Which statement correctly describes the second law of thermodynamics?

The correct answer is B: Entropy of an isolated system never decreases; it increases for irreversible processes. The Clausius statement of second law: For isolated systems, dS_universe ≥ 0. dS = 0 for reversible, dS > 0 for irreversible processes.

A vertical pipe of diameter 40 mm has water flowing upward at 2.5 m/s. If the friction factor is 0.035 and pipe length is 50 m, what is the total head loss (friction + elevation)?

The correct answer is C: 51.12 m. h_f = 0.035 × (50/0.04) × (6.25/19.62) = 1.12 m; total = 1.12 + 50 = 51.12 m

Which type of bearing is preferred for applications requiring self-alignment capability?

The correct answer is B: Spherical roller bearing. Spherical roller bearings have a spherical outer raceway that allows self-alignment up to 2-3 degrees, making them ideal for misaligned shaft applications.

In a cam-follower system, which type of follower motion profile minimizes shock and vibration?

The correct answer is C: Cycloidal motion. Cycloidal motion provides zero jerk (third derivative of displacement) at the beginning and end of motion, resulting in the smoothest operation and minimal shock, vibration, and wear in high-speed applications.

In an Otto cycle engine with compression ratio r = 9 and γ = 1.4, the thermal efficiency is approximately:

The correct answer is B: 58%. Otto cycle efficiency: η = 1 - 1/r^(γ-1) = 1 - 1/9^0.4 = 1 - 1/2.28 ≈ 0.558 or 55.8% ≈ 58%.

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Mechanical Engineering

Thermodynamics, hydraulics, machine design

123 Q 3 Topics Take Mock Test

Difficulty: All Easy Medium Hard 111–120 of 123

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All Thermodynamics 100 Fluid Mechanics 79 Machine Design 80

Q.111 Medium Thermodynamics

What is the dryness fraction (quality) of steam at a state where internal energy u = 2400 kJ/kg, u_f = 1317.3 kJ/kg, and u_fg = 1753.7 kJ/kg?

A 0.62

B 0.45

C 0.55

D 0.38

Correct Answer: A. 0.62

EXPLANATION

u = u_f + x × u_fg, so x = (u - u_f)/u_fg = (2400 - 1317.3)/1753.7 = 1082.7/1753.7 ≈ 0.62

Test

Q.112 Medium Thermodynamics

Which of the following processes is impossible according to the second law of thermodynamics?

A A process where a system loses heat to surroundings at lower temperature

B A process where total entropy of universe remains constant

C A process where a system absorbs heat and converts it completely into work

D A process where entropy of universe decreases

Correct Answer: D. A process where entropy of universe decreases

EXPLANATION

The second law states that entropy of an isolated system must increase or remain constant (reversible). A decrease in total entropy violates the second law and is impossible.

Test

Q.113 Medium Thermodynamics

In the Rankine cycle, which process involves expansion in a turbine?

A Isobaric process

B Isochoric process

C Isentropic process

D Isothermal process

Correct Answer: C. Isentropic process

EXPLANATION

In an ideal Rankine cycle, turbine expansion is isentropic (reversible and adiabatic), maximizing work output. Real turbines follow this closely but with some irreversibilities.

Test

Q.114 Medium Thermodynamics

A piston-cylinder device contains 0.5 kg of steam at 200°C. Heat is removed and the steam condenses to saturated liquid at the same temperature. The latent heat of vaporization at 200°C is 1941 kJ/kg. Calculate heat removed.

A 970.5 kJ

B 1941 kJ

C 3882 kJ

D 485.25 kJ

Correct Answer: A. 970.5 kJ

EXPLANATION

Q = m × L_fg = 0.5 kg × 1941 kJ/kg = 970.5 kJ heat is removed during condensation

Test

Q.115 Medium Thermodynamics

In a vapour compression refrigeration cycle, the throttling device (expansion valve) causes:

A Decrease in enthalpy

B Increase in enthalpy

C No change in enthalpy

D Decrease in entropy significantly

Correct Answer: C. No change in enthalpy

EXPLANATION

The throttling process is isenthalpic (constant enthalpy). The enthalpy remains the same before and after the expansion valve.

Test

Q.116 Medium Thermodynamics

The specific heat capacity at constant volume (Cv) for a monatomic ideal gas is:

A (3/2)R

B (5/2)R

C (7/2)R

D R

Correct Answer: A. (3/2)R

EXPLANATION

For a monatomic ideal gas with 3 translational degrees of freedom: Cv = (3/2)R. For diatomic: (5/2)R; for polyatomic: (7/2)R.

Test

Q.117 Medium Thermodynamics

Two bodies at different temperatures are brought into contact in an isolated system. The total entropy of the system will:

A Decrease

B Remain constant

C Increase

D Become zero

Correct Answer: C. Increase

EXPLANATION

For an irreversible process in an isolated system, total entropy increases (ΔS_total > 0). Heat transfer between bodies at different temperatures is irreversible.

Test

Q.118 Medium Thermodynamics

In a closed system, the total energy balance equation for a time interval is:

A ΔE = Q + W

B ΔE = Q - W

C Q = ΔE + W

D W = Q - ΔE

Correct Answer: B. ΔE = Q - W

EXPLANATION

First Law: ΔE_system = Q - W, where Q is heat added to system and W is work done by system. This is the IUPAC convention.

Test

Q.119 Medium Thermodynamics

The polytropic process for an ideal gas follows PV^n = constant. When n = 0, the process is:

A Isobaric

B Isochoric

C Isothermal

D Adiabatic

Correct Answer: A. Isobaric

EXPLANATION

When n = 0, PV⁰ = P = constant, which is an isobaric process. When n = 1 (isothermal), n = γ (adiabatic), n = ∞ (isochoric).

Test

Q.120 Medium Thermodynamics

For a real gas, the compressibility factor Z is defined as:

A Z = PV/nRT

B Z = nRT/PV

C Z = P/VT

D Z = V/nRT

Correct Answer: A. Z = PV/nRT

EXPLANATION

Compressibility factor Z = PV/(nRT). For ideal gas, Z = 1. For real gases, Z ≠ 1 due to molecular interactions.

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