question_answer

A copper sphere is suspended in an evacuated chamber maintained at\[300K\]. The sphere is maintained at constant temperature of \[900K\] by heating electrically. A total of \[300W\]electric power is needed to do this. When half of the surface of the copper sphere is completely blackened, \[600W\] is needed to maintain the same temperature of sphere. What is the emissivity of copper?

Answer:

1/3