Electrical Engineering Multiple Choice Questions andAnswers

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Electromagnetic Induction

Electrical Engineering Multiple Choice Questions andAnswers

1. The property of coil by which a counter e.m.f. is induced in it when the currentthrough the coil changes is known as
(a) self-inductance
(b) mutual inductance
(c) series aiding inductance
(d) capacitance
Ans: a

2. As per Faraday’s laws of electromagnetic induction, an e.m.f. is inducedinaconductor whenever it
(a) lies perpendicular to the magnetic flux
(b) lies in a magnetic field
(e) cuts magnetic flux
(d) moves parallel to the direction of the magnetic field
Ans: c

3. Which of the following circuit element stores energy in the electromagneticfield ?
(a) Inductance
(b) Condenser
(c) Variable resistor
(d) Resistance
Ans: a

4. The inductance of a coil will increase under all the following conditions except(a) when more length for the same number of turns is provided
(6) when the number of turns of the coil increase
(c) when more area for each turn is provided
(d) when permeability of the core increases
Ans: a

5. Higher the self-inductance of a coil,
(a) lesser its weber-turns
(b) lower the e.m.f. induced
(c) greater the flux produced by it
(d) longer the delay in establishing steady current through it
Ans: d

6. In an iron cored coil the iron core is removed so that the coil becomes anaircored coil. The inductance of the coil will
(a) increase
(b) decrease
(c) remain the same
(d) initially increase and then decrease
Ans: b

7. An open coil has
(a) zero resistance and inductance
(b) infinite resistance and zero inductance
(c) infinite resistance and normal inductance
(d) zero resistance and high inductance
Ans: b

8. Both the number of turns and the core length of an inductive coil are doubled. Its self-inductance will be
(a) unaffected
(b) doubled
(c) halved
(d) quadrupled
Ans: b

9. If current in a conductor increases then according to Lenz’s lawself-inducedvoltage will
(a) aid the increasing current
(b) tend to decrease the amount of cur-rent
(c) produce current opposite to the in-creasing current
(d) aid the applied voltage
Ans: c

10. The direction of induced e.m.f. can be found by
(a) Laplace’s law
(b) Lenz’s law
(c) Fleming’s right hand rule
(d) Kirchhoff s voltage law
Ans: b

11. Air-core coils are practically free from
(a) hysteresis losses
(b) eddy current losses
(c) both (a) and (b)
(d) none of the above
Ans: c

12. The magnitude of the induced e.m.f. in a conductor depends on the
(a) flux density of the magnetic field
(b) amount of flux cut
(c) amount of flux linkages
(d) rate of change of flux-linkages
Ans: d

13. Mutually inductance between two magnetically-coupled coils depends on
(a) permeability of the core
(b) the number of their turns
(c) cross-sectional area of their common core
(d) all of the above
Ans: d

14. A laminated iron core has reduced eddy-current losses because
(a) more wire can be used with less D.C. resistance in coil
(b) the laminations are insulated from each other
(c) the magnetic flux is concentrated in the air gap of the core
(d) the laminations are stacked vertfcally
Ans: b

15. The law that the induced e.m.f. and current always oppose the cause producing them is due to
(a) Faraday
(b) Lenz
(c) Newton
(d) Coulomb
Ans: b

16. Which of the following is not a unit of inductance ?
(a) Henry
(b) Coulomb/volt ampere
(c) Volt second per ampere
(d) All of the above
Ans: b

17. In case of an inductance, current is proportional to
(a) voltage across the inductance
(b) magnetic field
(c) both (a) and (b)
(d) neither (a) nor (b)
Ans: b

18. Which of the following circuit elements will oppose the change in circuit current ?
(a) Capacitance
(b) Inductance
(c) Resistance
(d) All of the above
Ans: b

19. For a purely inductive circuit which of the following is true ?
(a) Apparent power is zero
(b) Relative power is.zero
(c) Actual power of the circuit is zero
(d) Any capacitance even if present in the circuit will not be charged
Ans: c

20. Which of the following is unit of inductance ?
(a) Ohm
(b) Henry
(c) Ampere turns
(d) Webers/metre
Ans: b

21. An e.m.f. of 16 volts is induced in a coil of inductance 4H. The rate of changeof current must be
(a) 64 A/s
(b) 32 A/s
(c) 16 A/s
(d) 4 A/s
Ans: d

22. The core of a coil has a length of 200 mm. The inductance of coil is 6mH. If the core length is doubled, all other quantities, remaining the same, the inductance will be
(a) 3 mH
(b) 12 mH
(c) 24mH
Ans: a

23. The self inductances of two coils are 8 mH and 18 mH. If the co-efficientsof coupling is 0.5, the mutual inductance of the coils is
(a) 4 mH
(b) 5 mH
(c) 6 mH
(d) 12 mH
Ans: c

24. Two coils have inductances of 8 mH and 18 mH and a co-efficient of couplingof 0.5. If the two coils are connected in series aiding, the total inductance will be(a) 32 mH
(b) 38 mH
(c) 40 mH
(d) 48 mH
Ans: b

25. A 200 turn coil has an inductance of 12 mH. If the number of turns is increased to 400 turns, all other quantities (area, length etc.) remaining thesame, the inductance will be
(a) 6 mH
(b) 14 mH
(c) 24 mH
(d) 48 mH
Ans: d

26. Two coils have self-inductances of 10 H and 2 H, the mutual inductancebeingzero. If the two coils are connected in series, the total inductance will be
(a) 6 H
(b) 8 H
(c) 12 H
(d) 24 H
Ans: c

27. In case all the flux from the current in coil 1 links with coil 2, the co-efficient of coupling will be
(a) 2.0
(b) 1.0
(c) 0.5
(d) zero
Ans: b

28. A coil with negligible resistance has 50V across it with 10 mA. The inductivereactance is
(a) 50 ohms
(b) 500 ohms
(c) 1000 ohms
(d) 5000 ohms
Ans: d

29. A conductor 2 meters long moves at right angles to a magnetic fieldof fluxdensity 1 tesla with a velocity of 12.5 m/s. The induced e.m.f. in the conductorwill be
(a) 10 V
(6) 15 V
(c) 25V
(d) 50V
Ans: c

30. Lenz’s law is a consequence of the law of conservation of
(a) induced current
(b) charge
(c) energy
(d) induced e.m.f.
Ans: c

31. A conductor carries 125 amperes of current under 60° to a magnetic fieldof 1.1tesla. The force on the conductor will be nearly
(a) 50 N
(b) 120 N
(c) 240 N
(d) 480 N
Ans: b

32. Find the force acting on a conductor 3m long carrying a current of 50amperesat right angles to a magnetic field having a flux density of 0.67 tesla.
(a) 100 N
(b) 400 N
(c) 600 N
(d) 1000 N
Ans: a

33. The co-efficient of coupling between two air core coils depends on
(a) self-inductance of two coils only
(b) mutual inductance between two coils only
(c) mutual inductance and self inductance of two coils
(d) none of the above
Ans: c

34. An average voltage of 10 V is induced in a 250 turns solenoid as a result of achange in flux which occurs in 0.5 second. The total flux change is
(a) 20 Wb
(b) 2 Wb
(c) 0.2 Wb
(d) 0.02 Wb
Ans: d

35. A 500 turns solenoid develops an average induced voltage of 60 V. Over whattime interval must a flux change of 0.06 Wb occur to produce such a voltage?
(a) 0.01 s
(b) 0.1 s
(c) 0.5 s
(d) 5 s
Ans: c

36. Which of the fpllowing inductor will have the least eddy current losses ?
(a) Air core
(b) Laminated iron core
(c) Iron core
(d) Powdered iron core
Ans: a

37. A coil induces 350 mV when the current changes at the rate of 1 A/s. Thevalue of inductance is
(a) 3500 mH
(b) 350 mH
(c) 250 mH
(d) 150 mH
Ans: b

38. Two 300 uH coils in series without mutual coupling have a total inductanceof
(a) 300 uH
(b) 600 uH
(c) 150 uH
(d) 75 uH
Ans: b

39. Current changing from 8 A to 12 A in one second induced 20 volts inacoil. The value of inductance is
(a) 5 mH
(b) 10 mH
(c) 5 H
(d) 10 H
Ans: c

40. Which circuit element(s) will oppose the change in circuit current ?
(a) Resistance only
(b) Inductance only
(c) Capacitance only
(d) Inductance and capacitance
Ans: b

41. A crack in the magnetic path of an inductor will result in
(a) unchanged inductance
(b) increased inductance
(c) zero inductance
(d) reduced inductance
Ans: d

42. A coil is wound on iron core which carries current I. The self-inducedvoltagein the coil is not affected by
(a) variation in coil current
(b) variation in voltage to the coil
(c) change of number of turns of coil
(d) the resistance of magnetic path
Ans: b

43. A moving magnetic field will produce the same effect as a conductor that ismoving.
(a) Yes
(b) No

44. The polarity of the induced voltage can be determined by using the left-handgenerator rule.
(a) Yes
(b) No
Ans: a

45. Increasing the field or increasing the current will decrease the force ontheconductor.
(a) Yes
(b) No
Ans: b

46. Reversing the field or the current will reverse the force on the conductor.
(a) Yes
(b) No
Ans: a

49. When a conductor moves in the field, so that it makes an angle 8 withthelines of flux, the force F is given as : F = Bl sin2 0.
(a) Yes
(b) No
Ans: b

50. The self-inductance of the coil may be defined as equal to the e.m.f. inducedin volts when the current in the circuit changes at the rate of unit Weber turns.
(a) Yes
(b) No
Ans: b