# 290+ Electromagnetic Theory Solved MCQs

Chapters

35
57.9k
Chapter: Magnetism and Electromagnetism
201.

A. 2
B. 4
C. 6
D. 8
202.

A. 2 AT
B. 4 AT
C. 6 AT
D. 10 AT
203.

## A certain amount of current flows through a ring-shaped coil with fixed number of turns. How does the magnetic induction B varies inside the coil if an iron core is threaded into coil without dimensional change of coil ?

A. Decreases
B. Increases
C. Remains same
D. First increases and then decreases depending on the depth of iron in¬sertion
204.

## The magnetic reluctance of a material

A. decreases with increasing cross sectional area of material
B. increases with increasing cross-sec-tional area of material
C. does not vary with increasing cross-sectional area of material
D. any of the above
Answer» A. decreases with increasing cross sectional area of material
205.

## The initial permeability of an iron rod is

A. the highest permeability of the iron rod
B. the lowest permeability of the iron rod
C. the permeability at the end of the iron rod
D. the permeability almost in non-magnetised state
Answer» D. the permeability almost in non-magnetised state
206.

## How does the magnetic compass needle behave in a magnetic field ?

A. It assures a position right angle to magnetic field
B. It starts rotating
C. It assures a position which follows a line of magnetic flux
D. None of the above
Answer» C. It assures a position which follows a line of magnetic flux
207.

## In a simple magnetic field the strength of magnet flux

A. is constant and has same value in energy part of the magnetic field
B. increases continuously from initial value to final value
C. decreases continuously from initial value to final value
D. first increases and then decreases till it becomes zero
Answer» D. first increases and then decreases till it becomes zero
208.

## The stray line of magnetic flux is defined as

A. a line vertical to the flux lines
B. the mean length of a ring shaped coil
C. a line of magnetic flux in a non-uniform field
D. a line of magnetic flux which does not follow the designed path
Answer» D. a line of magnetic flux which does not follow the designed path
209.

## The bar magnet has

A. the dipole moment
B. monopole moment
C. (a) and (b) both
D. none of the above
210.

## Which of the following materials are dia-magnetic ?

A. Silver
B. Copper
C. Silver and copper
D. Iron
211.

## Which of the following type of materials are not very important for engineering applications ?

A. Ferromagnetic
B. Paramagnetic
C. Diamagnetic
D. None of the above
212.

A. KT3 and 1CT6
B. 1CT3 and 1CT7
C. KT4 and KT8
D. 10"2 and KT5
213.

## For which of the following materials the saturation value is the highest ?

A. Ferromagnetic materials
B. Paramagnetic materials
C. Diamagnetic materials
D. Ferrites
214.

## The magnetic materials exhibit the property of magnetisation because of

A. orbital motion of electrons
B. spin of electrons
C. spin of nucleus
D. either of these
215.

## For which of the following materials the net magnetic moment should be zero ?

A. Diamagnetic materials
B. Ferrimagnetic materials
C. Antiferromagnetic materials
D. Antiferrimagnetic materials
216.

## The attraction capacity of electromagnet will increase if the

A. core length increases i
B. core area increases
C. flux density decreases
D. flux density increases
217.

## Which of the following statements is correct ?

A. The conductivity of ferrites is better than ferromagnetic materials
B. The conductivity of ferromagnetic materials is better than ferrites
C. The conductivity of ferrites is very high
D. The conductivity of ferrites is same as that of ferromagnetic materials
Answer» A. The conductivity of ferrites is better than ferromagnetic materials
218.

## Temporary magnets are used in

A. loud-speakers
B. generators
C. motors
D. all of the above
Answer» D. all of the above
219.

## Main causes of noisy solenoid are

A. strong tendency of fan out of lami-nations at the end caused by repul¬sion among magnetic lines of force
B. uneven bearing surface, caused by dirt or uneven wear between moving and stationary parts
C. both of above
D. none of the above
220.

## Core of an electromagnet should have

A. low coercivity
B. high susceptibility
C. both of the above
D. none of the above
Answer» C. both of the above
221.

## Magnetism of a magnet can be destroyed by

A. heating
B. hammering
C. by inductive action of another magnet
D. by all above methods
Answer» D. by all above methods
Chapter: Magnetic Circuit
222.

## 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
223.

## 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
Answer» D. more than 100 or 1000
224.

## The unit of magnetic flux is

A. henry
B. weber
C. ampereturn/weber
D. ampere/metre
225.

A. resistance
B. resistivity
C. conductivity
D. conductance
226.

## 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
Answer» A. lowers their power efficiency
227.

A. 1
B. 1 H/m
C. 1/4JI
D. 4n x 10-' H/m
228.

A. alnico alloys
B. aluminium
C. cast iron
D. wrought iron
229.

A. 25
B. 50
C. 41.4
D. 100
230.

A. high, high
B. low, high
C. high, low
D. low, low
231.

## 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
Answer» B. at the start of the current flow
232.

## 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
Answer» A. time constant remains unchanged
233.

A. 50
B. 20
C. 0.05
D. 500
234.

## A material for good magnetic memory should have

A. low hysteresis loss
B. high permeability
C. low retentivity
D. high retentivity
235.

A. retentivity
B. resistivity
C. permeability
D. inductance
236.

## 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
237.

A. low, high
B. high, high
C. high, low
D. low, low
238.

## 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
239.

## 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
Answer» A. it has good residual magnetism
240.

## Silicon steel is used in electrical machines because it has

A. low co-ercivity
B. low retentivity
C. low hysteresis loss
D. high co-ercivity
241.

A. permeance
B. reluctance
C. flux
D. inductance
242.

## 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
243.

## The unit of retentivity is

A. weber
B. weber/sq. m
C. ampere turn/metre
D. ampere turn
244.

## Reciprocal of reluctance is

A. reluctivity
B. permeance
C. permeability
D. susceptibility
245.

## 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
246.

A. metre/henry
B. henry/metre
C. henry
D. 1/henry
247.

## A ferrite core has less eddy current loss than an iron core because

A. ferrites have high resistance
B. ferrites are magnetic
C. ferrites have low permeability
D. ferrites have high hysteresis
Answer» D. ferrites have high hysteresis
248.

## Hysteresis loss least depends on

A. volume of material
B. frequency
C. steinmetz co-efficient of material
D. ambient temperature
249.

## Laminated cores, in electrical machines, are used to reduce

A. copper loss
B. eddy current loss
C. hysteresis loss
D. all of the above
Chapter: Electromagnetic Induction
250.

## The property of coil by which a counter e.m.f. is induced in it when the current through the coil changes is known as

A. self-inductance
B. mutual inductance
C. series aiding inductance
D. capacitance
251.

## As per Faraday's laws of electromagnetic induction, an e.m.f. is induced in a conductor whenever it

A. lies perpendicular to the magnetic flux
B. lies in a magnetic field
C. cuts magnetic flux
D. moves parallel to the direction of the magnetic field
252.

## Which of the following circuit element stores energy in the electromagnetic field ?

A. Inductance
B. Condenser
C. Variable resistor
D. Resistance
253.

## The inductance of a coil will increase under all the following conditions except

A. when more length for the same number of turns is provided
B. when the number of turns of the coil increase
C. when more area for each turn is provided
D. when permeability of the core increases
Answer» A. when more length for the same number of turns is provided
254.

## Higher the self-inductance of a coil,

A. lesser its weber-turns
B. lower the e.m.f. induced
C. greater the flux produced by it
D. longer the delay in establishing steady current through it
255.

## In an iron cored coil the iron core is removed so that the coil becomes an air cored coil. The inductance of the coil will

A. increase
B. decrease
C. remain the same
D. initially increase and then decrease
256.

## An open coil has

A. zero resistance and inductance
B. infinite resistance and zero inductance
C. infinite resistance and normal inductance
D. zero resistance and high inductance
Answer» B. infinite resistance and zero inductance
257.

A. unaffected
B. doubled
C. halved
258.

## If current in a conductor increases then according to Lenz's law self-induced voltage will

A. aid the increasing current
B. tend to decrease the amount of cur-rent
C. produce current opposite to the in-creasing current
D. aid the applied voltage
Answer» C. produce current opposite to the in-creasing current
259.

## The direction of incViced e.m.f. can be found by

A. Laplace's law
B. Lenz's law
C. Fleming's right hand rule
D. Kirchhoff s voltage law
260.

## Air-core coils are practically free from

A. hysteresis losses
B. eddy current losses
C. both (a) and (b)
D. none of the above
Answer» C. both (a) and (b)
261.

## The magnitude of the induced e.m.f. in a conductor depends on the

A. flux density of the magnetic field
B. amount of flux cut
D. rate of change of flux-linkages
262.

## Mutually inductance between two magnetically-coupled coils depends on

A. permeability of the core
B. the number of their turns
C. cross-sectional area of their com-mon core
D. all of the above
Answer» D. all of the above
263.

## A laminated iron core has reduced eddy-current losses because

A. more wire can be used with less D.C. resistance in coil
B. the laminations are insulated from each other
C. the magnetic flux is concentrated in the air gap of the core
D. the laminations are stacked vertf-cally
Answer» B. the laminations are insulated from each other
264.

B. Lenz
C. Newton
D. Coulomb
265.

## Which of the following is not a unit of inductance ?

A. Henry
B. Coulomb/volt ampere
C. Volt second per ampere
D. All of the above
266.

## In case of an inductance, current is proportional to

A. voltage across the inductance
B. magnetic field
C. both (a) and (b)
D. neither (a) nor (b)
267.

## Which of the following circuit elements will oppose the change in circuit current ?

A. Capacitance
B. Inductance
C. Resistance
D. All of the above
268.

## For a purely inductive circuit which of the following is true ?

A. Apparent power is zero
B. Relative power is.zero
C. Actual power of the circuit is zero
D. Any capacitance even if present in the circuit will not be charged
Answer» C. Actual power of the circuit is zero
269.

A. Ohm
B. Henry
C. Ampere turns
D. Webers/metre
270.

A. 64 A/s
B. 32 A/s
C. 16 A/s
D. 4 A/s
271.

A. 3 mH
B. 12 mH
C. 24mH
D. 48mH
272.

A. 4 mH
B. 5 mH
C. 6 mH
D. 12 mH
273.

A. 32 mH
B. 38 mH
C. 40 mH
D. 48 mH
274.

A. 6 mH
B. 14 mH
C. 24 mH
D. 48 mH
275.

A. 6 H
B. 8 H
C. 12 H
D. 24 H
276.

A. 2.0
B. 1.0
C. 0.5
D. zero
277.

A. 50 ohms
B. 500 ohms
C. 1000 ohms
D. 5000 ohms
278.

A. 10 V
B. 15 V
C. 25V
D. 50V
279.

## Lenz's law is a consequence of the law of conservation of

A. induced current
B. charge
C. energy
D. induced e.m.f.
280.

A. 50 N
B. 120 N
C. 240 N
D. 480 N
281.

A. 100 N
B. 400 N
C. 600 N
D. 1000 N
282.

## The co-efficient of coupling between two air core coils depends on

A. self-inductance of two coils only
B. mutual inductance between two coils only
C. mutual inductance and self inductance of two coils
D. none of the above
Answer» C. mutual inductance and self inductance of two coils
283.

A. 20 Wb
B. 2 Wb
C. 0.2 Wb
D. 0.02 Wb
284.

A. 0.01 s
B. 0.1 s
C. 0.5 s
D. 5 s
285.

## Which of the fpllowing inductor will have the least eddy current losses ?

A. Air core
B. Laminated iron core
C. Iron core
D. Powdered iron core
286.

A. 3500 mH
B. 350 mH
C. 250 mH
D. 150 mH
287.

A. 300 uH
B. 600 uH
C. 150 uH
D. 75 uH
288.

A. 5 mH
B. 10 mH
C. 5 H
D. 10 H
289.

## Which circuit element(s) will oppose the change in circuit current ?

A. Resistance only
B. Inductance only
C. Capacitance only
D. Inductance and capacitance
290.

## A crack in the magnetic path of an inductor will result in

A. unchanged inductance
B. increased inductance
C. zero inductance
D. reduced inductance
291.

## A coil is wound on iron core which carries current I. The self-induced voltage in the coil is not affected by

A. variation in coil current
B. variation in voltage to the coil
C. change of number of turns of coil
D. the resistance of magnetic path
Answer» B. variation in voltage to the coil
292.

A. Yes
B. No
C. none
D. none
293.

A. Yes
B. No
C. none
D. none
294.

A. Yes
B. No
C. none
D. none
295.

A. Yes
B. No
C. none
D. none
296.

A. Yes
B. No
C. none
D. none