Thermodynamics – Objective Questions
Page 4 : Adiabatic vs Isothermal (Concept Traps)
Q31. For an ideal gas undergoing adiabatic expansion:
- A) Temperature remains constant
- B) Internal energy remains constant
- C) Temperature decreases
- D) Pressure remains constant
Answer: C
Explanation: In adiabatic expansion, work is done at the cost of internal energy → temperature falls.
Q32. In which process does heat exchange occur?
- A) Adiabatic only
- B) Isothermal only
- C) Both
- D) Neither
Answer: B
Explanation: Isothermal process requires heat exchange to maintain constant temperature.
Q33. For the same initial and final volumes, which curve lies above on a P–V diagram?
- A) Adiabatic
- B) Isothermal
- C) Both coincide
- D) Depends on gas
Answer: A
Explanation: Pressure falls more rapidly in adiabatic expansion, so curve lies above isothermal.
Q34. Work done in adiabatic expansion is _____ than isothermal expansion (same V change).
- A) Greater
- B) Less
- C) Equal
- D) Zero
Answer: B
Explanation: Area under adiabatic curve is less than isothermal curve.
Q35. Which relation is valid for an adiabatic process?
- A) PV = constant
- B) P/T = constant
- C) PVγ = constant
- D) V/T = constant
Answer: C
Explanation: Adiabatic condition: PVγ = constant.
Q36. For monoatomic gas, γ equals:
- A) 1.2
- B) 1.33
- C) 1.4
- D) 1.67
Answer: D
Explanation: γ = (f+2)/f = (3+2)/3 = 5/3.
Q37. If an ideal gas expands adiabatically, which of the following decreases?
- A) Temperature
- B) Pressure
- C) Internal energy
- D) All of these
Answer: D
Explanation: Expansion causes work → energy loss → drop in T, P, and U.
Q38. In isothermal expansion of ideal gas:
- A) Heat absorbed = Work done
- B) Heat absorbed > Work done
- C) Heat absorbed < Work done
- D) No heat absorbed
Answer: A
Explanation: ΔU = 0 ⇒ Q = W.
Q39. Which process is faster in real systems?
- A) Isothermal
- B) Adiabatic
- C) Isochoric
- D) Isobaric
Answer: B
Explanation: Rapid processes prevent heat exchange → adiabatic.
Q40. Which process is most difficult to achieve practically?
- A) Adiabatic
- B) Isothermal
- C) Isochoric
- D) Cyclic
Answer: B
Explanation: Maintaining constant temperature requires continuous heat control.
Focus: Adiabatic vs Isothermal traps
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