Explanation

(a) The Stefan’s law,
$E = \sigma T^{4}$ where $\sigma$ is Stefan’s constant,
Given, $T_{1} = 27℃ = 27 + 273 = 300$ K
$\ \ \ \ \ \ \ \ \ \ \ \ T_{2} = 84℃ = 273 + 84 = 357$ K
$\therefore$ $\frac{E_{1}}{E_{2}} = \frac{T_{1}^{4}}{T_{2}^{4}}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = \frac{(300)^{4}}{(357)^{4}} = \frac{1}{(1.19)^{4}}$
Rate of increase of energy is
$\frac{E_{2}}{E_{1}} = (1.19)^{4} = 2$