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)  

15. A solenoid with a core of relative permeability μr has 300 turns, a length of 0.5 meters, and carries a current of 2 A. If the permeability of free space is μ₀, what is the magnetic field inside the solenoid? (A) 600μ₁μ₀/0.5 (B) 300μ₁μ₀/0.5 (C) 600μ₁μ₀/2 (D) 0.5μ₁μ₀/600 (E) 0.5μ₁μ₀/300

14. In a region where there is an electric field E and a magnetic field B, a charged particle is observed to travel straight without deflection. What is the speed of the particle if the fields are perpendicular to each other? (A) E+B (B) B/E (C) EB (D) 1/(EB) (E) E/B 

13. A rectangular loop of wire lies in the xy-plane with current I flowing in a clockwise direction when viewed from the positive z-axis. The loop dimensions are a along the x-axis and b along the y-axis. A uniform magnetic field B₀ along the positive z-direction is applied in the region. Using Ampere's Law, determine the net magnetic field at the center of the loop. (A) B₀ along the positive z-direction (B) B₀ along the positive x-direction (C) B₀ along the negative y-direction (D) B₀ along the negative z-direction (E) 0