According to the First Law of Thermodynamics for a closed system: dU = δQ - δW. If a gas is compressed adiabatically, which is true?

The correct answer is A: ΔU = W (work done ON the gas increases internal energy). In adiabatic process, Q=0, so ΔU = -W or ΔU = W (depending on sign convention). Work done ON gas is positive, increasing internal energy.

A V-belt drive operates with a belt velocity of 20 m/s and transmits 30 kW. If the belt has a mass of 0.5 kg/m, what is the centrifugal tension in the belt?

The correct answer is D: 200 N. Centrifugal tension T_c = m × v² = 0.5 × 20² = 200 N

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.

What is the first law of thermodynamics in differential form?

The correct answer is A: dU = δQ - δW. The correct form is dU = δQ - δW, where δW = PdV for expansion work. This represents energy conservation in thermodynamic systems.

What is the relationship between specific heats C_p and C_v for an ideal gas?

The correct answer is B: C_p - C_v = R. The Mayer relation: C_p - C_v = R, where R is the specific gas constant. This relation holds for all ideal gases.

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Mechanical Engineering

Thermodynamics, hydraulics, machine design

123 Q 3 Topics Take Mock Test

Difficulty: All Easy Medium Hard 101–110 of 123

Topics in Mechanical Engineering

All Thermodynamics 100 Fluid Mechanics 79 Machine Design 80

Q.101 Medium Thermodynamics

For an ideal gas in a constant pressure (isobaric) process from state 1 to state 2, the change in specific entropy is:

A Cp ln(T2/T1)

B Cv ln(V2/V1)

C R ln(P2/P1)

D Cp ln(P2/P1)

Correct Answer: A. Cp ln(T2/T1)

EXPLANATION

For isobaric process: dS = dQ/T = nCp dT/T. Integrating: ΔS = nCp ln(T2/T1) or specific entropy Δs = Cp ln(T2/T1)

Test

Q.102 Medium Thermodynamics

In a turbocharger application, compressed air enters a turbine. If the isentropic efficiency of the turbine is 0.85, what does this indicate?

A 85% of input heat is converted to work

B Actual work output is 85% of isentropic work output

C 85% of the system is reversible

D The process is 85% adiabatic

Correct Answer: B. Actual work output is 85% of isentropic work output

EXPLANATION

Isentropic efficiency = Actual work output / Isentropic work output = 0.85. It compares real process with ideal isentropic process.

Test

Q.103 Medium Thermodynamics

During throttling of a real gas through an expansion valve, which thermodynamic property remains constant?

A Temperature

B Entropy

C Enthalpy

D Internal energy

Correct Answer: C. Enthalpy

EXPLANATION

Throttling is an isenthalpic process (constant enthalpy). Temperature may change for real gases due to Joule-Thomson effect.

Test

Q.104 Medium Thermodynamics

During an adiabatic expansion of an ideal gas, the temperature of the gas decreases. This is because:

A Heat is removed from the gas

B The gas does work on its surroundings, reducing internal energy

C The surroundings do work on the gas

D The gas absorbs heat from surroundings

Correct Answer: B. The gas does work on its surroundings, reducing internal energy

EXPLANATION

In adiabatic process, Q = 0. From first law: dU = -W. During expansion, W > 0, so dU < 0, meaning internal energy and temperature decrease.

Test

Q.105 Medium Thermodynamics

Which of the following statements is correct regarding the first law of thermodynamics?

A Energy can be created in an isolated system

B The internal energy of an isolated system can increase indefinitely

C Heat and work are forms of energy transfer, not properties of a system

D Internal energy is path-dependent

Correct Answer: C. Heat and work are forms of energy transfer, not properties of a system

EXPLANATION

The first law states that dU = Q - W, where heat (Q) and work (W) are energy transfer mechanisms, not state properties. Internal energy (U) is a state property.

Test

Q.106 Medium Thermodynamics

A reversible heat engine operates between two thermal reservoirs. If the temperature of the hot reservoir is 500 K and the cold reservoir is 300 K, what is the maximum possible efficiency of this engine?

A 40%

B 60%

C 50%

D 75%

Correct Answer: A. 40%

EXPLANATION

Maximum efficiency occurs in a Carnot engine: η = 1 - (T_cold/T_hot) = 1 - (300/500) = 1 - 0.6 = 0.4 or 40%

Test

Q.107 Medium Thermodynamics

A heat engine receives 2000 J from a hot reservoir and rejects 1200 J to a cold reservoir. What is the thermal efficiency and work output?

A η = 40%, W = 800 J

B η = 60%, W = 1200 J

C η = 40%, W = 1200 J

D η = 50%, W = 1000 J

Correct Answer: A. η = 40%, W = 800 J

EXPLANATION

Efficiency η = W/Q_in = (Q_in - Q_out)/Q_in = (2000-1200)/2000 = 800/2000 = 0.4 = 40%. Work output W = 800 J

Test

Q.108 Medium Thermodynamics

At what condition is the specific heat capacity at constant pressure equal to infinity?

A At critical point

B During phase change at constant temperature and pressure

C At absolute zero

D At very high pressures

Correct Answer: B. During phase change at constant temperature and pressure

EXPLANATION

During phase transitions (like vaporization at constant T and P), infinite heat can be absorbed without temperature change, making C_p → ∞.

Test

Q.109 Medium Thermodynamics

In an Otto cycle, the compression ratio is 10 and γ = 1.4. Calculate the thermal efficiency.

A 58.2%

B 60.2%

C 55.2%

D 62.2%

Correct Answer: B. 60.2%

EXPLANATION

Otto cycle efficiency = 1 - (1/r^(γ-1)) = 1 - (1/10^0.4) = 1 - (1/2.512) = 1 - 0.3981 = 0.602 ≈ 60.2%

Test

Q.110 Medium Thermodynamics

In a polytropic process PV^n = constant, if n = 1, the process is:

A Adiabatic

B Isothermal

C Isobaric

D Isochoric

Correct Answer: B. Isothermal

EXPLANATION

For polytropic process with n=1: PV = constant, which is the ideal gas law at constant temperature, making it isothermal (T = constant).

Test

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