Magnetostatic Fields MCQ

1. The flux-density at a distance of 0.1 m from a long straight wire, carrying a current of 200 A is

  1. 5 x 10-4 Wb/m2
  2. 4 x 10-4 Wb/m2
  3. 3 x 10-4 Wb/m2
  4. 2 x 10-4 Wb/m2

Answer. b

2. An iron rod of 10-3 m3 volume and relative permeability of 1150 is placed inside a long solenoid wound with 5 turns/cm. If a current of 0.5 A is allowed to pass through the solenoid, the magnetic moment of the rod is

  1. 2.87 x 104 A-m2
  2. 28.7 x 103 A-m2
  3. 2.87 x 102 A-m2
  4. 28.7 x 102 A-m2

Answer. c

3. The magnetic susceptibility of aluminium is 2.1 x 10-5. The permeability and relative permeability are, respectively

  1. 12.6 x 10-7 and 1.0021
  2. 1.26 x 10-7 and 1.0021
  3. 12.6 x 10-7 and 1.000021
  4. 1.26 x 10-7 and 1.000021

Answer. c

4. The magnitude of magnetic field strength H is independent of

  1. current only
  2. distance only
  3. permeability of the medium only
  4. both current and distance

Answer. c

5. A conductor of length 100 cm moves at right angle to a uniform field flux density of 1.5 Wb/m with a velocity of 50 m/s. The e.m.f. induced in conductor will be

  1. 150 V
  2. 75 V
  3. 50 V
  4. 37.5 V

Answer. b

6. Two conductors of a transmission line carry equal current I in opposite directions. The force on each conductor is proportional to

  1. I
  2. I2
  3. the distance between the conductors
  4. I3

Answer. b

7. The law which states that the line integral of the magnetic field around a closed curve is equal to the free current through a surface, is

  1. Tellegen’s theorem
  2. Gauss’ law
  3. Ampere’s law
  4. Coulomb’s law

Answer. c

8. If the current density inside a straight conductor is uniform over its cross-section, the flux density variation inside the conductor at different distances from its centre is

  1. linear
  2. square of the distance
  3. inverse of the distance
  4. exponential

Answer. a

9. For electromechanical energy conversion, a magnetic field is employed as the medium rather than electric field because

  1. the stored energy density for practicable field strength is low in the electric field
  2. the electric field presents insulation problem
  3. the specific magnetic loss is more than the specific dielectric loss
  4. None of the above

Answer. a

10. A magnetic ring has a mean circumference of 20 cm and a cross-section of 20 cm2 and has 800 numbers of turns of wire. When the exciting current is 5 A, the flux is 2 mWb. The relative permeability of iron is nearly

  1. 3.98
  2. 398
  3. 39.8
  4. 0.398

Answer. c

11. Consider the following statements:

A current I flows through a circular coil of one turn of radius R in the counter-clockwise direction.

  1. The magnetic field at the center is inversely proportional to R.
  2. The magnetic moment of the coil is directly proportional to R2.
  3. The magnetic field at its center is directly proportional to R2.

Which of the above statements is/are correct?

  1. i and ii
  2. i only
  3. ii and iii
  4. iii only

Answer. a

12. Along straight wire carries a current I = 10 A, the magnetic field at a distance of 1.59 m is

  1. 0.1 Am-1
  2. 1 Am-1
  3. 10 Am-1
  4. 100 Am-1

Answer. b

13. If the magnetic flux through each turn of the coil consisting of 200 turns is (t2 – 3t) milli-Webers, where t is in seconds, then the induced emf in the coil at t= 4 sec is

  1. -1 V
  2. 1 V
  3. -0.1 V
  4. 0.1 V

Answer. a

14. The unit of magnetic flux density is

  1. gauss
  2. tesla
  3. bohr
  4. weber/sec

Answer. b

15. How much current must flow in a loop of radius 1 m to produce a magnetic field 1 mAm-1?

  1. 1.5 mA
  2. 1.0 mA
  3. 2.5 mA
  4. 2.0 mA

Answer. d

16. A long straight wire carries a current I = 1A. At what distance is the magnetic field 1 Am-1?

  1. 1.59 m
  2. 0.159 m
  3. 0.00159 m
  4. 0.0159 m

Answer. b

17. A long straight wire carries a current I = 10 A. At what distance is the magnetic field H= 1 Am-1?

  1. 1.39 m
  2. 1.19 m
  3. 1.59 m
  4. 1.79 m

Answer. c

18. If a very flexible wire is laid out in the shape of a hairpin with its two ends secured, what shape will the wire tend to assume if a current is passed through it?

  1. Parabolic
  2. Straight line
  3. Circle
  4. Ellipse

Answer. b

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