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)