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OBJECTIVE TYPE QUESTIONS

  1. A.       Choose the correct answer:
  2. 1.       An air gap is usually inserted in magnetic circuits to

(a) increase m.m.f

(b) increase the flux

(c) prevent saturation

(d) none of the above.

  1. 2.       The relative permeability of a ferromagnetic material is

(a) less than one

(b) more than one

(c) more than 10

(d) more than 100 or 1000.

  1. 3.       The unit of magnetic flux is

(a) henry

(b) weber

(c) ampere turn/metre

(d) ampere/metre.

  1. 4.       Permeability in a magnetic circuit corresponds to …. in an electric circuit.

(a) resistance

(b) resistivity

(c) conductivity

(d) conductance.

  1. 5.       Point out the wrong statement. Magnetic leakage is undesirable in electric machines because it

(a) lowers their power efficiency

(b) increases their cost of manufacture

(c) leads to their increased weight

(d) produces fringing.

  1. 6.       Relative permeability of vacuum is

(a) 1

(b) 1 H/m     

(c) 1/4

(c) 1/4Π         

(d) 4Π × 10-7 H/m.

  1. 1.       The magnetising force (H)  and magnetic flux density (B) are connected by the relation

a) B = µrH/µ0

(b) B = µH

(c) B = H/µ0µr

(d) B = µ0H/µr

  1. 2.       Permanent magnets are normally made of

(a) alnico alloys

(b) aluminium

(c) cast iron

(d) wrought iron.

  1. 3.       Energy stored by a coil is doubled when its current is increased by percent.

(a) 25

(b) 50

(c) 41.4

(d) 100.

  1. 4.       Those magnetic materials are best suited for making armature and transformer cores which have permeability and….. hysteresis loss.

(a) high, high

(b) low, high

(c) high, low

(d) low, low.

  1. 5.       The rate of rise of current through an inductive coil is maximum

(a) at 63.2% of its maximum steady value

(b) at the start of the current flow

(c) after one time constant

(d) near the final maximum value of current.

  1. 6.       When both the inductance and resistance of a coil are doubled the value of

(a) time constant remains unchanged

(b) initial rate of rise of Current is doubled

(c) final steady current is doubled

(d) time constant is halved.

  1. 7.       The initial rate of rise of current through a coil of inductance 10 H when suddenly connected to D.C. supply of 200 V is….. A/s

(a) 50

(b) 20

(c) 0.05

(d) 500.

  1. 8.       A material for good magnetic memory should have

(a) low hysteresis loss

(b) high permeability

(c) low retentivity

(d) high retentivity

  1. 9.       Conductivity is analogous to

(a) retentivity

(b) resistivity

(c) permeability

(d) inductance

  1. 10.    In a magnetic material hysteresis loss taken place due to

(a) rapid reversals of its magnetisation

(b) flux density lagging behind magnetising force

(c) molecular friction

(d) its high retentivity

  1. 11.    Those materials are well suited for making permanent magnets which have ….. retentivity and ……..

(a) low, high

(b) high, high

(c) high, low

(d) low, low.

  1. 12.    If the area of hysteresis loop of a material is large, the hysteresis loss in this material will be

(a) zero

(b) small

(c) large

(d) none of the above.

  1. 13.    Hard steel is suitable for making permanent magnets because

(a) it has good residual magnetism

(b) its hysteresis loop has large area

(c) its mechanical strength is high

(d) its mechanical strength is low.

  1. 14.    Silicon steel is used in electrical machines because it has

(a) low coercitity

(b) low retentivity

(c) low hysteresis loss

(d) high coercivity.

  1. 15.    Conductance is analogous to

(a) permeance

(b) reluctance

(c) flux

(d) inductance.

  1. 16.    The property of a material which opposes the creation of magnetic flux in it is known as

(a) reluctivity

(b) magnetomotive force

(c) permeance

(d) reluctance.

  1. 17.    The unit retentivity is

(a) weber     

(b) weber/sq. m

(c) ampere turn/metre

(d) ampere turn.

  1. 18.    Reciprocal of reluctance is

(a) reluctivity

(b) permeance

(c) permeability

(d) susceptibility.

  1. 19.    While comparing magnetic and electric circuits, the flux of magnetic circuit is compared with which parameter of electrical circuit?

(a) E.m.f

(b) Current

(c) Current density

(d) Conductivity.

  1. 20.    The unit of reluctance is

(a) metre/henry

(b) henry/metre

(c) henry

(d) l/henry.

  1. 21.    A ferrite core has less eddy current loss than an iron core because

(a) ferrites have high resistance

(b) ferrites are magnetic

(c) ferrites have low permeability

(d) ferrites have high hysteresis.

  1. 22.    A ferromagnetic core subjected to cycles of magnetisation will exhibit hysteresis when the cycle is

(a) rotating

(b) alternating

(c) pulsating

(d) any of the above.

  1. 23.    In order to minimise loss due to hysteresis, the magnetic material should have

(a) high resistivity

(b) low hysteresis co-efficient

(c) large B-H loop area

(d) high retentivity.

  1. 24.    Hysteresis loss depends on

(a) volume of material

(b) frequency

(c) steinmetz co-efficient of material

(d) ambient temperature.

  1. 25.    Laminated cores, in electrical machines, are used to reduce

(a) copper loss

(b) eddy current loss

(c) hysteresis loss

(d) all of the above.

  1. 26.    The area of hysteresis loss is a measure of

(a) perittivity

(b) permeance

(c) energy loss per cycle

(d) magnetic flux.

  1. 27.    Permeance is to reluctance as conductance is to

(a) ampere turns

(b) inductance

(c) capacitance

(d) resistance.

  1. 28.    According to steinmetz hysteresis law, hysteresis loss in a material is proportional to

(a) B3.6

(b) B1.6          

(c) B1.2

(c) B2.6

  1. 29.    The transformer cores operating at microwave frequency range, are usually made of

(a) carbon

(b) copper

(c) silicon steels

(d) ferrites.

  1. 30.    The property of coil by which a counter e.m.f. is induced in it when the current through the coil changes is known as

(a) self-inductance

(b) mutual inductance

(c) series aiding inductance

(d) capacitance.

  1. 31.    As per Faraday’s laws of electromagnetic induction, an e.m.f. is induced in a conductor whenever it

(a) lies perpendicular to the magnetic flux

(b) lies in a magnetic field

(c) cuts magnetic flux

(d) moves parallel to the direction of the magnetic field.

  1. 32.    Which of the following circuit element stores energy in the electromagnetic field?

(a) Inductance

(b) Condenser

(c) Variable resistor

(d) Resistance.

  1. 33.    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

(b)  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.

  1. 34.    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.

  1. 35.    In an iron cored coil the iron core is removed so that the coil becomes an air cored coil. The inductance of the coil will

(a) increase

(b) decrease

(c) remain the same

(d) initially increase and then decrease.

  1. 36.    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.

  1. 37.    Both the number of turns and the core length of an inductive coil are doubled. Its self-inductance will

(a) unaffected

(b) doubled

(c) halved

(d) quadrupled.

  1. 38.    If current in a conductor increases then according to Lenz’s law self-induced voltage will

(a) air the increasing current

(b) tend to decrease the amount of current

(c) produce current opposite to the increasing current

(d) aid the applied voltage.

  1. 39.    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) Kirchhoffs voltage law.

  1. 40.    Air-core coils are practically free from

(a) hysteresis losses

(b) eddy current losses

(c) both (a) and (b)

(d) none of the above

  1. 41.    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.

  1. 42.    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.

  1. 43.    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 vertically.

  1. 44.    The co-efficient of self-inductance for a coil is given as

(a) NI/ɸ

(b) Nɸ/I

(c) NI2

(d) Nɸ/I2

  1. 45.    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

(b) Coulomb.

  1. 46.    Which of the following is not a unit of inductance ?

(a) Henry

(b) Coulomb/volt ampere

(c) Volt second per ampere

(b) All of the above.

  1. 47.    In case of an inductance, current is proportional to

(a) voltage across the inductance

(b) magnetic field

(c) both (a) and (b)

(b) neither (a) and (b)

  1. 48.    Which of the following circuit elements will oppose the change in circuit current?

(a) Capacitance

(b) Inductance

(c) Resistance

(d) All of the above.

  1. 49.    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 changed.

  1. 50.    Which of the following is unit of inductance?

(a) Ohm

(b) Henry

(c) Ampere turns

(d) Webers/metre.

  1. 51.    An e.m.f., of 16 volts is induced in a coil of inductance 4 H. The rate of change of current must be

(a) 64A/s

(b) 32 A/s

(c) 16 A/s

(d) 4 A/s.

  1. 52.    The core of a coil has a length of 200 mm. The inductance of coil is 6 mH. If the core length is doubled, all other quantities, remaining the same, the inductance will be

(a) 3 mH

(b) 12 mH

(c) 24 mH

(d) 48 mH

  1. 53.    The self-inductances of two coils are 8 mH and 18 mH, If the co-efficients of coupling is 0.5, the mutual inductance of the coils is

(a) 4 mH

(b) 5 mH

(c) 6 mH

(d) 12 mH.

  1. 54.    Two coils have inductances of 8 mH and 18 mH and a co-efficient of coupling of 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

  1. 55.    A 200 turns coil has an inductance of 12 mH. If the number of turns is increased to 400 turns, all other quantities (area, length etc.) remaining the same, the inductance will be

(a) 6 mH

(b) 14 mH

(c) 24 mH

(d) 48 mH

  1. 56.    Two coils have self-inductances of 10 H and 2 H, the mutual inductance being zero. If the two coils are connected in series, the total inductance will be

(a) 6 H

(b)8H

(c)12H

(d) 24H.

  1. 57.    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.

  1. 58.    The energy in joules stored in the magnetic field of 0.15 H inductance with a 180 mA current will be

(a) 2.43

(b) 2.43 × 10-3

(c) 2.43 × 10-6

(d) 2.43 × 10-9

  1. 59.    A coil with negligible resistance has 50 V across it with 10 mA. The inductive reactance is

(a) 50 ohms

(b) 500 ohms

(c) 1000 ohms

(d) 5000 ohms.

ANSWERS

  1. 1.       (c)
  2. 2.       (d)
  3. 3.       (b)
  4. 4.       (c)
  5. 5.       (a)
  6. 6.       (a)
  7. 7.       (b)
  8. 8.       (a)
  9. 9.       (c)
  10. 10.    (c)
  11. 11.    (b)
  12. 12.    (a)
  13. 13.    (b)
  14. 14.    (d)
  15. 15.    (c)
  16. 16.    (d)
  17. 17.    (b)
  18. 18.    (c)
  19. 19.    (a)
  20. 20.    (c)
  21. 21.    (a)
  22. 22.    (d)
  23. 23.    (b)
  24. 24.    (b)
  25. 25.    (b)
  26. 26.    (d)
  27. 27.    (d)
  28. 28.    (d)
  29. 29.    (b)
  30. 30.    (d)
  31. 31.    (b)
  32. 32.    (c)
  33. 33.    (d)
  34. 34.    (b)
  35. 35.    (d)
  36. 36.    (a)
  37. 37.    (c)
  38. 38.    (a)
  39. 39.    (a)
  40. 40.    (d)
  41. 41.    (b)
  42. 42.    (b)
  43. 43.    (b)
  44. 44.    (c)
  45. 45.    (b)
  46. 46.    (c)
  47. 47.    (d)
  48. 48.    (d)
  49. 49.    (b)
  50. 50.    (b)
  51. 51.    (b)
  52. 52.    (b)
  53. 53.    (b)
  54. 54.    (b)
  55. 55.    (c)
  56. 56.    (b)
  57. 57.    (d)
  58. 58.    (a)
  59. 59.    (c)
  60. 60.    (b)
  61. 61.    (d)
  62. 62.    (c)
  63. 63.    (b)
  64. 64.    (b)
  65. 65.    (d)