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A conducting loop is placed above a long
straight conductor carrying current from left to
right. What will be the direction of the induced
current in loop if current in conductor is
increased?
The direction of magnetic field intensity of
induced current will be such that it will oppose
the magnetic field intensity of conductor. Thus
the current in conductor will be in clockwise
direction.
An artificial satellite with metal surface is
always launched in equatorial plane why?
This is done because if the satellite moves in
equatorial plane flux linked with the satellite is
not changing thus the induced emf will be zero.
Does Lenz’s law violate the law of
conservation of energy?
No, it does not violate the law of conservation
as the work done to overcome the opposing
force of induced emf will be used to generate
electrical energy.
When a magnet falls through the vertical coil,
will its acceleration be different from
acceleration due to gravity?
Yes, because when the magnet falls through
the coil induced emf will be generated which
will be oppose the motion of the magnet. Thus
acceleration of magnet is less than the
acceleration due to gravity.
Why the soft iron core is used in dead beat
galvanometer?
Soft iron core is used in dead beat
galvanometer because the eddy currents in the
soft iron core will oppose the motion of the coil
and it comes to rest instantly.
two identical loops one of copper and other of
iron are rotated with same angular velocity. In
which case will be the induced emf more?
Induced emf doesn’t depend upon the nature
of material but depends only on the magnetic
field, angular velocity and radius of the loop.
A lamp is connected in parallel with the coil
having large self inductance glows brilliantly
before going off. Why?
The lamp glows brilliantly before going off due
to large induced emf and thus large induced
current.
A wire kept along north south direction is
allowed to fall freely. Will there be induced emf
in wire?
There will be no induced emf as magnetic flux
is not changing.
A metal block and a brick of same mass are
falling from the same height above the ground.
Which will reach the ground earlier and why?
brick will reach the ground earlier as induced
emf will be there in the metal block which will
oppose the motion of the block and its
acceleration will be reduced.
Sanjay Chopra M.Sc[Hons] Physics
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Why a thick metal plate oscillating about
horizontal axis stops when a strong magnetic
field is applied on the plate?
This happens because when the magnetic field
is switched on the induced emf and thus eddy
currents are produced in metal plate due to
motion of the plate. The direction of eddy
currents will be such that they opposes the
motion of the plate and it stops.
What are the factors on which the self
inductance of coil depends?
Self inductance of the coil depends upon the
dimensions of the coil, number of turns and
medium used as core in coil.
how does the self inductance of the coil
changes when iron is used as core in place of
air?
Permeability of iron is much greater than the
permeability of air. Thus self inductance with
iron core will also increase.
How does the mutual inductance between the
coils changes when distance between the coils
is increased?
On increasing the distance the flux linkage
between the coil decreases i.e. lesser number
of field lines of primary passes through
secondary. Thus mutual inductance will
decrease.
Two similar current carrying coils carry equal
currents in same direction. If the loops are
brought closer, what will happen to the
currents in them?
when the loops are brought closer, the
magnetic flux linked with the coils increases.
Thus induced emf will be produce to oppose
this increase and current in coils will decrease.
An iron bar falling through hollow region of the
cylindrical shell made of copper experiences a
retarding force. What can you conclude about
the iron bar?
The motion of iron bar is retarded because flux
linked with it is changes which will induce emf.
The induced emf will oppose motion of iron
bar. But flux can change only if the iron bar is
magnet.
How can a space traveler detect the presence
of magnetic field on an unknown planet with
the help of galvanometer and coil of wire?
The traveler can fix coil with galvanometer. If
the coil is rotated and galvanometer shows
deflection, it implies presence of magnetic
field. But if there is no magnetic field then the
deflection in galvanometer will not be there.
An induced emf has no direction of its own.
Comment.
Induced emf has no direction of its own as it
always opposes the change the flux. Thus
direction of induced emf is governed by the
cause of change in flux.
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A glass rod of length l moves with velocity v in
uniform magnetic field B. what is the emf
induced emf in the rod?
Induced emf in the rod will be zero as glass is
an insulator and no movement of charges
takes place in it due to magnetic field.
A conductor is placed in magnetic field is
moved parallel to the magnetic field B. What is
the emf induced in the loop?
No, emf is induced in the loop as motional emf
is induced if the rod is moved perpendicular to
the magnetic field.
two coils are being moved out of magnetic
field-one coil is moved rapidly and the other
slowly. In which case is more work done and
why?
More work will be done in case of rapidly
moving coil. This is because according to
Faraday’s law the induced emf is directly
proportional to the rate of change of flux linked
with the coil.
Why the inductance per unit length for solenoid
near its center different from inductance per
unit length near the ends?
Self inductance depends upon the magnetic
field of solenoid and field at the center is
double of the field at the center of solenoid.
Thus self inductance per unit length will also
be different.
Will an induced current be always produced
when there is change in magnetic flux linked
with the coil?
No, induced current is not essential, whenever
flux changes emf is induced but the induced
current flows only when the conducting circuit
is closed.
A closed conducting loop moves normal to the
electric field between the plates of large
capacitor. Is a current induced in the loop
when it is [a]wholly inside the capacitor [b]
partially inside the capacitor?
No, in both the cases induced current or
induced emf will not be there because induced
current flows due to the changing magnetic
flux and not due the changing electric flux.
Why birds fly off a high tension wire when
current is switched on?
When current begins to increase from zero to
maximum value, the magnetic lines of force
and thus magnetic flux through the body of bird
changes. The change in flux induces current in
the body of bird which produces repulsive
force and birds fly off.
An electron moves in circle with uniform speed
in stationary magnetic field normal to the plane
of the circle. If the field is made to increase
with time, will electron speed up or speed
down? Will it continue to revolve in the same
circle?
Sanjay Chopra M.Sc[Hons] Physics
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Using Lenz’s law it is easy to see that the
electron speeds up is B increases with time.
The electron will not stay in same circular orbit
because the speed of electron may not
increase in proportion to the magnetic field
intensity.
A copper ring is held horizontal and a bar
magnet is made to fall through the ring with the
length along the axis of the ring. Will the
acceleration of the falling magnet be equal to
greater than or less than g?
When the magnet approaches the ring, flux
through the ring changes. Induced emf is
produced which will oppose the cause of
change in flux. As the cause is motion of
magnet therefore its motion is opposed and
less than g.
A small resistor is usually put in parallel with
the current carrying coil of an electromagnet.
What purpose does this serve?
When the current in the coil of large
electromagnet is suddenly switched off, flux
changes from large value to zero in short time.
Thus large induced emf is produced. A low
resistor provides conducting path through
which current can flow thus reducing risks of
sparks.
Why the oscillations of copper disc in magnetic
field are highly damped?
The eddy currents are produced in the copper
disc which will oppose the motion of the disc
thus its motion is highly damped.
Why does capacitor locks dc while ac is
allowed to pass through it?
Capacitive reactance i.e. resistance offered by
the capacitor to the flow of current through it is
inversely proportional to frequency  I,e, Xc =
1/C. For dc  = 0 , thus capacitive reactance
is infinity and dc gets blocked.
Why ac is more dangerous than dc of the
same voltage?
AC is more dangerous than dc of the same
voltage because peak value of ac is 2 times
the rms value. Thus 220V ac will have a peak
value of 310V.
What are capacitive and inductive reactance
and what is phase difference between them?
Capacitive reactance is the resistance offered
by the capacitor to the flow of current through it
and inductive reactance is resistance offered
by inductor to the flow of ac through it. Phase
difference between inductive and capacitive
reactance is 1800.
Which is best method of reducing current in an
ac circuit and why?
the best method of reducing current in an a
circuit making use of choke coil. If the choke
coil is assumed to be resistance less, then
there is no dissipation of energy.
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Why cannot we use ordinary moving coil
galvanometer for measuring ac currents and
voltages?
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Because it measures the average value of
emf/current and the average value of ac is
zero. Therefore it cannot measure ac current
and voltages.
Q.34 Can we use 20c/s for lightning purposes?
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yes 20c/s can be used for lightning purposes.
The fluctuations in the current will be so
rapid[40 times/s] that the bulb will appear to
glow continuously.
Q.35 Is any power consumed by ideal choke coil?
A.35
Power consumed by a device is directly
proportional to the power factor. For choke coil
power factor cos , for choke coil  =900, thus
power factor is zero and power consumed is
zero.
Q.36 Why alternating current measuring instruments
has a non-linear scale?
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Because these instruments are based on
heating effects of currents and heat generated
is proportional to I2.
Q.37 What is the significance of time constant in an
LR circuit?
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The time constant of LR circuit tells us how
fast or slow the growth or decay of current
takes place in an LR circuit.
Q.38 In any ac circuit, is the applied voltage equal to
the algebraic sum of instantaneous voltages
across different circuit elements? Is the same
true for rms voltages also?
A.38
Yes applied instantaneous voltage is equal to
algebraic sum of instantaneous voltages
across circuit elements.
No, this is not true for rms values because
such voltages across different circuit elements
are not in same phase.
Q.39 Can a capacitor of suitable capacitance be
used in place of choke coil?
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Yes, because average power consumed b
capacitor also in one complete cycle of ac is
zero.
Q.40 The dc and ac both can be measured by hot
wire instrument. Why?
A.40
Hot wire instruments are based on the
principle of generation of heat, which is
proportional to the square of current flowing
through it.
Q.41 A lamp is connected in series with a capacitor.
Predict your observations for dc an ac
connections. What happens in each case if the
capacity is reduced?
A.41
Capacitor blocks dc therefore bulb will not glow
even if the capacity is reduced if it connected
to dc. In ac connections on decreasing C
capacitive reactance increases thus the
intensity of glow deceases.
Q.42 An electric heater is used with ac and dc turn
by turn. If the potential difference is kept
Sanjay Chopra M.Sc[Hons] Physics
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constant in the two cases. Will the heat
produced be same in two cases?
An electric heater is in the form of coil or
inductor with inductive reactance L. For dc
inductive reactance is zero. Thus current flow
in the case of dc is more than the current flow
in the case of ac. Thus heat produced in the
case of ac will be less than dc.
At very high frequency of ac, capacitor
behaves as pure conductor. Why?
The capacitive reactance is inversely
proportional to the frequency of ac. Thus for
very high frequency the capacitive reactance
reduces to zero and it behaves as a pure
conductor.
What is the minimum value of power factor?
When does this occur?
Minimum value of power factor is zero and it
occurs for purely inductive or capacitive circuit.
What is the phase difference between the
voltages across L and C in series LCR circuit
connected to ac source?
Potential difference across L and C in series
LCR circuit is 180.
Power factor can be improved by using
capacitor of appropriate value in the circuit.
Explain.
Power factor in LCR circuit is R/Z. When
capacitance is changed for one particular
value of C, inductive and capacitive reactance
is equal, then Z=R and power factor becomes
maximum.