partthreethermodynamics第三部热力学.pptVIP

Wien‘s displacement law 維恩位移定律 : ?max = b / T ? P ? T4 ? Radiation dominates at high T. 輻射在高温時為主導 Wavelength of peak radiation becomes shorter as T increases. 輻射高峯的波長隨 T 增大而變短 Sun ~ visible light. 太陽 ~ 可見光 Near room T ~ infrared. 室温 ~ 紅外線 Stefan-Boltzmann law 史特凡-波茲曼定律 : GOT IT 懂嗎 ? 16.3. Name the dominant form of heat transfer from 指出下列熱傳遞的主導機制 a red-hot stove burner with nothing on it. 上無一物的紅熱爐灶。 a burner in direct contact with a pan of water. 火爐上放一煱水。 the bottom to the top of the water in the pan once it boils. 水開後從水底到水面 輻射 Radiation 傳導 conduction 對流 convection Example 16.5. Sun’s Temperature 太陽的温度 The sun radiates energy at the rate P = 3.9?1026 W, its radius is 7.0 ?108 m. 太陽以 P = 3.9?1026 W 的功率輻射能量，它的半徑是 7.0 ?108 m. Treating it as a blackbody ( e = 1 ), find its surface temperature. 把它當成是一個黑體 ( e = 1 ) ，找出它的表面温度。 ? = 5.67?10?8 W / m2 K4 Conceptual Example 15.1. Energy-Saving Windows 省能窗户 Why do double-pane windows reduce heat loss greatly compared with single-paned windows? 為甚麼雙層玻璃窗的隔熱効果比單層的大很多 ? Why is a window’s R-factor higher if the spacing between panes is small? 為甚麼窗户的 R-因子會在玻璃間隔小的時候增大? And why do the best windows have “low-E” coatings? 為甚麼最好的窗户都有 “低 E” 外鏌 ? Thermal conductivity (see Table 16.2) 熱導率 (見表 16.2) : Glass 玻璃 k ~ 0.8 W/m?K Air 空氣 k ~ 0.026 W/m?K Layer of air reduces heat loss greatly increases the R-factor . 加一層空氣可以減少散熱和增大 R-因子。 This is so unless air layer is so thick that convection current develops. 這需要空氣層不能太厚，以免產生對流。 “low-E” means low emissivity, which reduces energy loss by radiation. “低 E” 表示低放射率，以減少幅射散熱。 Making the Connection 連起來 Compare the for a single pane window made from 3.0-mm-thick glass with that of a double-pane window make from the same glass with a 5.0-mm air gap between panes. 比較一片 3.0-mm 厚的單層玻璃窗和一片用同樣玻璃，中留 5.0-mm 空隙的雙層玻璃窗。 Glass 玻璃 k ~ 0.8 W/m?K Air 空氣 k ~ 0.026 W/m ? K 16.4. Thermal Energy Balance 熱能的平衡 A house in th