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A composite wall consists of two slabs, A and B, of equal thickness but made of different materials. Their thermal conductivities satisfy K_A = 3K_B. If the total temperature difference across the wall is 20°C, then in steady thermal equilibrium:

1. the temperature drop across slab A is 15°C
2. the temperature drop across slab A is 5°C
3. the temperature drop across slab A is 10°C
4. the heat flow rate through slab A is greater than that through slab B

Correct answer: the temperature drop across slab A is 5°C

Solution

Same Q through both: dT_A/dT_B = K_B/K_A = 1/3. With dT_A + dT_B = 20, dT_A = 20/4 = 5 C. Heat flow rates are equal (series), so the 'greater through A' option is false.

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