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# heat transfer

(1) 如果一個輻射儀量測到某地區海表面輻射出的長波輻射為370 W/m^2, 試估算海表面的溫度為多少? ( 請自行查詢相關所需參數，並明確說明 )

(2)假設某個物體的長波輻射率為0.9，溫度為攝氏25 度，試比較下列兩個因子變化對於長波輻射的影響 (i) 長波輻射率減少20% (ii)該物體絕對溫度下降 5%

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### 1 Answer

- 天同Lv 76 years agoFavorite Answer
(1) Using Stefan's Law,

370 = (5.7x10^-8)T^4

where (5.7x10^-8 W/m^2.T^4 is the Stefan's Constant), and T is the absolute temperature of the sea surface.

Hence, T = 284 K = 10.8 deg.C

(2)(i) New emissivity (輻射率) = 0.9 x 0.8 = 0.72

hence, intensity of radiation

= 0.72 x (5.7x10^-8) x (273+25)^4 W/m^2 = 324 W/m^2

Original intensity = 0.9 x (5.7x10^-8) x (273+25)^4 W/m^2 = 405 W/m^2

Hence, percentage decrease in intensity

= [(405 - 324)/405] x 100% = 20%

(ii) New temperature = (273+25) x 0.95 K = 283.1 K

hence, intensity of radiation = 0.9 x (5.7x10^-8) x 283.1^4 W/m^2

= 330 W/m^2

Percentage decrease in intensity

= [(405 - 330)/405] x 100%

= 18.5%

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