MAGNETIC CIRCUIT

1. An air gap is usually inserted in magnetic circuits to

1. increase m.m.f.
2. increase the flux
3. prevent saturation
4. none of the above

Ans: c

2. The relative permeability of a ferromagnetic material is

1. less than one
2. more than one
3. more than 10
4. more than 100 or 1000

Ans: d

3. The unit of magnetic flux is

1. henry
2. weber
3. ampereturn/weber
4. ampere/metre

Ans: b

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

1. resistance
2. resistivity
3. conductivity
4. 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

1. 1
2. 1 H/m
3. 1/4JI
4. 4n x 10-' H/m

Ans: a

7. Permanent magnets are normally made of

1. alnico alloys
2. aluminium
3. cast iron
4. wrought iron

Ans: a

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

1. 25
2. 50
3. 41.4
4. 100

Ans: c

9. Those magnetic materials are best suited for making armature and transformer cores which have     permeability and                                  hystersis loss.

1. high, high
2. low, high
3. high, low
4. low, low

Ans: c

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

1. at 63.2% of its maximum steady value
2. at the start of the current flow
3. after one time constant
4. near the final maximum value of current

Ans: b

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

1. time constant remains unchanged
2. initial rate of rise of current is doubled
3. final steady current is doubled
4. 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

1. 50
2. 20
3.  0.05
4. 500

Ans: b

13. A material for good magnetic memory should have

1. low hysteresis loss
2. high permeability
3. low retentivity
4. high retentivity

Ans: d

14. Conductivity is analogous to

1. retentivity
2. resistivity
3. permeability
4. inductance

Ans: c

15. In a magnetic material hysteresis loss takes place primarily due to

1. rapid reversals of its magnetisation
2. flux density lagging behind magnetising force
3. molecular friction
4. it high retentivity

Ans: d

16. Those materials are well suited for making permanent magnets which have      retentivity and           coercivity.

1. low, high
2. high, high
3. high, low
4. low, low

Ans: b

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

1. zero
2. small
3. large
4. none of the above

Ans: c

18. Hard steel is suitable for making permanent magnets because

1. it has good residual magnetism

2. its hysteresis loop has large area
3. its mechanical strength is high
4. its mechanical strength is low

Ans: a

19. Silicon steel is used in electrical machines because it has

1. low co-ercivity
2. low retentivity
3. low hysteresis loss
4. high co-ercivity

Ans: c

20. Conductance is analogous to

1. permeance
2. reluctance
3. flux
4. inductance

Ans: a

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

1. reluctivity
2. magnetomotive force
3. permeance
4. reluctance

Ans: d

22. The unit of retentivity is

1. weber
2. weber/sq. m
3. ampere turn/metre
4. ampere turn

Ans: b

23. Reciprocal of reluctance is

1. reluctivity
2. permeance
3. permeability
4. susceptibility

Ans: b

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

1. E.m.f.
2. Current
3. Current density
4. Conductivity