thermodynamics – MCQs with Answers & Explanations

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1. The zeroth law of thermodynamics establishes the concept of:

 A) Internal energy
 B) Temperature
 C) Entropy
 D) Enthalpy
Answer: B
Explanation: It states that if A is in thermal equilibrium with B and B with C, then A is
with C, defining temperature as a measurable property.

2. A closed system is one in which:

 A) No mass or energy crosses the boundary
 B) Energy crosses but not mass
 C) Mass crosses but not energy
 D) Both mass and energy cross
Answer: B
Explanation: Closed systems exchange energy (heat/work) but not mass; isolated systems
exchange neither.

3. An isolated system:

 A) Exchanges heat only
 B) Exchanges work only
 C) Exchanges both heat and work
 D) Exchanges neither heat nor work nor mass
Answer: D
Explanation: Isolated systems are cut off from surroundings for both mass and energy.

4. Specific heat at constant pressure is defined as:

 A) cp = (∂u/∂T)p
 B) cp = (∂h/∂T)p
 C) cp = (∂u/∂p)T
 D) cp = (∂h/∂p)T
Answer: B
Explanation: By definition, cp is the rate of change of enthalpy with temperature at
constant pressure.

5. For an ideal gas, the difference cp − cv equals:

 A) R
 B) 0
 C) γ

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 D) 1/R
Answer: A
Explanation: Mayer’s relation: cp − cv = R for ideal gases.

6. Enthalpy h is defined as:

 A) u + pv
 B) u − pv
 C) pv − u
 D) Tds
Answer: A
Explanation: h = u + pv, a convenient property for constant-pressure processes.

7. The first law of thermodynamics for a closed system undergoing a process is:

 A) δQ = du + δW
 B) δQ = du − δW
 C) δW = du + δQ
 D) dU = δW − δQ
Answer: A
Explanation: Energy conservation: δQ − δW = du ⇒ δQ = du + δW.

8. Work done during a quasi-static expansion of a gas is:

 A) ∫ v dp
 B) ∫ p dv
 C) ∫ T ds
 D) ∫ u dv
Answer: B
Explanation: Boundary work for a quasi-equilibrium process is the area under the p–v
curve: ∫ p dv.

9. For an ideal gas, internal energy depends on:

 A) Pressure only
 B) Volume only
 C) Temperature only
 D) Both pressure and volume
Answer: C
Explanation: For ideal gases, u = u(T) and h = h(T).

10. The Kelvin–Planck statement of the second law prohibits:

 A) Heat flow from hot to cold
 B) Complete conversion of heat from a single reservoir into work
 C) Work to heat conversion

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 D) Heat engines
Answer: B
Explanation: No heat engine can convert all heat from one reservoir into work in a cyclic
process.

11. The Clausius statement of the second law prohibits:

 A) Heat engines
 B) Refrigerators
 C) Spontaneous heat flow from cold to hot
 D) Spontaneous heat flow from hot to cold
Answer: C
Explanation: Heat cannot spontaneously flow from a colder to a hotter body.

12. Entropy is defined for a reversible process by:

 A) ds = δQ/T
 B) ds = δW/T
 C) ds = du/T
 D) ds = dh/T
Answer: A
Explanation: For reversible heat transfer, δQrev = T ds.

13. For any adiabatic reversible process in an isolated system, entropy:

 A) Increases
 B) Decreases
 C) Remains constant
 D) Is undefined
Answer: C
Explanation: Reversible adiabatic process is isentropic; in isolated systems entropy is
constant if reversible.

14. The entropy of the universe for any real process:

 A) Decreases
 B) Remains constant
 C) Increases
 D) Is conserved
Answer: C
Explanation: Second law: total entropy increases for irreversible processes.

15. The Carnot cycle efficiency depends on:

 A) Working fluid
 B) Pressures

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 C) Reservoir temperatures
 D) Compression ratio
Answer: C
Explanation: ηCarnot = 1 − TL/TH, independent of working substance.

16. The thermal efficiency of a heat engine is:

 A) Wout/Qin
 B) Qin/Wout
 C) Wout/Win
 D) Qout/Qin
Answer: A
Explanation: Efficiency is the fraction of input heat converted to net work.

17. A process at constant temperature is:

 A) Isobaric
 B) Isochoric
 C) Isothermal
 D) Isentropic
Answer: C
Explanation: Isothermal means constant T.

18. A process with no heat transfer is:

 A) Isothermal
 B) Adiabatic
 C) Isentropic
 D) Isobaric
Answer: B
Explanation: Adiabatic means δQ = 0.

19. For an ideal gas undergoing a reversible adiabatic process, p v^γ is:

 A) Constant
 B) Proportional to T
 C) Zero
 D) Increasing
Answer: A
Explanation: Isentropic relation: p v^γ = constant.

20. Polytropic process definition:

 A) p v = constant
 B) p v^n = constant
 C) T v = constant