Thermodynamics – Objective Questions
Page 8 : GOLD Questions (Multi-Concept • IIT Level)
💎 GOLD QUESTIONS = Questions where 2–3 laws act together. These decide top ranks.
Q66. An ideal gas undergoes a process in which temperature remains constant while pressure doubles. What happens to volume?
- A) Doubles
- B) Becomes half
- C) Remains same
- D) Becomes four times
Answer: B
Logic: Isothermal ⇒ PV = constant
Q67. In an adiabatic expansion of ideal gas:
- A) Temperature increases
- B) Temperature decreases
- C) Temperature constant
- D) Internal energy constant
Answer: B
Key idea: Work done comes from internal energy.
Q68. A gas expands freely into vacuum. Choose correct statements:
- 1) Work done = 0
- 2) Heat absorbed = 0
- 3) Temperature changes
- 4) Internal energy remains constant
Answer: 1, 2, 4
Trap: Free expansion is not adiabatic by insulation.
Q69. Which P–V curve encloses maximum area for same initial & final states?
- A) Adiabatic
- B) Isothermal
- C) Isochoric
- D) Free expansion
Answer: B
Reason: Maximum heat intake ⇒ maximum work.
Q70. Two Carnot engines work between same reservoirs. Which is correct?
- A) Efficiencies differ
- B) Efficiency depends on gas
- C) Efficiencies are equal
- D) Depends on pressure
Answer: C
Golden Rule: Carnot efficiency depends only on temperatures.
Q71. In a cyclic process, which quantity is path-dependent?
- A) Internal energy
- B) Temperature
- C) Work
- D) Pressure
Answer: C
Concept: Work & heat are path functions.
Q72. For ideal gas, Cp − Cv equals:
- A) R
- B) 2R
- C) γR
- D) Zero
Answer: A
Must-remember: Cp − Cv = R (always)
Q73. If γ = 1, the process behaves like:
- A) Adiabatic
- B) Isothermal
- C) Isochoric
- D) Free expansion
Answer: B
Reason: γ = Cp/Cv = 1 ⇒ Cv → ∞ ⇒ isothermal
Q74. Which graph represents adiabatic expansion?
- A) Steep curve in P–V
- B) Straight line in P–V
- C) Horizontal line
- D) Vertical line
Answer: A
Q75. Refrigerator works on which principle?
- A) First law
- B) Zeroth law
- C) Second law
- D) Third law
Answer: C
Important: Heat cannot flow cold → hot without work.
These are Rank-deciding Thermodynamics questions
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