Page 361 - Schaum's Outline of Theory and Problems of Applied Physics

346 ELECTROMAGNETIC INDUCTION [CHAP. 28

28.4. Upon which one or more of the following does the magnetic energy of a coil that carries a current I depend?
(a) I
(b) the number of turns in the coil
(c) the resistance of the coil
(d) the presence of an iron core in the coil

28.5. The horizontal steel cargo boom of a freighter traveling at 10 m/s is 7.0 m long and is at an angle of 75 relative
◦
to the direction of the ship’s motion. The magnetic field of the earth in that region has a vertical component of
4.0 × 10 −5 T. The potential difference between the ends of the boom is
(a) 0.06 mV (c) 2.7 mV
(b) 0.72 mV (d) 2.8 mV

2
28.6. A wire loop that encloses an area of 15 cm is perpendicular to a magnetic field of 0.10 T. If the field drops to 0.04 T
in 0.2 s, the average emf induced in the loop is
(a) 0.3 mV (c) 4 mV
(b) 0.45 mV (d) 4.5 V

2
28.7. A 200-turn coil whose resistance is 4 encloses an area of 20 cm . A changing magnetic field parallel to the coil
axis induces a current of 1.2 A in the coil. How rapidly is the magnetic field changing?
(a) 0.75 T/s (c) 14.4 T/s
(b) 12 T/s (d) 30 T/s

28.8. A transformer has 300 turns in its primary coil and 75 turns in its secondary coil. When the current in the secondary
coil is 20 A, the current in the primary coil is
(a) 5 A (c) 80 A
(b) 25 A (d) 6.4 kA

28.9. If the power input to the transformer of Question 28.7 is 40 W, the power output is
(a) 2.5 W (c) 40 W
(b) 10 W (d) 160 W

28.10. The self-induced emf in a 0.1-H coil in which the current is changing at 200 A/s is
(a) 10 V (c) 0.1 kV
(b) 20 V (d) 2 kV

28.11. While the current in a circuit is falling from 8 to 2 A in 20 ms, the average induced emf in the circuit is 12 V. The
inductance of the circuit is
(a) 0.04 H (c) 100 H
(b) 0.12 H (d) 3.6 kH

28.12. An 80-mH coil whose resistance is 4 is connected to a 12-V battery of negligible internal resistance. The current
in the coil begins to increase at a rate of
(a) 0.15 A/s (c) 7.5 A/s
(b) 3 A/s (d) 150 A/s

28.13. The time needed for the current in the coil of Question 28.12 to reach 63 percent of its final value is
(a) 0.02 s (c)20 s
(b) 0.05 s (d)50 s