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Magnetic Conceptual XII Class
Q.1
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Q.2
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Q.3
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ALFA PHYSICS CLASSES
The North pole of a magnet is brought near a negatively charged conductor. Will the pole
experience any force?
The pole experiences force only if it is placed in some external magnetic field. As the
stationary charge doesn’t produce any magnetic field, the force acting on the pole will be
zero.
Under what condition the force acting on the charge particle moving in the magnetic field
maximum?
The force will have a magnitude F=qvB sin , thus it will be maximum if sin is maximum.
Thus angle between velocity and magnetic field should be 90 or the charge particle moves
perpendicular to the velocity vector.
Under what condition the force acting on charge particle moving in the magnetic field
minimum?
Minimum force or no force acts on the charge particle if it is either moving parallel or
antiparallel to the magnetic field intensity. F=qvBsin, which implies angle should be either
zero[parallel] or 180[antiparallel]
Q.4
A.4
What is the work done by the magnetic field on the moving charge and why?
No work is done by the magnetic field on the moving charge. The magnetic force acts in such
a way that the direction of the magnetic force and velocity are always perpendicular to each
other. If force and velocity are perpendicular force and displacement are also perpendicular,
thus W= FScos, if =90, work done will be zero.
Q.5
A proton enters in a straight line in a uniform magnetic field along the field direction. How will
its path and velocity change?
As the proton is moving along the field direction, thus no force acts on it and the velocity and
the path will not change. It will keep on moving with the same speed along the field direction.
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Q.6
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Q.7
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Q.8
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A charge particle enters a uniform magnetic field perpendicular to the direction of magnetic
field. How will its kinetic energy and momentum change?
As the charge particle enters perpendicular to the magnetic field, the force will act
perpendicular to the velocity. Thus the particle starts moving in the circular path. As force
and displacement are perpendicular the work done will be zero and kinetic energy will not
change. But as it moves in circular path direction of momentum will change but magnitude is
unchanged.
A positive charge is moving vertically upwards in magnetic field towards south. In which
direction will it be deflected?
If positive charge is moving upwards the corresponding current is also upwards, the magnetic
field is towards south. Using Fleming’s left hand rule force is towards east and deflection
towards east.
If electron is not delected in passing through a certain region of space, can we be sure that
there is no magnetic field?
No, the electron will not deflect if it is moving parallel or antiparallel to the direction of
magnetic field or if magnetic force acting on the electron is balanced by some other force.
Q.9
A.9
Does magnetic field exert force on a static charge? Explain
No, if a charge particle is placed in steady magnetic field it will not experience any force.
Q.10
Free electrons always keep on moving in the conductor. Even then no magnetic force acts on
them in a magnetic field. Why?
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Magnetic Conceptual XII Class
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Q.11
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ALFA PHYSICS CLASSES
In the absence of external potential difference the motion of the electrons inside the
conductor is totally random. Thus although the electrons are moving their velocity comes out
to be zero. Thus, magnetic force which acts on them will always be zero.
What is the nature of the magnetic field generated by a current carrying straight conductor?
The magnetic field of long straight conductor is in the circular magnetic lines of force. The
center of these imaginary lines lies on the wire. The plane of magnetic lines of force is
perpendicular to the length of the conductor and is given by right hand thumb rule.
Q.12
A.12
What can be the various sources for generation of the magnetic field?
the magnetic field is generated by the [a] current carrying wire [b] charge particle in motion
[c] magnets [d] time varying electric field
Q.13
The net charge on the conductor placed in magnetic field is zero, but still it experiences force
in the magnetic field?
The net charge is zero because of equal positive and negative charges. But the positive
changes being stationary don’t experience any force whereas the free electrons are moving in
the current carrying conductor experiences force.
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Q.14
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Q.15
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How will you identify if the magnetic field at a given location is due to earth or some other
conductor?
The magnetic field of earth at all places is always in north south direction. Thus if a magnet
suspended at a place always align in north south direction it is the magnetic field of earth
whereas if the magnetic field is due to some conductor, its magnitude ad direction changes
with the change in position.
Does magnetic field exert force on the charge particle in the direction of magnetic field
intensity?
The moving charge particle experience force in external magnetic field and its direction is
always perpendicular to the direction of external magnetic field.
Q.16
A.16
In what respect the current carrying wire different from the wire which carries no current?
In current carrying wire the electrons are moving with some velocity in direction opposite to
the electric field inside the wire. These electrons constitutes current and generates magnetic
field. But if no current is flowing it will not generate magnetic field.
Q.17
In wire in which there is no current flowing, the electrons are in thermal motion. What is the
value of magnetic field near the wire?
As the current is not flowing and motion of electrons is due to temperature, the motion wll be
totally random. Thus, although the electrons are moving with high speed their velocity is
zero. Thus the magnetic field near the wire is also zero.
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Q.18
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A charge particle trapped in a agnetic bottle leaks out after some time. What is the total work
done in the time the particle is trapped?
no, work is done on the charge particle because the magnetic force which acts on the charge
particle is a radial force[perpendicular to the velocity], it can change the direction but not the
speed.
Q.19
A.19
Can we compare the masses of the charged particles moving in the magnetic field?
The radius of the charge particle moving in magnetic field is r= mv/qB. This implies that the
radius depends on the charge to mass ratio. Thus, by comparing the radii of their paths in
magnetic field we can compare the masses if the charges are known.
Q.20
Can magnetic field exert force on the static charge and make it move?
Residence: 86, Chotti Baradari Part –1,[Near Medical College], Garha Road, Jalandhar #98152-15362
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Magnetic Conceptual XII Class
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Q.21
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Q.22
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ALFA PHYSICS CLASSES
If the magnetic field varies with time it will generate electric field, this electric field can exert
force on the static charge.
If a charge particle is deflected in a field, can we find if deflection is due to electric or
magnetic field?
Yes, if the deflection is due to the electric field the path followed is parabolic and the kinetic
energy will also change. But, if the deflection is due to magnetic field the path followed is
either circular or helical and kinetic energy remains constant.
A current carrying ring is placed is placed in uniform magnetic field, what will be its position
of equilibrium?
The equilibrium position of the ring is one in which the torque acting on the ring is zero. Thus,
area vector and the magnetic field intensity should be parallel to each other.
Q.23
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State two properties of the material required as suspension wire of moving coil galvanometer.
The two properties of the suspension are [a] small value of restoring torque per unit angular
twist and [b] non brittle conducting nature.
Q.24
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What is the principle of the moving coil galvanometer?
It is based on the principle that whenever current carrying coil is placed in magnetic field it
will experience a torque.
Q.25
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Why is ammeter always connected in series in the circuit?
Ammeters are always connected in series so that total current of the circuit flows through it
and it should not change resistance of the circuit.
Q.26
Equal currents are flowing in two infinitely long wires in opposite direction. What will be the
magnetic field at the point midway between them?
The magnetic field at the point midway between them is twice the magnetic field due to one
wire, because at midway point the field due to both the wires is in the same direction and
adds up.
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Q.27
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A cyclotron is not suitable to accelerate the electron. Why?
Cyclotron can’t be used to accelerate the electrons because the mass of the electron is small
and at very low values of kinetic energies they acquire velocity comparable to light. Thus
their mass begins to vary. The time taken to complete semicircular orbit is variable not in
synchronization with the oscillating field.
Q.28
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Can cyclotron accelerate uncharged particles?
No, cyclotron can’t accelerate the uncharged particles because neither electric nor magnetic
force acts on the uncharged particles.
Q.29
An electron passes through a region of crossed electric and magnetic field of intensities E and
B respectively. For what value of the electron speed will the beam remain undeflected?
The beam remain undeflected if the electric and magnetic force acting on the electron
balance each other. This means that have same magnitude and opposite direction. Thus v=
E/B.
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Q.30
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An electron and proton enter a uniform magnetic field perpendicular to the direction of the
field with same velocity. Will the force on them be same?
Yes, the force acting on them is equal in magnitude but the direction of force will be
different[opposite]. Also, the radius of the path followed by both will be different as radius is
directly proportional to the mass of the charged particle.
Residence: 86, Chotti Baradari Part –1,[Near Medical College], Garha Road, Jalandhar #98152-15362
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Magnetic Conceptual XII Class
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Q.32
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ALFA PHYSICS CLASSES
What are primary function of electric and magnetic field in the cyclotron?
The magnetic field is responsible for making the charge particle move in the same electric
field again and again. The electric field is used to increase the speed or kinetic energy of the
charge particle.
The frequency of the charged particle circulating inside the dees of the cyclotron does not
depend on the speed of the charge particle. Why?
With the increase in the speed of the charge particle the radius of the circular path also
increases, thus the time taken to complete the circular path or the frequency remains
unchanged.
Q.33
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Is any work done by the magnetic field on the moving charge?
Work done by the magnetic field on the moving charge is always zero, because if the charge
particle moves in the magnetic field the force acts on the charge particle in direction
perpendicular to the direction of velocity of the charge particle. As force and velocity are
perpendicular the force and displacement are also perpendicular thus work done is zero.
Q.34
Does time spend by the charge particle inside the dee of cyclotron depends on the [a] radius
of circular path [b] velocity of the circular path?
The time spend by the charge particle in any dee depends only on the specific charge and
magnetic field intensity. It is independent of the velocity of the charge particle or the radius
of the circular path.
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Q.35
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What is magnetic lorentz force?
The force acting on the charge particle moving inside the magnetic field is called magnetic
Lorentz force. F = qvB sin  is the magnitude of the force.
Q.36
Under what conditions electron moving in the uniform magnetic field experiences maximum
force?
The electron experience maximum force when moving inside the magnetic field if the velocity
of the electron is perpendicular to the direction of the magnetic field intensity.
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Q.37
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Q.38
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Under what condition the force acting on the charge particle in the magnetic field of intensity
B minimum?
The force will be minimum if the charge particle moves collinear with the direction of
magnetic field intensity i.e. either parallel to the field or antiparallel to the magnetic field.
An electron is projected with its velocity in the direction of the magnetic field. Will its motion
be affected?
No the motion of the electron will not be affected by the magnetic field as the charge particle
moves parallel to the direction of the magnetic field the force acting on the charge particle is
zero.
Q.39
A.39
Why to solenoid tends to contract when current passes through it?
When current flows through the solenoid the current in the adjacent turns is parallel to each
other. Since parallel currents attract each other they tend to come closer. Thus, solenoids
tends to contract when current flows through it.
Q.40
A loop of irregular shape carrying current I is placed in uniform magnetic field. If the wire is
flexible why does it take circular shape?
It will assume circular shape with its plane normal to the field in order to maximize the
magnetic flux through it. The reason is that for a given perimeter, of all the geometrical
shapes circle has maximum area.
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Residence: 86, Chotti Baradari Part –1,[Near Medical College], Garha Road, Jalandhar #98152-15362
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Magnetic Conceptual XII Class
Q.41
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Q.42
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ALFA PHYSICS CLASSES
Why should ammeter have low resistance?
for measuring the current the ammeter is connected in series in the circuit. So the current
remains practically unchanged on connecting the ammeter, the resistance of the ammeter
should be as small as possible. For ideal ammeters the resistance should be zero.
A small coil carrying current is placed in uniform magnetic field. How does the coil tend to
orient itself relative to the magnetic field?
When current carrying coil is placed in the magnetic field torque acts on the coil. The torque
acting on the coil will be zero if the coil and the magnetic field are perpendicular. Thus it
orients such that the plane of coil is perpendicular to the field direction.
Q.43
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Does current carrying coil produce uniform magnetic field?
No, the current carrying coil doesn’t produce uniform magnetic field but it can be assumed to
be uniform at the center of the circular coil.
Q.44
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What kind of magnetic field is produced by a straight solenoid?
Straight solenoid produces magnetic field which is identical to the magnetic
magnet.
field of a bar
Q.45 Define Bohr magneton.
A,45 Bohr magneton is defined as the magnetic dipole moment associated with the atom due to
orbital motion of electrons in the first Bohr orbit. Its value is 9.27x10 -24Am2.
Q.46
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Why ordinary piece of iron doesn’t behave as a magnet?
In ordinary piece of irons the magnetic dipole moments are randomly oriented thus the net
magnetic dipole moment is zero. Thus it can’t behave as a magnet.
Q.47
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What do you mean by the directive property of magnetic dipole?
Directive property of the magnetic dipole is that the freely suspended magnet always aligns
itself in the direction of the magnetic field.
Q.48
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What is the basic difference between electric and magnetic lines of force?
Electric lines of force originate from positive and ends on the negative charge whereas the
magnetic lines of force are in the form of closed loops. Magnetic lines never originate or ends
anywhere.
Q.49
The earth’s core contains iron, yet it is not considered the cause of Earth’s magnetic field.
Why?
As we move inside the earth from the surface, the temperature of earth increases. At the
core of earth temperature is assumed to be of the order of thousands of Kelvin. No substance
can retain its magnetic properties at such a high temperature.
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Q.50
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Can current loop be treated as a magnetic dipole?
The current loop can be treated as a magnetic dipole. If the current I flows in a loop of area A
then the magnetic dipole moment is given by
M= iA
Residence: 86, Chotti Baradari Part –1,[Near Medical College], Garha Road, Jalandhar #98152-15362
5
Magnetic Conceptual XII Class
ALFA PHYSICS CLASSES
Residence: 86, Chotti Baradari Part –1,[Near Medical College], Garha Road, Jalandhar #98152-15362
6