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Question
Energy is emitted from a hole in an electric furnace at the rate of 20 W when the temperature of the furnace is 727°C. What is the area of the hole? (Take Stefan’s constant σ to be 5.7 × 10-8 Js-1 m-2K-4.)
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Solution
`"Q"/"t" = 20`W, T = 273 + 727 = 1000 K
σ = 5.7 × 10-8 Js-1 m-2K-4.
`"Q"/"t" = sigma"AT"^4`
∴ The area of the hole,
A = `("Q"//"t")/(sigma"T"^4) = 20/((5.7 xx 10^-8)(10^3)^4)` m2
`= (20xx10^-4)/5.7 = 3.509 xx 10^-4` m2
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