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.
Text Solution
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According to Stefan,s law,
Temperature difference 
Let the emissivity of copper be 
.
….(i)
When the surface of the copper sphere is completely blackened, 700 W is needed to maintain the temperature of the sphere. For a black body,
On dividing equation (i) by equation (ii) we have,
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