A copper sphere is suspended in an evacuated chamber maintained at

PHYSICS · Numerical · CBSE · Physics · Thermal Properties of Matter · Thermal Properties of Matter

Problem

300\ \mathrm{K}

. The sphere is maintained at a constant temperature of

500\ \mathrm{K}

by electrical heating. A total electric power of

210\ \mathrm{W}

is required. When the surface of the copper sphere is completely blackened,

700\ \mathrm{W}

is required to maintain the same temperature. Calculate the emissivity of copper.