CHAPTER 13 REVIEW K/U Knowledge/Understanding T/I Thinking/Investigation C Communication A Application Knowledge For each question, select the best answer from the four alternatives. 1. Which of the following determines the amount of current produced in a conductor by a changing magnetic field? (13.1) K/U (a) the rate of change of the magnetic field (b) the strength of the magnetic field (c) coiling the conductor (d) all of the above 2. Which of the following materials would work best for a pot used in an induction cooker? (13.1) K/U (a) aluminum (b) glass (c) iron (d) plastic 3. Which of the following devices most likely uses alternating current? (13.3) K/U (a) a computer (b) a calculator (c) a light bulb in your home (d) an LCD television 4. Which of the following safety systems can be used only once? (13.3) K/U (a) fuses (b) circuit breakers (c) ground fault circuit interrupters (d) arc fault circuit interrupters 5. For a single-loop AC electric generator, the current produced is zero when the plane of the loop is at which angles relative to the magnetic field? (13.4) K/U (a) 0 and 180 (b) 45 and 135 (c) 90 and 270 (d) 90 and 180 6. Which of the following transformer ratios, p s, will produce the most current in the secondary circuit? (13.5) K/U 7. For a given amount of power and transmission line resistance, at which of the following voltages should a power plant transmit electricity in order to minimize the amount of power lost? (13.6) K/U (a) 20 V (b) 12 kv (c) 250 kv (d) 500 V Indicate whether each statement is true or false. If you think the statement is false, rewrite it to make it true. 8. Increasing the number of loops in a coil decreases the amount of current induced by a changing magnetic field. (13.1) K/U 9. A rapidly changing magnetic field induces more current in a conductor than a slowly changing magnetic field. (13.1) K/U 10. When a magnetic field induces a current in a coil, the electric current produces a magnetic field that opposes the change that produced it. (13.2) K/U 11. Circuit breakers are devices that prevent sparking or arcing that could cause a fire. (13.3) K/U 12. Most homes in orth America use direct current. (13.3) K/U 13. Coils are used in AC generators to increase the amount of electricity produced. (13.4) K/U 14. A DC generator has the same design as a DC motor, and can be considered a motor in reverse. (13.4) K/U 15. Transformers can only operate using direct current. (13.5) K/U Understanding Write a short answer to each question. 16. Figure 1 shows a bar magnet with its north pole facing upward being dropped down a long coil. (13.2) T/I (a) In which direction does the induced magnetic field of the coil point? (b) In which direction does current flow through the coil? CH13Q-F001-OP11UB (a) 2 5 (b) 3 7 (c) 6 5 Figure 1 (d) 9 4 32 Chapter 13 Electromagnetic Induction EL
17. Figure 2 shows a bar magnet with its south pole facing upward being dropped down a long coil. (13.2) T/I (a) In which direction does the induced magnetic field of the coil point? (b) In which direction does current flow through the coil? CH13Q-F002-OP11UB 22. For the single-loop AC generator in Figure 4, the current is shown. Indicate in which direction the loop is spinning, and whether the current is at a maximum, zero, or neither. (13.4) T/I C CH13Q-F004-OP11UB Figure 2 18. Coil A has 50 windings, and coil B has 30 windings. If they are both placed in a changing magnetic field and positioned accordingly, which coil will induce more current? (13.2) K/U 19. A house has a hot tub on the back deck plugged into an outdoor outlet. Which safety device is most likely used with the outlet to prevent any electrical accidents? (13.3) K/U 20. A family has moved into an old house where the wiring may be deteriorating. What safety device should be installed to help prevent any fires or electrical damage while waiting for a certified electrician to change the wiring in the house? (13.3) K/U 21. For the single-loop AC generator shown in Figure 3, indicate in which direction the conventional current is travelling and whether it is at a maximum, zero, or neither. (13.4) T/I C CH13Q-F003-OP11UB Figure 3 Figure 4 23. Coil A has 100 windings, and coil B has 85 windings. How many more windings should be added to coil B so that it will generate twice the amount of electricity that coil A does in a changing magnetic field? (13.1) K/U CH13Q-F005-OP11UB T/I 24. Is the transformer in Figure 5 a step-up or a stepdown transformer? (13.5) K/U primary coil Figure 5 secondary coil 25. If the primary circuit of a transformer has 50 coils, how many coils should be in the secondary circuit to create a (a) step-up transformer? (b) step-down transformer? (13.5) K/U 26. If the secondary circuit of a transformer has 100 coils, how many coils should be in the primary circuit to create a (a) step-up transformer? (b) step-down transformer? (13.5) K/U 27. A power plant produces 1500 MW of power at a potential difference of 20.0 kv. How much current is produced? (13.6) T/I 28. A hydroelectric dam produces 8.0 3 10 2 MW of power at a current of 15 ka. What is the electric potential difference? (13.6) T/I EL Chapter 13 Review 33
29. A solar plant produces electricity that is stepped up to a voltage of 150 kv at a current of 40.0 A. How much power does the solar plant produce? (13.6) T/I 30. (a) A power plant produces power at a current of 40.0 ka. If the resistance in the transmission wire is 0.30 Ω, what is the total power loss due to transmission through the wire? (b) The same power plant uses a transformer to step up the voltage and transmit the current at 2.0 ka. If the total resistance in the transmission wire is the same, what is the total power loss due to transmission through the wire? Assume that there is no loss of power in the step-up transformer. (13.6) T/I 31. A power plant produces 5.0 3 10 2 MW of power that is transmitted at a current of 20.0 ka. If the resistance in the transmission wire is 0.20 Ω, what percentage of power is lost in the transmission line? (13.6) T/I 32. A coal-fired power plant produces 2300 MW of power that is transmitted at a current of 4.0 ka. If there is a 0.50 % loss of power from transmission, what is the total resistance in the transmission wire? (13.6) T/I 33. A nuclear power plant transmits 6.0 ka through a transmission line with a resistance of 0.50 Ω. If there is a 0.70 % loss of power due to transmission, how much power does the nuclear plant generate? (13.6) T/I Analysis and Application 34. In Figure 6 a magnetic field increases from zero to a maximum in the direction shown. Will a current be induced in the wire and, if so, in which direction? Explain. (13.2) T/I C CH13Q-F006-OP11UB (b) What is the direction of the magnetic field induced in the large coil by the current in the small coil? (c) What is the direction of the current induced in the large coil? CH13Q-F008-OP11UB Figure 8 37. In Figure 9, a small coil is placed inside a larger coil and a current is directed through the large coil in the direction shown. (13.1, 13.2) T/I (a) What is the direction of the current in the small coil induced by the magnetic field of the small coil? (b) What is the direction of the magnetic field induced by the current in the large coil? (c) What is the direction of the magnetic field induced by the small coil? CH13Q-F009-OP11UB Figure 6 35. In Figure 7 a magnetic field starts at a maximum in the direction shown and then decreases to zero. Is a current induced in the wire and, if so, in which direction? Explain. (13.2) T/I C CH13-F007-OP11UB Figure 9 Figure 7 36. In Figure 8, a small coil is placed inside a larger coil and a current is directed through the small coil in the direction shown. (13.1, 13.2) T/I (a) What is the direction of the magnetic field of the small coil? 38. Coil type A has 25 windings per centimetre, whereas coil type B has 10 windings per centimetre. If a student has 14 cm of coil type A, how much of coil type B is needed to produce the same amount of current in a changing magnetic field for both coils? Ignore the resistances of the wires. (13.2) T/I 34 Chapter 13 Electromagnetic Induction EL
39. Coil type A has 15 windings per centimetre, whereas coil type B has 9 windings per centimetre. If a student has 10 cm of coil type B, how much coil of type A is needed to produce 1.5 times the current in coil A than in coil B for a changing magnetic field? Ignore the resistances of the wires. (13.2) T/I 40. For the coil AC generators in Figure 10, what is the magnetic polar orientation of the shaded region, and in which direction is the conventional current? (13.4) T/I CH13Q-F010-OP11UB CH13Q-F011-OP11UB Figure 10 41. For the coil AC generator in Figure 11, if the current is moving in the direction indicated, what is the magnetic polar orientation of the shaded region, and in which direction is the coil spinning? (13.4) T/I CH13Q-F012-OP11UB Figure 11 42. The primary circuit in a transformer delivers 5 A of current at 200 V. The secondary circuit delivers 10 A of current. (13.5) T/I (a) What is the voltage of the secondary circuit? (b) What is the ratio of the number of windings of the primary circuit to the number of windings of the secondary circuit? 43. The primary circuit in a transformer has 200 coils and a potential difference of 3.0 3 10 2 V. The secondary circuit has 300 coils. (13.5) T/I (a) What is the voltage of the secondary circuit? (b) What is the ratio of the current in the primary circuit to the current in the secondary circuit? 44. The primary circuit in a transformer has 120 coils and a potential difference of 60 V. The secondary circuit has 160 coils and a current of 5.0 A. (13.5) T/I (a) What is the voltage of the secondary circuit? (b) What is the current of the primary circuit? 45. The primary circuit of a transformer has 60 coils and 8.0 A of current. The secondary circuit has 12 A of current and a potential difference of 20 V. (13.5) T/I (a) What is the voltage of the primary circuit? (b) How many coils does the secondary circuit have? 46. The primary circuit of a transformer has a current of 5.0 A and a potential difference of 60.0 V. The secondary circuit has 120 coils and a potential difference of 25 V. (13.5) T/I (a) How many coils are in the primary circuit? (b) What is the current in the secondary circuit? 47. The primary circuit of a transformer has a potential difference of 200 V with 120 coils. The secondary circuit has a potential difference of 50 V. (13.5) T/I (a) How many coils are in the secondary circuit? (b) If the total resistance in the primary circuit is 10 Ω, what is the current in both circuits? 48. The primary circuit of a transformer carries 30.0 A of current and has 50 coils. The secondary circuit has 150 coils. (13.5) T/I (a) What is the current in the secondary circuit? (b) If the total resistance in the secondary circuit is 4.2 Ω, what are the potential differences of both circuits? 49. The primary circuit of a transformer has 70 coils and a total resistance of 25 Ω. The secondary circuit has 280 coils and a potential difference of 7.00 3 10 2 V. (13.5) T/I (a) What is the potential difference in the primary circuit? (b) What is the resistance in the secondary circuit? 50. The primary circuit of a transformer has a potential difference of 2.0 kv and a total resistance of 5.00 3 10 2 Ω. The secondary circuit has a potential difference of 1.0010 2 V and 50 coils. (13.5) T/I (a) How many coils are in the primary circuit? (b) What is the resistance in the secondary circuit? 51. A power plant produces 1500 MW of power that is stepped up to a potential difference of 3.00 3 10 2 kv for transmission. If the total resistance in the wire EL Chapter 13 Review 35
is 0.150 Ω and there is no loss of power in the transformer, what is the percentage loss of power due to transmission? (13.5, 13.6) T/I 52. A wind power plant produces 12 MW of power that is stepped up to a potential difference of 1.00 3 10 2 kv for transmission. If there is a 0.90 % loss of power due to transmission, what is the total resistance in the transmission wire? Assume there is no power loss in the transformer. (13.5, 13.6) T/I 53. A hydroelectric dam produces power that is stepped up to a potential difference of 220 kv. There is a 0.70 % loss of power due to transmission through a wire with a total resistance of 0.40 Ω. How much power does the dam produce? Assume there is no power loss in the transformer. (13.6) T/I 54. A power plant produces 1800 MW of power at a current of 30.0 ka. The power is then stepped up to a voltage of 240 kv. If the primary circuit used in the transformer has 100 windings, how many windings are used in the secondary circuit? Assume there is no power loss in the transformer. (13.5, 13.6) T/I 55. A power plant produces 2500 MW of power with a potential difference of 40.0 kv. A step-up transformer with 100 windings in the primary circuit and 500 windings in the secondary circuit is used to step up the potential difference for transmission. With what potential difference and current is the power transmitted? Assume there is no power loss in the transformer. (13.5, 13.6) T/I 56. A nuclear power plant produces power with a current of 24 ka. A transformer with 100 windings in the primary circuit and 800 windings in the secondary circuit is used to step up the potential difference to 240 kv. How much power does the nuclear plant produce? (13.5, 13.6) T/I 57. A power plant produces 1200 MW of power that is sent through a transmission line with a resistance of 2 Ω. A transformer is used that has 120 windings 3 in the primary circuit and 600 windings in the secondary circuit. If this changes the power loss to 0.20 %, what are the current and potential difference both before and after the transformer change? Assume there is no power loss in the transformer. (13.5, 13.6) T/I Evaluation 58. tate the law of electromagnetic induction. Using Ohm s law, explain which materials should be used to induce the most current from a changing magnetic field. (13.1) T/I C 59. Using your knowledge of alternating current and electromagnetic induction, explain in your own words why you see a blue spark when you pull a plug out of an outlet too fast. Include an explanation about why it is important to turn off a device before unplugging it. (13.1, 13.2, 13.3) C A 60. Why are superconductors not used to transmit power? If superconductors were available at the time of Edison and Tesla, would Edison have won the energy battle? What would change if superconductors could be used economically in our electrical grid? (13.3, 13.6) C A 61. The modified transformer in Figure 12 has a normal primary circuit, but two secondary circuits. The same winding-to-voltage ratio holds for each segment. (13.5) T/I (a) tate the equations comparing the primary voltage to the output voltage for each of the coil segments. (b) Energy must be conserved. Knowing this, derive an equation for the input power compared to the output power and use it to find an equation relating the input current and voltage to the output currents and voltages. (c) If V p 5 120 V, I p 5 5.0 A, p 5 100, 1 5 5, and 2 5 20, what are the two secondary voltages V 1 and V 2? (d) Why is this type of transformer useful? primary coil Figure 12 secondary coil CH13Q-F013-OP11UBA 62. When a magnet is brought into a copper loop it induces a current for a short time. (13.2) T/I C A (a) Why does the current stop? (b) What would happen if the loop were made of a superconducting material? What would the resulting net magnetic field be? 63. Two coils, A and B, are placed in the same magnetic field. Coil A has twice as many coils as coil B, but the material used in coil B has half the resistance of the material used to make coil A. The number of loops in a coil is directly proportional to the magnitude of the electric current induced. Knowing this, determine which coil would have more current if the magnetic field were to suddenly drop to zero. (13.1, 13.2, 13.6) T/I C 36 Chapter 13 Electromagnetic Induction EL