Magnetic Circuits MCQ

1. In Fig. given below, the magnetic circuit is the path

(i) DAB
(ii) ABCDA
(iii) ABC
(iv) ABCD

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2. If l is the magnetic path in the above Fig., then magnetising force is

(i) Nl
(ii) Nl x l
(iii) l ÷ NI
(iv) NI ÷ l

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3. The reluctance of the magnetic circuit shown in the above Fig.is

(i) NI ÷ l
(ii) Ø ÷  NI
(iii) NI ÷ Ø
(iv)  Ø ÷  l

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4. The SI unit of reluctance is

(i) AT/Wb
(ii) AT/m
(iii) AT
(iv) N/Wb

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5. A magnetic circuit has a mmf of 400 AT and a reluctance of 2 x 105 AT/Wb. The magnetic flux in the magnetic circuit is

(i) 3 x 10-5 Wb
(ii) 2 x 10-3 Wb
(iii) 1.5 x10-2 Wb
(iv) 2.5 x 10-4 Wb

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6. A 2 cm long coil has 10 turns and carries a current of 750 mA. The magnetising force of the coil is

(i) 225 AT/m
(ii) 675 AT/m
(iii) 450 AT/m
(iv) 375 AT/m

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7. A magnetic device has a core with cross-section of 1 inch2. If the flux in the core is 1 mWb, then flux density (1 inch = 2.54 cm) is

(i) 2.5 T
(ii) 1.3 T
(iii) 1.55T
(iv) 0.25 T

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8. The reluctance of a magnetic circuit varies as ……..

(i) length x area
(ii) length ÷  area
(iii) area ÷  length
(iv) (length)2 + area

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9. The reluctance of a magnetic circuit is ….. relative permeability of the material comprising the circuit

(i) directly proportional to
(ii) inversely proportional to
(iii) independent of
(iv) none of the above

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10. M.M.F. in a magnetic circuit corresponds to …….. in an electric circuit.

(i) voltage drop
(ii) potential difference
(iii) electric intensity
(iv) em.f.

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11. Permeance of a magnetic circuit is ………… area of x-section of the circuit.

(i) inversely proportional to
(ii) directly proportional to
(iii) independent of
(iv) none of the above.

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12. The magnitude of AT required for air gap is much greater than that required for iron part of a magnetic circuit because ………

(i) air is a gas
(ii) air has the lowest relative permeability
(iii) air is a conductor of magnetic flux
(iv) none of the above

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13. In electro-mechanical conversion devices (e.g. motors and generators), a small air gap is left between the rotor and stator in order to 

(i) complete the magnetic path
(ii) decrease the reluctance of magnetic path
(iii) permit mechanical clearance
(iv) increase flux density in air gap

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14. A magnetic circuit carries a flux Øi in the iron part and a flux Øg in the air gap. Then leakage co-efficient is 

(i) Øig
(ii) Øg/Øi
(iii) Øg x Øi
(iv) none of the above

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15. The value of leakage co-efficient for electrical machines is usually about 

(i) 0.5 to 1
(ii) 4 to 10
(iii) above 10
(iv) 1.15 to 1.25

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16. The reluctance of a magnetic circuit depends upon 

(i) current in the coil
(ii) no. of turns of coil
(iii) flux density in the circuit
(iv) none of the above

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17. The B-H curve for ……….. will be a straight line passing through the origin.

(i) air
(ii) soft iron
(iii) hardened steel
(iv) silicon steel

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18. Whatever may be the flux density in   ………. the material will never saturate.

(i) soft iron
(ii) cobalt steel
(iii) air
(iv) silicon steel

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19. The B-H curve of ….. will not be a straight line.

(i) air
(ii) copper
(iii) wood
(iv) soft iron

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20. The B-H curve is used to find the m.m.f. of  ………. in a magnetic circuit.

(i) air gap
(ii) iron part
(iii) both air gap and iron part
(iv) none of the above

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