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A copper sphere hangs in an evacuated chamber held at 300 K. To keep the sphere at a steady 500 K it must be supplied 210 W of electrical power. After its surface is fully blackened, 700 W is required to hold the same temperature. Find the emissivity of the copper surface.

1. 0.3
2. 0.4
3. 0.5
4. 0.6

Correct answer: 0.3

Solution

Net radiation power is proportional to emissivity, so e = 210/700 = 0.3.

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