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QPages 98-102: Q46: HC Verma: Concepts of Physics - Volume 2 Heat Transfer Pages 98-102: Q46 Solution: A copper sphere is suspended in an evacuated chamber maintained …

A copper sphere is suspended in an evacuated chamber maintained at 300 K. The sphere is maintained at a constant temperature of 500 K by heating it electrically. A total of 210 W of electric power is needed to do it. When the surface of the copper sphere is completely blackened, 700 W is needed to maintain the same temperature of the sphere. Calculate the emissivity of copper.

Step-by-Step Verified Solution

**Answer:** Emissivity of copper = 0.30. **Step-BY-Step Explanation:** For copper sphere: P_copper = 210 W. For blackened: P_black = 700 W.

e_{copper} = \frac{P_{copper}}{P_{black}} = \frac{210}{700} = 0.30

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