NEET 2025 · PhysicsSpecific heatPrevious Year Question
Two gases A and B are filled at the same pressure in separate cylinders with movable pistons of radius r_A and r_B, respectively. On supplying an equal amount of heat to both systems reversibly under constant pressure, the pistons of gas A and gas B are displaced by 16 cm and 9 cm, respectively. If the change in their internal energy is the same, then the ratio r_A/r_B is equal to:
- A.2/3
- B.√3/2
- C.4/3
- D.3/4✓
Correct Answer
(D) 3/4
Solution & Explanation
Correct Answer (D): 3/4 At constant pressure the first law gives Q = ΔU + W. Both gases receive equal heat Q and have equal ΔU, so they must do equal work W = P ΔV. At the same pressure P, equal work means equal volume change ΔV = (area) × (displacement) = π r² L. Setting r_A² L_A = r_B² L_B with L_A = 16 cm and L_B = 9 cm gives r_A/r_B = √(L_B/L_A) = √(9/16) = 3/4.
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