90 percent of the exhaust steam from stage 1 enters stage 2. What is the overall thermal efficiency of the combined cycle?

What is the change in specific entropy of a block of copper (Cp = 0.385 kJ/kgK) when it is heated from 25 degrees Celsius to 100 degrees Celsius?

Ammonia in a piston-cylinder device is cooled from 80 degC, 2000 kPa to a saturated liquid at 15 degC. The process is irreversible. The specific entropy generation for the process is found to be 0.3089 kJ/kgK. What does this value signify?

A rigid, well-insulated tank is divided into two compartments. Compartment A contains 3 kg of CO2 at 150 kPa, 300 K. Compartment B contains 2 kg of CO2 at 220 kPa, 500 K. The partition is removed and the gas mixes. What is the final temperature of the mixture?

In the mixing process of CO2 described in the previous question (3 kg at 300 K mixed with 2 kg at 500 K), what happens to the total entropy of the CO2 in the tank?

What is the primary role of the condenser in a Rankine cycle heat engine?

The thermal efficiency of a real power plant is projected to be 35 percent. The plant produces 500 MW of power. How much heat (Q_L) is rejected to the cooling water in the condenser?

Which component of a vapor-compression refrigeration cycle is responsible for absorbing heat from the cold space?

An isentropic process for an ideal gas follows the relation P * v^k = constant. This is a special case of the more general polytropic process relation P * v^n = constant. What is the value of the polytropic exponent 'n' for an isentropic process?

A heat pump provides 20 kW of heating to a house. Its actual COP is 5.5. How much power is required to drive the heat pump?

Which statement correctly distinguishes entropy (S) from entropy generation (S_gen)?

An inventor claims to have built a heat engine that receives 10 kW of heat from a source, produces 5 kW of net work, and rejects 3 kW of heat to a sink. Does this engine satisfy the laws of thermodynamics?

What is the primary function of an intercooler in a multi-stage compressor?

Air is compressed adiabatically in a two-stage compressor. In stage 1, the specific work is 140.7 kJ/kg. The air is then cooled and enters stage 2, where the specific work is 152.31 kJ/kg. What is the total specific work of the two-stage compressor?

When applying the second law to a closed system, the term representing entropy transfer is approximated by the sum of Q_k / T_k. What does the temperature T_k in this term represent?

An air compressor operates in a polytropic process with n=1.35. Air enters at 100 kPa and exits at 300 kPa. If the inlet temperature is 20 degrees Celsius (293.15 K), what is the outlet temperature?

R134a enters an evaporator at -10 degrees Celsius with a quality of 0.1 and exits as a saturated vapor at the same temperature. The specific enthalpy of the saturated liquid (hf) is 186.70 kJ/kg and the specific enthalpy of the saturated vapor (hg) is 392.67 kJ/kg. What is the specific enthalpy of the R134a at the evaporator inlet?

A throttling valve is a device used to reduce the pressure of a fluid. Assuming the process is adiabatic with negligible changes in kinetic and potential energy, what property of the fluid remains constant across the valve?

Why is it impossible for any heat engine to achieve a better performance than a Carnot heat engine operating between the same two temperature reservoirs?

In a subsonic nozzle, how do the fluid velocity and pressure change as the fluid flows from the inlet to the outlet?

For a steady flow device like a nozzle, the energy conservation equation can be written as h_i + (1/2)V_i^2 = h_e + (1/2)V_e^2. This assumes several conditions. Which of the following is NOT one of the standard assumptions for an ideal nozzle?

An adiabatic two-stage compressor with an intercooler has a total specific entropy generation of 0.246 kJ/kgK. The entropy generation in each of the two adiabatic compression stages is 0.0913 kJ/kgK and 0.0921 kJ/kgK respectively. Why is the total system entropy generation greater than the sum of the entropy generated in the two stages?

For the same overall pressure ratio, why does a two-stage compressor with an intercooler generally require less work input than a single-stage compressor?

What does a perpetual-motion machine of the second kind (PMM2) attempt to do?

An irreversible process and a reversible process both occur between the same initial and final states. Which of the following statements is true regarding their change in entropy (delta S)?