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Two infinite parallel plates are placed at a certain distance apart. An infinite radiation shield is inserted between the plates without touching any of them to reduce heat exchange between the plates. A assume that the emissivities of plates and radiation shield are equal. The ratio of the net heat exchange between the plates with and without the shield is

1. (A) 1/2
2. (B) 1/3
3. (C) 1/4
4. (D) 1/8

Correct answer: (A) 1/2

Solution

For two infinite parallel plates with N radiation shields of equal emissivity, the net heat exchange is reduced by a factor 1/(N+1). With one shield (N=1) the ratio of heat exchange with shield to without shield is 1/2.

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