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Two large parallel plates exchange heat by radiation. If each plate is a grey body with emissivity 0.5 (instead of being a perfect black body), by what factor does the net rate of heat transfer per unit area between them change?

1. 1/2
2. 1/3
3. 1/4
4. 1/5

Correct answer: 1/3

Solution

The grey-body factor 1/(1/0.5 + 1/0.5 - 1) = 1/3, so the net radiative transfer is one-third of the black-body value.

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