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Electrical Engineering
Magnetic Circuit
Quiz 2
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Q.1
Relative permeability of vacuum is
1
1 H/m
1/4JI
4n × 10' H/m
Q.2
In a magnetic material hysteresis loss takes place primarily due to
Rapid reversals of its magnetization
Flux density lagging behind magnetizing force
Molecular friction
It high retentivity
Q.3
Permeability in a magnetic circuit corresponds to_________ in an electric circuit.
Resistance
Resistivity
Conductivity
Conductance
Q.4
The unit for reluctance is
tesla
At/Wb
At/m
Wb
Q.5
Those materials are well suited for making permanent magnets whichhave______retentivity and _______coercivity.
low, high
high, high
high, low
low, low
Q.6
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
Q.7
Permeability in a magnetic circuit corresponds to______ in an electric circuit.
resistance
resistivity
conductivity
conductance
Q.8
If a loop in a basic dc generator suddenly begins rotating at a faster speed, the induced voltage
remains unchanged
reverses polarity
increases
decreases
Q.9
What is the magnetomotive force (mmf) of a wire with 8 turns carrying three amperes of current?
2,400 At
240 At
24 At
2.4 At
Q.10
When a solenoid is activated, the force that moves the plunger is
an electromagnetic field
a permanent magnetic field
varying voltage
a steady current
Q.11
An electromagnetic field exists only when there is
an increasing current
decreasing current
voltage
current
Q.12
The direction of a magnetic field within a magnet is
from south to north
from north to south
back to front
front to back
Q.13
When the north poles of two bar magnets are brought close together, there will be
no force
a downward force
a force of attraction
a force of repulsion
Q.14
The ability of a material to remain magnetized after removal of the magnetizing force is known as
permeability
reluctance
hysteresis
retentivity
Q.15
The induced voltage across a stationary conductor in a stationary magnetic field is
zero
reversed in polarity
increased
decreased
Q.16
What is the magnetomotive force in a 75-turn coil of wire when there are 4 A of current through it?
18.75 At
30 At
300 At
187 At
Q.17
The unit for permeability is
Wb/At × m
At/m
At/Wb
Wb
Q.18
A basic one-loop dc generator is rotated atrev/s. How many times each second does the dc output voltage peak (reach maximum)?
90
180
270
360
Q.19
When the current through the coil of an electromagnet reverses, the
direction of the magnetic field reverses
direction of the magnetic field remains unchanged
magnetic field expands
magnetic field collapses
Q.20
If the cross-sectional area of a magnetic field increases, but the flux remains the same, the flux density
increases
decreases
remains the same
doubles
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