NEET 2025 · PhysicsPrevious Year Question
Three identical heat-conducting rods are connected in series as shown. The two side rods have thermal conductivity 2K and the middle rod has thermal conductivity K. The left end is maintained at 3T and the right end at T; the rods are insulated from the sides. In steady state the left junction is at T₁ and the right junction is at T₂. The ratio T₁ : T₂ is:
- A.5/3✓
- B.5/4
- C.3/2
- D.4/3
Correct Answer
(A) 5/3
Solution & Explanation
Thermal resistance R = L/(KA). Side rods: R_s = L/(2KA) = r; middle: R_m = L/(KA) = 2r. Series total = r + 2r + r = 4r. Heat current H = (3T − T)/4r = T/(2r). T₁ = 3T − H·r = 3T − T/2 = 5T/2; T₂ = T + H·r = T + T/2 = 3T/2. T₁/T₂ = (5T/2)/(3T/2) = 5/3.
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