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A two-layer furnace wall carries the same heat flow through both layers, whose thicknesses x1 and x2 are equal. The outer temperatures T1 and T3 are held constant with T1 > T3, and the temperature drop is larger across layer 1. Which statement is correct?

1. k1 > k2
2. k1 < k2
3. k1 = k2 but heat flow through material (1) is larger then through (2)
4. k1 = k2 but heat flow through material (1) is less than that through (2)

Correct answer: k1 < k2

Solution

Since Q and x are the same, k is inversely related to its temperature drop; the larger drop across layer 1 means k1 < k2.

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