PlacementPrepThe first law of thermodynamics is a statement of the conservation of:
Solution
Energy can change form but is neither created nor destroyed.
Answer: (A) Energy
Technical · Mechanical Engineering
Thermodynamics MCQs for placement tests
20 technical questions of moderate difficulty, as asked in campus placement and company tests. Each has a short explanation.
The Kelvin-Planck statement relates to the impossibility of:
Solution
Some heat must always be rejected to a colder reservoir.
Answer: (B) A heat engine converting all heat from one reservoir into work
The efficiency of a Carnot engine working between 500 K and 300 K is:
Solution
η = 1 − T₂/T₁ = 1 − 300/500 = 0.4.
Answer: (C) 40%
In an isothermal process for an ideal gas, which quantity stays constant?
Solution
Iso-thermal means constant temperature.
Answer: (B) Temperature
In an adiabatic process:
Solution
Q = 0; work changes the internal energy.
Answer: (A) No heat is transferred
Entropy of an isolated system during an irreversible process:
Solution
This is the second law of thermodynamics.
Answer: (D) Increases
The Otto cycle is the ideal cycle for:
Solution
Heat is added at constant volume.
Answer: (D) Petrol (spark-ignition) engines
The Diesel cycle adds heat at constant:
Solution
Combustion in the ideal diesel cycle occurs at constant pressure.
Answer: (B) Pressure
For the same compression ratio, which cycle has the highest efficiency?
Solution
For a given compression ratio, Otto > Dual > Diesel.
Answer: (A) Otto cycle
The Rankine cycle is the ideal cycle for:
Solution
It uses water and steam as the working fluid.
Answer: (C) Steam power plants
The Brayton cycle is the ideal cycle for:
Solution
Heat is added and rejected at constant pressure.
Answer: (A) Gas turbines
The COP of a refrigerator is the ratio of:
Solution
COP = Q_L/W.
Answer: (B) Heat removed from the cold space to work input
A refrigerator removes 300 kJ of heat using 100 kJ of work. Its COP is:
Solution
COP = 300/100 = 3.
Answer: (D) 3
The specific heat ratio γ for air is about:
Solution
γ = cp/cv ≈ 1.4 for diatomic gases like air.
Answer: (C) 1.4
Which property is a path function?
Solution
Heat and work depend on the process path; properties do not.
Answer: (D) Work
In a throttling process, which quantity remains constant?
Solution
Throttling is an isenthalpic process.
Answer: (D) Enthalpy
The zeroth law of thermodynamics is the basis for:
Solution
Two systems in thermal equilibrium with a third are in equilibrium with each other.
Answer: (D) Temperature measurement
1 ton of refrigeration is approximately:
Solution
1 TR = 211 kJ/min ≈ 3.5 kW.
Answer: (D) 3.5 kW
The triple point of water is:
Solution
At 273.16 K (0.01 °C), ice, water and vapour coexist.
Answer: (B) 273.16 K
The work done in a constant-volume process is:
Solution
Work = ∫P dV = 0 when volume does not change.
Answer: (B) Zero
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