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Question:

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