EIT - Engineer In Training Thermodynamics Questions and Answers

Question 1

A piston-cylinder device contains a gas that undergoes a process where 50 kJ of heat is added to the system. During this process, the gas expands and performs 20 kJ of work on the surroundings. Assuming changes in kinetic and potential energy are negligible, what is the change in the internal energy of the gas?

Accepted Answer

30 kJ

Answer

The First Law of Thermodynamics for a closed system is given by the equation ΔU = Q - W, where ΔU is the change in internal energy, Q is the heat added to the system, and W is the work done by the system. [3, 7] In this problem, Q = +50 kJ (heat is added) and W = +20 kJ (work is done by the system). Therefore, ΔU = 50 kJ - 20 kJ = 30 kJ. [9]

Question 2

A heat engine operates between a high-temperature reservoir at 800°C and a low-temperature reservoir at 100°C. What is the maximum possible thermal efficiency (Carnot efficiency) for this engine?

Accepted Answer

65.2%

Answer

The maximum theoretical efficiency of a heat engine is the Carnot efficiency, calculated as η_carnot = 1 - (T_L / T_H). [16, 21] The temperatures must be in an absolute scale, such as Kelvin. First, convert the Celsius temperatures to Kelvin: T_H = 800°C + 273.15 = 1073.15 K; T_L = 100°C + 273.15 = 373.15 K. [16] Then, calculate the efficiency: η_carnot = 1 - (373.15 K / 1073.15 K) ≈ 0.6523, or 65.2%.

Question 3

Air, behaving as an ideal gas with k = 1.4, undergoes a reversible and adiabatic (isentropic) compression from an initial state of 100 kPa and 300 K to a final pressure of 700 kPa. What is most nearly the final temperature?

Accepted Answer

522 K

Answer

For an isentropic process of an ideal gas, the relationship between temperature and pressure is given by T₂/T₁ = (P₂/P₁)^((k-1)/k). [8, 13] First, calculate the exponent: (k-1)/k = (1.4-1)/1.4 = 0.4/1.4 ≈ 0.2857. Now, solve for T₂: T₂ = T₁ * (P₂/P₁)^((k-1)/k) = 300 K * (700 kPa / 100 kPa)^(0.2857) = 300 K * (7)^(0.2857) ≈ 300 K * 1.74 = 522 K.

Question 4

A rigid 0.5 m³ tank contains a saturated liquid-vapor mixture of water at 150 kPa. If the mass of the saturated liquid is 1.2 kg and the mass of the saturated vapor is 0.8 kg, what is the quality (x) of the mixture?

Accepted Answer

0.40

Answer

The quality (x) of a saturated liquid-vapor mixture is defined as the ratio of the mass of the vapor to the total mass of the mixture. The total mass (m_total) is the sum of the liquid mass (m_f) and the vapor mass (m_g). m_total = 1.2 kg + 0.8 kg = 2.0 kg. The quality is x = m_g / m_total = 0.8 kg / 2.0 kg = 0.40. The volume and pressure data are not needed for this specific calculation.

Question 5

Which of the following describes the process occurring in the turbine of an ideal Rankine cycle?

Accepted Answer

Isentropic expansion

Answer

The ideal Rankine cycle consists of four processes: 1) Isentropic compression in the pump, 2) Isobaric (constant pressure) heat addition in the boiler, 3) Isentropic expansion in the turbine, and 4) Isobaric (constant pressure) heat rejection in the condenser. [1, 2, 4] Therefore, the process in the turbine is an isentropic expansion.

(EIT) Engineer In Training Certification Practice Test

(EIT) Engineer In Training Certification Practice Test

EIT - Engineer In Training Thermodynamics Questions and Answers