Last updated: October 22, 2008 PHY 2054 Discussion – Fall ‘08 Practice Exam Problems (Chapter 20) 1. A loop of area 0.250 m2 is in a uniform 0.0200-T magnetic field. If the flux through the loop is 3.83×10-3 Tm2, what angle does the normal to the plane of the loop make with the direction of the magnetic field? (Magnetic Flux) 2. a. 40.0˚ b. 50.0˚ c. 37.5˚ d. This is not possible. A planer loop consisting of four turns of wire, each of which encloses 200 cm 2, is oriented perpendicularly to a magnetic field that increases uniformly in magnitude form 10 mT to 25 mT in a time of 5.0 ms. That is the resulting induced current in the coil if the resistance of the coil is 5.0 Ω? (Induced Current) 3. a. 60 mA b. 12 mA c. 0.24 mA d. 48 mA A 10-turn square coil of area 0.036 m2 and a 20-turn circular coil are both placed perpendicular to the same changing magnetic field. The voltage induced in each of the coils is the same. What is the area a. 0.072 m2 of the circular coil? (Faraday’s Law) 4. b. 0.60 m2 c. 0.018 m2 d. 0.036 m2 A bar magnet is falling through a loop of wire with constant velocity. The south pole enters first. As the magnet leaves the wire, the induced current (as viewed from above): (Lenz’s Law) clockwise 5. b. is counterclockwise c. is zero a. is d. is along the length of the magnet A large jetliner with a wingspan of 40 m flies horizontally and due north at a speed of 300 m/s in a region where the magnetic field of the earth is 60 μT directed 50˚ below the horizontal. What is the magnitude of the induced emf between the ends of the wing? (Motional Emf) a. 250 mV b. 350 mV c. 550 mV d. 750 mV 6. A straight wire of length l is oriented east-west and in a magnetic field B pointing north. The wire is moving downward at a constant speed v. Which end of the rod is positively charged? (Polarity of Motional Emf) 7. a. neither b. the east end c. the west end d. both ends Two loops of wire are arranged so that a changing current in one will induce a current in the other. If the current if the first is increasing clockwise by 1.0 A every second, the induced current in the second loop will: (Minus Sign in Faraday’s Law) constant. 8. c. increase clockwise also. a. be increasing counterclockwise. b. stay d. stay zero. A 500-turn circular coil with an area of 0.050 0 m2 is mounted on a rotating frame, which turns at a rate of 20.0 rad/s in the presence of a 0.050 0 T uniform magnetic field that is perpendicular to the axis of rotation. What is the instantaneous emf in the coil at the moment that the normal to its plane is parallel to the field? (Generators) 9. a. zero b. 125 V c. 216 V d. 250 V An electric motor draws a current of 2.50 A from a 15.0-V battery when it runs at normal speed. If the motor’s armature is “jammed” so that it cannot rotate, the current suddenly rises to 10.0 A. What is the back emf of the motor when running at normal speed? (Back Emf) a. 18.8 V b. 10.0 V c. 12.5 V d. 11.3 V 10. A coil with a self-inductance of 0.75 mH experiences a constant current buildup from zero to 10 A in 0.25 s. What is the induced emf during this interval? (Self-Inductance) a. 0.045 V b. 0.030 V c. 0.47 V d. 0.019 V 11. A 12-V battery is connected in series with a switch, resistor and coil. If the circuit’s time constant is 2.0×10-4 s and the final steady current after the switch is close becomes 1.0 A, what is the value fo the inductance? (RL Circuit) a. 1.2 mH b. 2.4 mH c. 9.6 mH d. 48 mH 12. Superconductors have been discussed as a means for electrical energy storage. Because they are resistanceless a current once started in a loop would continue without loss. If a current of 1.0×104 A were started in a huge toroidal coil of radius 1.0 km and inductance 50 H, how much electrical energy (in kWh) could be stored? (Energy Stored in Inductors) kWh a. 300 kWh d. 840 kWh Answers: 1-a 2-d 3-c 4-b 5-c 6-b 7-b 8-a 9-d 10-b 11-b 12-c b. 480 kWh c. 690

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# Last updated: October 22, 2008