A copper sphere is suspended within an evacuated chamber kept at...

According to the Stefan-Boltzmann law, the net rate of heat loss by radiation from a body is given by $P = e \sigma A (T^4 - T_0^4)$.

For the copper sphere, the power required to maintain its temperature is:

$P_1 = e \sigma A (T^4 - T_0^4) = 210 \text{ W}$

When the sphere is completely blackened, it behaves as a black body with an emissivity of $1$. The power required is:

$P_2 = (1) \sigma A (T^4 - T_0^4) = 700 \text{ W}$

Dividing the equation for the copper sphere by the equation for the blackened sphere, we get:

$e = \dfrac{P_1}{P_2} = \dfrac{210}{700} = 0.3$

Thus, the emissivity of copper is $0.3$.