A copper sphere is suspended in an evacuated chamber maintained at

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