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MAGNETIC CIRCUIT Multiple Choice Questions and, Answers pdf :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, Ans: c, 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, Ans: d, 3. The unit of magnetic flux is, (a) henry, (b) weber, (c) ampereturn/weber, (d) ampere/metre, Ans: b, 4. Permeability in a magnetic circuit corresponds to______ in an electric circuit., (a) resistance, (b) resistivity, (c) conductivity, (d) conductance, Ans: c, 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, Ans: a, 6. Relative permeability of vacuum is, (a) 1, (b) 1 H/m, (c) 1/4JI, (d) 4n x 10-‘ H/m, Ans: a

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7. Permanent magnets are normally made of, (a) alnico alloys, (b) aluminium, (c) cast iron, (d) wrought iron, Ans: a, 8. Energy stored by a coil is doubled when its current is increased by percent., (a) 25, (b) 50, (c)41.4, (d) 100, Ans: c, 9. Those magnetic materials are best suited for making armature and transformer, cores which have____permeability and_______hystersis loss., (a) high, high, (b) low, high, (c) high, low, (d) low, low, Ans: c, 10. 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, Ans: b, 11. 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, Ans: a, 12. The initial rate of rise of current through a coil of inductance 10 H when, suddenly connected to a D.C. supply of 200 V is_______Vs, (a) 50, (b) 20, (c) 0.05, (d) 500, Ans: b, 13. A material for good magnetic memory should have, (a) low hysteresis loss, (b) high permeability

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(c) low retentivity, (d) high retentivity, Ans: d, 14. Conductivity is analogous to, (a) retentivity, (b) resistivity, (c) permeability, (d) inductance, Ans: c, 15. In a magnetic material hysteresis loss takes place primarily due to, (a) rapid reversals of its magnetisation, (b) flux density lagging behind magnetising force, (c) molecular friction, (d) it high retentivity, Ans: d, 16. Those materials are well suited for making permanent magnets which have, _____ retentivity and _______ coercivity., (a) low, high, (b) high, high, (c) high, low, (d) low, low, Ans: b, 17. 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, Ans: c, 18. 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, Ans: a, 19. Silicon steel is used in electrical machines because it has, (a) low coercivity, (b) low retentivity, (c) low hysteresis loss, (d) high coercivity, Ans: c

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20. Conductance is analogous to, (a) permeance, (b) reluctance, (c) flux, (d) inductance, Ans: a, 21. 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, Ans: d, 22. The unit of retentivity is, (a) weber, (b) weber/sq. m, (c) ampere turn/meter, (d) ampere turn, Ans: b, 23. Reciprocal of reluctance is, (a) reluctivity, (b) permeance, (c) permeability, (d) susceptibility, Ans: b, 24. 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, Ans: b, 25. The unit of reluctance is, (a) metre/henry, (b) henry/metre, (c) henry, (d) 1/henry, Ans: d, 26. A ferrite core has less eddy current loss than an iron core because, (a) ferrites have high resistance, (b) ferrites are magnetic

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(c) ferrites have low permeability, (d) ferrites have high hysteresis, Ans: a, 27. Hysteresis loss least depends on, (a) volume of material, (b) frequency, (c) steinmetz coefficient of material, (d) ambient temperature, Ans: d, 28. Laminated cores, in electrical machines, are used to reduce, (a) copper loss, (b) eddy current loss, (c) hysteresis loss, (d) all of the above, Ans: b