Question 1:
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: _________
Question 2:
Conductance is analogous to
A.
Permeance
B.
Reluctance
C.
Flux
D.
Inductance
Answer: _________
Question 3:
Permanent magnets are normally made of
A.
Alnico alloys
B.
Aluminium
C.
Cast iron
D.
Wrought iron
Answer: _________
Question 4:
A material for good magnetic memory should have
A.
Low hysteresis loss
B.
High permeability
C.
Low retentivity
D.
High retentivity
Answer: _________
Question 5:
Reciprocal of reluctance is
A.
Reluctivity
B.
Permeance
C.
Permeability
D.
Susceptibility
Answer: _________
Question 6:
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: _________
Question 7:
Silicon steel is used in electrical machines because it has
A.
Low coercivity
B.
Low retentivity
C.
Low hysteresis loss
D.
High coercivity
Answer: _________
Question 8:
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
Answer: _________
Question 9:
The unit of retentivity is
A.
Weber
B.
Weber/sq. m
C.
Ampere turn/meter
D.
Ampere turns
Answer: _________
Question 10:
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
Answer: _________
Question 11:
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: _________
Question 12:
Energy stored by a coil is doubled when its current is increased by percent.
A.
25
B.
50.14
C.
41.4
D.
100
Answer: _________
Question 13:
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: _________
Question 14:
The unit of reluctance is
A.
$$frac{{{ ext{Meter}}}}{{{ ext{Henry}}}}$$
B.
$$frac{{{ ext{Henry}}}}{{{ ext{Meter}}}}$$
C.
$${ ext{Henry}}$$
D.
$$frac{1}{{{ ext{Henry}}}}$$
Answer: _________
Question 15:
Hysteresis loss least depends on
A.
Volume of material
B.
Frequency
C.
Steinmetz’s coefficient of material
D.
Ambient temperature
Answer: _________
Question 16:
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: _________
Question 17:
The property of a material which opposes the creation of magnetic flux in it is known as
A.
Reluctivity
B.
Magneto-motive force
C.
Permeance
D.
Reluctance
Answer: _________
Question 18:
Conductivity is analogous to
A.
Retentivity
B.
Resistivity
C.
Permeability
D.
Inductance
Answer: _________
Question 19:
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
Answer: _________
Question 20:
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: _________
Question 21:
The unit of magnetic flux is
A.
Henry
B.
Weber
C.
Ampere-turn/weber
D.
Ampere/metre
Answer: _________
Question 22:
Laminated cores, in electrical machines, are used to reduce
A.
Copper loss
B.
Eddy current loss
C.
Hysteresis loss
D.
All of the above
Answer: _________
Question 23:
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
Answer: _________
Question 24:
Those magnetic materials are best suited for making armature and transformer cores which have ________permeability and _________hysteresis loss.
A.
High, high
B.
Low, high
C.
High, low
D.
Low, low
Answer: _________
Question 25:
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
Answer: _________
Question 26:
Relative permeability of vacuum is
A.
1
B.
1 H/m
C.
$$4pi imes {10^{ - 7}}$$
D.
$$frac{1}{4}$$
Answer: _________
Question 27:
In a magnetic material hysteresis loss takes place primarily due to
A.
Rapid reversals of its magnetization
B.
Flux density lagging behind magnetizing force
C.
Molecular friction
D.
It high retentivity
Answer: _________
Question 28:
Permeability in a magnetic circuit corresponds to_________ in an electric circuit.
A.
Resistance
B.
Resistivity
C.
Conductivity
D.
Conductance
Answer: _________
Answer Key
1:
A
Solution: A ferrite core has less eddy current loss than an iron core because ferrites have high resistance . Eddy currents are circulating currents that are induced in a conducting material (like a core) when it is exposed to a changing magnetic field. These currents lead to energy loss in the form of heat due to the electrical resistance of the material. Ferrite cores are made from materials that have high electrical resistance, which means they offer more opposition to the flow of eddy currents. As a result, the eddy current losses in ferrite cores are significantly lower compared to materials with lower resistance, like iron cores. This property makes ferrite cores suitable for applications in high-frequency transformers and other electronic devices where minimizing energy losses is important.
2:
A
3:
A
4:
D
5:
B
6:
A
7:
C
8:
C
9:
B
10:
C
11:
D
12:
C
13:
A
14:
D
15:
D
16:
B
17:
D
18:
C
19:
B
20:
A
21:
B
22:
B
23:
B
24:
C
25:
B
26:
A
27:
B
Solution: Hysteresis loss occurs in a magnetic material due to the lagging of flux density (B) behind the magnetizing force (H) when the material undergoes cyclic magnetization. This delay in response leads to energy dissipation in the form of heat , which is represented by the area enclosed by the hysteresis loop . Why Not the Other Options? Option A: Rapid reversals of its magnetization – This contributes to hysteresis loss but is not the root cause . The loss happens due to the delay in realigning the magnetic domains, not just the rapid changes themselves. Option C: Molecular friction – This is a misleading term . While there is internal resistance in domain movement, hysteresis loss is primarily due to flux lag rather than physical friction at the molecular level. Option D: Its high retentivity – Retentivity refers to a material’s ability to retain magnetization after the external field is removed. A material with high retentivity does not necessarily have high hysteresis loss . Retentivity influences the shape of the hysteresis loop but is not the direct cause of hysteresis loss . Thus, Option B is the correct answer.
28:
C