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A cavity wall is built of two brick layers separated by an air gap, all three layers of equal thickness. The thermal conductivity of brick is far greater than that of air. With the left layer hotter than the right and steady state reached, which graph correctly shows how temperature T varies with distance x across the wall? (Refer to the temperature-distance options.)

1. (A)
2. (B)
3. (C)
4. (D)

Correct answer: (B)

Solution

Most of the temperature drop occurs across the poorly conducting air gap (steep slope there) with very small drops across the highly conductive brick layers, matching graph (B).

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