A body at a temperature of 728°C and has surface area 5 cm², radiates 300 J of energy each minute. The emissivity is (Given Boltzmann constant = 5.67 × 10⁻⁸ Wm⁻²K⁻⁴)
e=0.02
e=0.2
e=0.18
e=0.15
Solution:
According to the Stefan-Boltzmann law, P = eσAT⁴ ⇒ e = P/σAT⁴ = 300/(60 × 0.0005 × 5.67 × 10⁻⁸ × (728 + 273)⁴) = 0.18