20. A circuit consists of a resistor R=10 Ω and an inductor L=1 H in series connected to a direct circuit source of 5 V. The time constant of the circuit is   (20)  

19. A Carnot engine operates between two heat reservoirs at temperatures of 400 K and 800 K. The maximum efficiency of the engine is   (19)   

18. Light in an optical fiber undergoes total internal reflection. If the index of refraction of the fiber is 1.5 and that of the surrounding air is 1.0, the critical angle for the light traveling in the fiber is   (18)  

80.下列哪些處所依配線裝置規則應裝置漏電斷路器？(A)浴室插座分路(B)乾燥處所之110V電燈分路(C)電熱水器分路(D)臨時用電。

79.下列哪些是造成管路壓降的原因？(A)管內流速(B)管路長度(C)管內表面粗糙度(D)管徑大小。

78.低壓積液器安裝與功能，下列敘述哪些正確？(A)防止蒸發器內未蒸發完之液態冷媒進入壓縮機(B)裝置於冷凝器出口與膨脹閥入口之間(C)裝置於蒸發器出口與壓縮機入口之間(D)防止氣態冷媒進入膨脹閥。