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A wall is made of two layers A and B of equal thickness but different materials. The thermal conductivity of A is twice that of B. In steady state the temperature drop across layer B is 36 deg C. Find the temperature drop across layer A.

1. 6 deg C
2. 12 deg C
3. 18 deg C
4. 72 deg C

Correct answer: 18 deg C

Solution

With equal thickness and the same heat flow, delta T is inversely proportional to conductivity; A has twice the conductivity of B, so its drop is half of B's, i.e. 18 deg C.

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