DC Pandey Solutions: Magnetics

# DC Pandey Solutions: Magnetics | Physics Class 12 - NEET PDF Download

``` Page 1

Introductory Exercise 23.1
Q.1. Write the dimensions of E/B. Here, E is the electric field and B the
magnetic field.
Sol. qE = Bqv sin ?
[E/B] = [v] = [LT ]
Q.2. In the relation which pairs are always perpendicular to
each other.
Sol. From the property of cross product  is always perpendicular to both
Q.3. If a beam o f electrons travels in a straight line in a certain region. Can
we say there is no magnet ic field?
Sol.  both may be present and may be possible that,
Q.4. A charge q = -4µC has an instantaneous velocity
in a uniform magnetic field
What is the force on the charge?
Sol.
Here, q has to be substituted with sign.
Introductory Exercise 23.2
-1
Page 2

Introductory Exercise 23.1
Q.1. Write the dimensions of E/B. Here, E is the electric field and B the
magnetic field.
Sol. qE = Bqv sin ?
[E/B] = [v] = [LT ]
Q.2. In the relation which pairs are always perpendicular to
each other.
Sol. From the property of cross product  is always perpendicular to both
Q.3. If a beam o f electrons travels in a straight line in a certain region. Can
we say there is no magnet ic field?
Sol.  both may be present and may be possible that,
Q.4. A charge q = -4µC has an instantaneous velocity
in a uniform magnetic field
What is the force on the charge?
Sol.
Here, q has to be substituted with sign.
Introductory Exercise 23.2
-1
Q.1. Can a charged particle be accelerated by a magnetic field. Can its
speed be increased?
Sol Magnetic force may be non-zero. Hence acceleration due to magnetic force may
be non-zero. Magnetic force is always perpendicular to velocity. Hence its power is
always zero or work done by magnetic force is always zero. Hence it can be change
the speed of charged particle.
Q.2. An electron beam projected along positive x-axis deflects along the
positive y-axis. If this deflection is caused by a magnetic field, what is the
direction of the field?
Sol:
is along position y -direction, q is negative and  is along positive x-direction.
Therefore  should be along positive z-direction.
Q.3. An electron and a proton are projected with same velocity
perpendicular to a magnetic field,
(a) Which particle will describe the smaller circle?
(b) Which particle will have greater frequency?
Sol.  as other factors are same.
Q.4. In a region of space a uniform magnetic field 6 is along positive x-axis.
Electrons are emitted from the origin with speed v at different angles. Show
that the paraxial electrons are refocused on the x-axis at a distance
Here, m is the mass of electron and e the charge on it.
Sol.
Electrons touches the x-axis again after every pitch. Therefore the asked distance is
d = p = v T
11
Page 3

Introductory Exercise 23.1
Q.1. Write the dimensions of E/B. Here, E is the electric field and B the
magnetic field.
Sol. qE = Bqv sin ?
[E/B] = [v] = [LT ]
Q.2. In the relation which pairs are always perpendicular to
each other.
Sol. From the property of cross product  is always perpendicular to both
Q.3. If a beam o f electrons travels in a straight line in a certain region. Can
we say there is no magnet ic field?
Sol.  both may be present and may be possible that,
Q.4. A charge q = -4µC has an instantaneous velocity
in a uniform magnetic field
What is the force on the charge?
Sol.
Here, q has to be substituted with sign.
Introductory Exercise 23.2
-1
Q.1. Can a charged particle be accelerated by a magnetic field. Can its
speed be increased?
Sol Magnetic force may be non-zero. Hence acceleration due to magnetic force may
be non-zero. Magnetic force is always perpendicular to velocity. Hence its power is
always zero or work done by magnetic force is always zero. Hence it can be change
the speed of charged particle.
Q.2. An electron beam projected along positive x-axis deflects along the
positive y-axis. If this deflection is caused by a magnetic field, what is the
direction of the field?
Sol:
is along position y -direction, q is negative and  is along positive x-direction.
Therefore  should be along positive z-direction.
Q.3. An electron and a proton are projected with same velocity
perpendicular to a magnetic field,
(a) Which particle will describe the smaller circle?
(b) Which particle will have greater frequency?
Sol.  as other factors are same.
Q.4. In a region of space a uniform magnetic field 6 is along positive x-axis.
Electrons are emitted from the origin with speed v at different angles. Show
that the paraxial electrons are refocused on the x-axis at a distance
Here, m is the mass of electron and e the charge on it.
Sol.
Electrons touches the x-axis again after every pitch. Therefore the asked distance is
d = p = v T
11
For paraxial electrons ? ˜ 0° and q = e
?
Q.5. A particle of mass m and charge q is projected into a region having a
perpendicular magnetic field 6. Find the angle of deviation of the particle as
it comes out of the magnetic field if the width of the region is,

Sol.
and ? = p if L = r
Q.6. An electron is accelerated through a PD of 100 V and then enters a
region where it is moving perpendicular to a magnetic field S = 0.2 T. Find
the radius of the circular path. Repeat this problem for a proton.
Sol.
Q.7. A proton, a deutron and an a-particle have equal kinetic energies.
Compare the radii of their paths when a normal magnetic field is applied.
Page 4

Introductory Exercise 23.1
Q.1. Write the dimensions of E/B. Here, E is the electric field and B the
magnetic field.
Sol. qE = Bqv sin ?
[E/B] = [v] = [LT ]
Q.2. In the relation which pairs are always perpendicular to
each other.
Sol. From the property of cross product  is always perpendicular to both
Q.3. If a beam o f electrons travels in a straight line in a certain region. Can
we say there is no magnet ic field?
Sol.  both may be present and may be possible that,
Q.4. A charge q = -4µC has an instantaneous velocity
in a uniform magnetic field
What is the force on the charge?
Sol.
Here, q has to be substituted with sign.
Introductory Exercise 23.2
-1
Q.1. Can a charged particle be accelerated by a magnetic field. Can its
speed be increased?
Sol Magnetic force may be non-zero. Hence acceleration due to magnetic force may
be non-zero. Magnetic force is always perpendicular to velocity. Hence its power is
always zero or work done by magnetic force is always zero. Hence it can be change
the speed of charged particle.
Q.2. An electron beam projected along positive x-axis deflects along the
positive y-axis. If this deflection is caused by a magnetic field, what is the
direction of the field?
Sol:
is along position y -direction, q is negative and  is along positive x-direction.
Therefore  should be along positive z-direction.
Q.3. An electron and a proton are projected with same velocity
perpendicular to a magnetic field,
(a) Which particle will describe the smaller circle?
(b) Which particle will have greater frequency?
Sol.  as other factors are same.
Q.4. In a region of space a uniform magnetic field 6 is along positive x-axis.
Electrons are emitted from the origin with speed v at different angles. Show
that the paraxial electrons are refocused on the x-axis at a distance
Here, m is the mass of electron and e the charge on it.
Sol.
Electrons touches the x-axis again after every pitch. Therefore the asked distance is
d = p = v T
11
For paraxial electrons ? ˜ 0° and q = e
?
Q.5. A particle of mass m and charge q is projected into a region having a
perpendicular magnetic field 6. Find the angle of deviation of the particle as
it comes out of the magnetic field if the width of the region is,

Sol.
and ? = p if L = r
Q.6. An electron is accelerated through a PD of 100 V and then enters a
region where it is moving perpendicular to a magnetic field S = 0.2 T. Find
the radius of the circular path. Repeat this problem for a proton.
Sol.
Q.7. A proton, a deutron and an a-particle have equal kinetic energies.
Compare the radii of their paths when a normal magnetic field is applied.
Sol.
Page 5

Introductory Exercise 23.1
Q.1. Write the dimensions of E/B. Here, E is the electric field and B the
magnetic field.
Sol. qE = Bqv sin ?
[E/B] = [v] = [LT ]
Q.2. In the relation which pairs are always perpendicular to
each other.
Sol. From the property of cross product  is always perpendicular to both
Q.3. If a beam o f electrons travels in a straight line in a certain region. Can
we say there is no magnet ic field?
Sol.  both may be present and may be possible that,
Q.4. A charge q = -4µC has an instantaneous velocity
in a uniform magnetic field
What is the force on the charge?
Sol.
Here, q has to be substituted with sign.
Introductory Exercise 23.2
-1
Q.1. Can a charged particle be accelerated by a magnetic field. Can its
speed be increased?
Sol Magnetic force may be non-zero. Hence acceleration due to magnetic force may
be non-zero. Magnetic force is always perpendicular to velocity. Hence its power is
always zero or work done by magnetic force is always zero. Hence it can be change
the speed of charged particle.
Q.2. An electron beam projected along positive x-axis deflects along the
positive y-axis. If this deflection is caused by a magnetic field, what is the
direction of the field?
Sol:
is along position y -direction, q is negative and  is along positive x-direction.
Therefore  should be along positive z-direction.
Q.3. An electron and a proton are projected with same velocity
perpendicular to a magnetic field,
(a) Which particle will describe the smaller circle?
(b) Which particle will have greater frequency?
Sol.  as other factors are same.
Q.4. In a region of space a uniform magnetic field 6 is along positive x-axis.
Electrons are emitted from the origin with speed v at different angles. Show
that the paraxial electrons are refocused on the x-axis at a distance
Here, m is the mass of electron and e the charge on it.
Sol.
Electrons touches the x-axis again after every pitch. Therefore the asked distance is
d = p = v T
11
For paraxial electrons ? ˜ 0° and q = e
?
Q.5. A particle of mass m and charge q is projected into a region having a
perpendicular magnetic field 6. Find the angle of deviation of the particle as
it comes out of the magnetic field if the width of the region is,

Sol.
and ? = p if L = r
Q.6. An electron is accelerated through a PD of 100 V and then enters a
region where it is moving perpendicular to a magnetic field S = 0.2 T. Find
the radius of the circular path. Repeat this problem for a proton.
Sol.
Q.7. A proton, a deutron and an a-particle have equal kinetic energies.
Compare the radii of their paths when a normal magnetic field is applied.
Sol.
Introductory Exercise 23.3
Q 1.  In a certain region uniform electric field
0
ˆ
E E k ??
?
and magnetic field
0
ˆ
B B k ??
?
are present. At
time t = 0 a particle of mass m and charge q is given a velocity
00
ˆˆ
v v j v k ??
?
.

Find the minimum
speed of the particle and the time when it happens so.

Q 2.  A particle of mass m and charge q is lying at the origin in a uniform magnetic field B directed
along x-axis. At time t = 0, it is given a velocity v
0
at an angle ? with the y-axis in the xy-plane.
Find the coordinates of the particle after one revolution.
Q 3.  A wire of length l carries a current i along the x-axis. A magnetic field
0
ˆˆ
B B (j k) ??
?
exists in the
space. Find the magnitude of the magnetic force acting on the wire.
Q 4.  In the above problem will the answer change if magnetic field becomes

0
ˆ ˆ ˆ
B B (i j k) ? ? ?
?

Solutions
1.  (along negative z-direction)
Electric field will make z-component of velocity zero. At that time speed of the particle will be
minimum and that minimum speed is the other component i. e., v
0
. This is minimum when,

2.  Path is helix and after one rotation only x-coordinate will change by a distance equal to pitch.

3.

4.  No it will not change, as the new component of is in the direction of
Introductory Exercise 23.4
Q 1.  A charge q is uniformly distributed on a nonconducting disc of radius Ft. It is rotated with an
angular speed a about an axis passing through the centre of mass of the disc and perpendicular to
its plane. Find the magnetic moment of the disc.
[Hint: Magnetic moment =
q
2m
??
??
??
(angular momentum)]
Q 2.  Find the magnetic moment of the current carrying loop OABCO shown in figure.
Given that, i = 4.0 A, OA = 20 cm and AB =10 cm.
```

## Physics Class 12

105 videos|425 docs|114 tests

## FAQs on DC Pandey Solutions: Magnetics - Physics Class 12 - NEET

 1. What are some common applications of magnetics in daily life?
Ans. Magnetics have various applications in daily life, such as in electric motors, transformers, speakers, and magnetic storage devices like hard drives and credit cards. These applications make use of magnetic fields and the properties of magnetic materials to perform specific tasks.
 2. How does magnetic levitation work?
Ans. Magnetic levitation, also known as maglev, works by using the repulsive or attractive forces between magnets to suspend an object in mid-air. This is achieved by using magnets with opposite poles facing each other, creating a magnetic field that counters the force of gravity. Maglev trains and floating displays are some examples of applications that utilize magnetic levitation.
 3. What is the difference between ferromagnetic and paramagnetic materials?
Ans. Ferromagnetic materials, such as iron and nickel, have a strong attraction to magnetic fields and can retain their magnetization even after the external magnetic field is removed. On the other hand, paramagnetic materials, like aluminum and platinum, are weakly attracted to magnetic fields and lose their magnetization once the external field is removed.
 4. How does electromagnetic induction work?
Ans. Electromagnetic induction is the process of generating an electric current in a conductor by varying the magnetic field around it. This can be achieved by moving a magnet through a coil of wire or by varying the current in a nearby coil. The changing magnetic field induces an electric current in the conductor according to Faraday's law of electromagnetic induction.
 5. What are some factors that affect the strength of an electromagnet?
Ans. The strength of an electromagnet depends on various factors, including the number of turns in the coil, the amount of current flowing through the coil, the material used as the core, and the presence of a ferromagnetic material within the core. Increasing any of these factors enhances the magnetic field strength produced by the electromagnet.

## Physics Class 12

105 videos|425 docs|114 tests

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