Determine the amount of heat radiated per second by a body having a...

The temperature of the body in thermal equilibrium with the room is T = 20^{}^{{}^{\circ}} \text{C} = 293 \text{ K}.

The surface area of the body is $A = 12 \text{ cm}^2 = 12 \times 10^{-4} \text{ m}^2$.

According to the Stefan-Boltzmann law, the amount of heat radiated per second by the body is given by:

$P = e \sigma A T^4$

Substituting the given values:

$P = 0.80 \times (6.0 \times 10^{-8}) \times (12 \times 10^{-4}) \times (293)^4$

$P = 57.6 \times 10^{-12} \times 7.37 \times 10^9$

$P \approx 0.4245 \text{ W}$

Rounding off, the heat radiated per second is $0.42 \text{ W}$.