Two rods are connected end to end. Rod 1 has length l and thermal conductivity 2K. Rod 2 has length 2l and thermal conductivity K. Both rods have the same cross-sectional area. What is the effective thermal conductivity of the combination?

(A) 6K/5
(B) 4K/5
(C) 3K/4
(D) 2K/3

Correct answer: (A) 6K/5

Solution

When two rods are connected end to end (series), their thermal resistances add. The effective conductivity is found from the total resistance equation K_eff = L_total / (R_total * A).

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