Top Most Magnetic Circuit Objective Questions
1.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
50
20
0.05
500
ANSWER : : 20
2.The unit of reluctance is
metre/henry
henry/metre
henry
1/henry
ANSWER : : 1/henry
3.Those magnetic materials are best suited for making armature and transformer cores which have____permeability and_______hystersis loss.
high, high
low, high
high, low
low, low
ANSWER : : high, low
4.While comparing magnetic and electric circuits, the flux of magnetic circuit is compared with which parameter of electrical circuit ?
E.m.f.
Current
Current density
Conductivity
ANSWER : : Current
5.Those materials are well suited for making permanent magnets which have______retentivity and _______coercivity.
low, high
high, high
high, low
low, low
ANSWER : s: b
6.The rate of rise of current through an inductive coil is maximum
at 63.2% of its maximum steady value
at the start of the current flow
after one time constant
near the final maximum value of current
ANSWER : : at the start of the current flow
7.If the area of hysteresis loop of a material is large, the hysteresis loss in this material will be
zero
small
large
none of the above
ANSWER : : large
8.The unit of retentivity is
weber
weber/sq. m
ampere turn/metre
ampere turn
ANSWER : : weber/sq. m
9.Silicon steel is used in electrical machines because it has
low co-ercivity
low retentivity
low hysteresis loss
high co-ercivity
ANSWER : : low hysteresis loss
10.Point out the wrong statement. Magnetic leakage is undesirable in electric machines because it
lowers their power efficiency
increases their cost of manufacture
leads to their increased weight
produces fringing
ANSWER : : lowers their power efficiency
11.Permeability in a magnetic circuit corresponds to______ in an electric circuit.
resistance
resistivity
conductivity
conductance
ANSWER : : conductivity
12.The relative permeability of a ferromagnetic material is
less than one
more than one
more than 10
more than 100 or 1000
ANSWER : : more than 100 or 1000
13.A material for good magnetic memory should have
low hysteresis loss
high permeability
low retentivity
high retentivity
ANSWER : : high retentivity
14.Permanent magnets are normally made of
alnico alloys
aluminium
cast iron
wrought iron
ANSWER : : alnico alloys
15.Energy stored by a coil is doubled when its current is increased by percent.
25
50
41.4
100
ANSWER : : 41.4
16.Relative permeability of vacuum is
1
1 H/m
1/4JI
4n x 10-‘ H/m
ANSWER : : 1
17.When both the inductance and resistance of a coil are doubled the value of
time constant remains unchanged
initial rate of rise of current is doubled
final steady current is doubled
time constant is halved
ANSWER : : time constant remains unchanged
18.A ferrite core has less eddy current loss than an iron core because
ferrites have high resistance
ferrites are magnetic
ferrites have low permeability
ferrites have high hysteresis
ANSWER : : ferrites have high hysteresis
19.Conductance is analogous to
permeance
reluctance
flux
inductance
ANSWER : : permeance
20.Hard steel is suitable for making permanent magnets because
it has good residual magnetism
its hysteresis loop has large area
its mechanical strength is high
its mechanical strength is low
ANSWER : : it has good residual magnetism
21.Conductivity is analogous to
retentivity
resistivity
permeability
inductance
ANSWER : : permeability
22.The property of a material which opposes the creation of magnetic flux in it is known as
reluctivity
magnetomotive force
permeance
reluctance
ANSWER : : reluctance
23.Reciprocal of reluctance is
reluctivity
permeance
permeability
susceptibility
ANSWER : : permeance
24.In a magnetic material hysteresis loss takes place primarily due to
rapid reversals of its magnetisation
flux density lagging behind magnetising force
molecular friction
it high retentivity
ANSWER : : it high retentivity
25.The unit of magnetic flux is
henry
weber
ampereturn/weber
ampere/metre
ANSWER : : weber
26.Laminated cores, in electrical machines, are used to reduce
copper loss
eddy current loss
hysteresis loss
all of the above
ANSWER : : eddy current loss