Chapter 25

1. Chapter 25 Electromagnetic Induction

2. Voltage will be induced in a wire loop when a magnetic field within that loop • changes. • aligns with the electric field. • is at right angles to the electric field. • converts to magnetic energy.

3. Voltage will be induced in a wire loop when a magnetic field within that loop • changes. • aligns with the electric field. • is at right angles to the electric field. • converts to magnetic energy.

4. When you thrust a bar magnet to and fro into a coil of wire, you induce • direct current. • alternating current. • neither dc nor ac. • alternating voltage only, not current.

5. When you thrust a bar magnet to and fro into a coil of wire, you induce • direct current. • alternating current. • neither dc nor ac. • alternating voltage only, not current. Explanation: Indeed alternating voltage is induced, which produces alternating current, so we can say that current as well as voltage is induced in a metal coil. Hence b is correct.

6. When a magnet is moved into a wire coil, an induced current in the coil produces a magnetic field that • resists motion of the magnet. • attracts the magnet. • has negligible effect on the magnet. • interferes with the electric field.

7. When a magnet is moved into a wire coil, an induced current in the coil produces a magnetic field that • resists motion of the magnet. • attracts the magnet. • has negligible effect on the magnet. • interferes with the electric field. Comment: This is shown in Figure 25.4.

8. Faraday’s law is the law • of energy conservation. • of charge conservation. • of electromagnetic induction. • that connects voltage and resistance to magnetic fields.

9. Faraday’s law is the law • of energy conservation. • of charge conservation. • of electromagnetic induction. • that connects voltage and resistance to magnetic fields. Comment: Answer a has merit, but answer c is central. Answer d is gibberish.

10. The underlying physics of an electric motor is that • electric and magnetic fields repel each other. • a current-carrying wire experiences force in a magnetic field. • like magnetic poles repel each other. • ac voltage is induced by a changing magnetic field.

11. The underlying physics of an electric motor is that • electric and magnetic fields repel each other. • a current-carrying wire experiences force in a magnetic field. • like magnetic poles repel each other. • ac voltage is induced by a changing magnetic field. Comment: Answer c is indirect, but answer b is more direct.

12. The essential physics concept in an electric generator is • Coulomb’s law. • Ohm’s law. • Faraday’s law. • Newton’s second law.

13. The essential physics concept in an electric generator is • Coulomb’s law. • Ohm’s law. • Faraday’s law. • Newton’s second law.

14. Within both a conventional ammeter and voltmeter you will find a • diode. • capacitor. • galvanometer. • tiny motor.

15. Within both a conventional ammeter and voltmeter you will find a • diode. • capacitor. • galvanometer. • tiny motor.

16. Both a motor and a generator operate via • similar concepts. • quite different concepts. • idealized transformers. • independent energy sources.

17. Both a motor and a generator operate via • similar concepts. • quite different concepts. • idealized transformers. • independent energy sources.

18. The major difference between a motor and generator is • input and output. • direction of windings of coils. • that one uses ac, and the other dc. • primarily cosmetic.

19. The major difference between a motor and generator is • input and output. • direction of windings of coils. • that one uses ac, and the other dc. • primarily cosmetic. Explanation: The main difference between a motor and a generator is the roles of input and output, which is opposite for each.

20. When a generator is used to light a lamp, the energy of the lit lamp originates in the • coils of wire in the generator. • magnet inside the generator. • lamp itself. • work done to turn the coils in the generator.

21. When a generator is used to light a lamp, the energy of the lit lamp originates in the • coils of wire in the generator. • magnet inside the generator. • lamp itself. • work done to turn the coils in the generator.

22. The purpose of a transformer is to transform • and create energy. • power at one voltage to the same power at another voltage. • current from one place to another. • voltage to useful applications.

23. The purpose of a transformer is to transform • and create energy. • power at one voltage to the same power at another voltage. • current from one place to another. • voltage to useful applications. Explanation: Answers c or d are not the best choices, as is b, the more direct answer. If you answered a, OUCH! … no device creates energy!

24. A transformer works by way of • Coulomb’s law. • Ohm’s law. • Faraday’s law. • Newton’s second law.

25. A transformer works by way of • Coulomb’s law. • Ohm’s law. • Faraday’s law. • Newton’s second law.

26. A step-up transformer in an electrical circuit can step up • voltage. • energy. • Both of these. • None of these.

27. A step-up transformer in an electrical circuit can step up • voltage. • energy. • Both of these. • None of these. Explanation: Stepping up energy is a big no-no in energy conservation!

28. A step-down transformer has a greater number of coils on the • input side. • output side. • side with lower power. • None of the above.

29. A step-down transformer has a greater number of coils on the • input side. • output side. • side with lower power. • None of the above. Comment: The fewer number of coils on the output side steps voltage down.

30. The workings of a transformer are consistent mainly with • Newton’s second law. • Coulomb’s law. • the conservation of momentum. • the conservation of energy.

31. The workings of a transformer are consistent mainly with • Newton’s second law. • Coulomb’s law. • the conservation of momentum. • the conservation of energy.

32. To minimize heat losses for power transported across the countryside, it is best that current in the wires is • low. • high. • not too low and not too high. • replaced with voltage.

33. To minimize heat losses for power transported across the countryside, it is best that current in the wires is • low. • high. • not too low and not too high. • replaced with voltage. Comment: High amperage produces large heat losses. So power at low current (and correspondingly high voltage) means less heat loss.

34. Lighting a lamp via electromagnetic induction • bypasses the need of work input. • requires work input. • may or may not require work input depending on efficiency. • produces a low-heat lamp.

35. Lighting a lamp via electromagnetic induction • bypasses the need of work input. • requires work input. • may or may not require work input depending on efficiency. • produces a low-heat lamp. Comment: Never forget an important fundamental of physics: Work is necessary to transform energy, whether the means of doing so is electromagnetic induction or otherwise.

36. A changing electric field can induce a changing • current loop. • voltage. • resonance that produces radio. • magnetic field.

37. A changing electric field can induce a changing • current loop. • voltage. • resonance that produces radio. • magnetic field. Comment: This was Maxwell’s generalization of electromagnetic induction.

38. If you change the magnetic field in a closed loop of wire, you induce in the loop a • current.   • voltage.   • electric field.   • All of these.  

39. If you change the magnetic field in a closed loop of wire, you induce in the loop a • current.   • voltage.   • electric field.   • All of these.  Explanation: Recall how Maxwell generalized Faraday’s law to include induction of an electric field.

40. The fact that electric and magnetic fields regenerate each other is important in • burglar alarms. • radio broadcasting. • metal detectors. • All of these. 

41. The fact that electric and magnetic fields regenerate each other is important in • burglar alarms. • radio broadcasting. • metal detectors. • All of these.  Explanation: A general answer would be electromagnetic waves, of which only radio broadcasting qualifies.