NCERT Solutions Class 12 Physics Chapter 4 Moving Charges And Magnetism Download In Pdf

Chapter 4 Moving Charges and Magnetism Download in pdf

**Question 4.1 **A circular coil of wire consisting of 100 turns, each of radius 8.0 cm
carries a current of 0.40 A. What is the magnitude of the magnetic
field B at the centre of the coil?

**Question 4.2 **A long straight wire carries a current of 35 A. What is the magnitude
of the field B at a point 20 cm from the wire?

**Question 4.3** A long straight wire in the horizontal plane carries a current of 50 A
in north to south direction. Give the magnitude and direction of B
at a point 2.5 m east of the wire.

**Question 4.4 **A horizontal overhead power line carries a current of 90 A in east to
west direction. What is the magnitude and direction of the magnetic
field due to the current 1.5 m below the line?

**Question 4.5** What is the magnitude of magnetic force per unit length ?

**Question 4.6** A 3.0 cm wire carrying a current of 10 A is placed inside a solenoid
perpendicular to its axis. The magnetic field inside the solenoid is
given to be 0.27 T. What is the magnetic force on the wire?

**Question 4.7** Two long and parallel straight wires A and B carrying currents of
8.0 A and 5.0 A in the same direction are separated by a distance of 4.0 cm. Estimate the force on a 10 cm section of wire A.

**Question 4.8** A closely wound solenoid 80 cm long has 5 layers of windings of 400
turns each. The diameter of the solenoid is 1.8 cm. If the current
carried is 8.0 A, estimate the magnitude of B inside the solenoid
near its centre.

**Question 4.9** A square coil of side 10 cm consists of 20 turns and carries a current
of 12 A. The coil is suspended vertically and the normal to the plane
of the coil makes an angle of 30º with the direction of a uniform
horizontal magnetic field of magnitude 0.80 T. What is the magnitude
of torque experienced by the coil?

**Question 4.10** Two moving coil meters, M1 and M2 have the following particulars:
R1 = 10 Ω, N1 = 30,
A1 = 3.6 × 10–3 m2, B1 = 0.25 T
R2 = 14 Ω, N2 = 42,
A2 = 1.8 × 10–3 m2, B2 = 0.50 T
(The spring constants are identical for the two meters).
Determine the ratio of (a) current sensitivity and (b) voltage
sensitivity of M2 and M1.

**Question 4.11** In a chamber, a uniform magnetic field of 6.5 G (1 G = 10–4 T) is
maintained. An electron is shot into the field

with a speed of
4.8 × 106 m s–1 normal to the field. Explain why the path of the
electron is a circle. Determine the radius of the circular orbit.
(e = 1.6 × 10–19 C, me = 9.1×10–31 kg)

**Question 4.12 **In Exercise 4.11 obtain the frequency of revolution of the electron in
its circular orbit. Does the answer depend on the speed of the
electron? Explain.

**Question 4.13** (a) A circular coil of 30 turns and radius 8.0 cm carrying a current
of 6.0 A is suspended vertically in a uniform horizontal magnetic
field of magnitude 1.0 T. The field lines make an angle of 60º with the normal of the coil. Calculate the magnitude of the
counter torque that must be applied to prevent the coil from
turning.

(b) Would your answer change, if the circular coil in

(a) were replaced
by a planar coil of some irregular shape that encloses the same
area? (All other particulars are also unaltered.)

**Question 4.14** Two concentric circular coils X and Y of radii 16 cm and 10 cm,
respectively, lie in the same vertical plane containing the north to
south direction. Coil X has 20 turns and carries a current of 16 A;
coil Y has 25 turns and carries a current of 18 A. The sense of the
current in X is anticlockwise, and clockwise in Y, for an observer
looking at the coils facing west. Give the magnitude and direction of
the net magnetic field due to the coils at their centre .

**Question 4.15** A magnetic field of 100 G (1 G = 10–4 T) is required which is uniform
in a region of linear dimension about 10 cm and area of cross-section
about 10–3 m2. The maximum current-carrying capacity of a given
coil of wire is 15 A and the number of turns per unit length that can
be wound round a core is at most 1000 turns m–1. Suggest some
appropriate design particulars of a solenoid for the required purpose.
Assume the core is not ferromagnetic.

**Question 4.16** For a circular coil of radius R and N turns carrying current I, the
magnitude of the magnetic field at a point on its axis at a distance x
from its centre is given by,
( )
2
0
2 2 3/2 2
IR N
B
x R
μ
=
+

(a) Show that this reduces to the familiar result for field at the
centre of the coil.

(b) Consider two parallel co-axial circular coils of equal radius R,
and number of turns N, carrying equal currents in the same
direction, and separated by a distance R. Show that the field on
the axis around the mid-point between the coils is uniform over
a distance that is small as compared to R, and is given by,
0.72 0
NI
B
R
μ
= , approximately.
[Such an arrangement to produce a nearly uniform magnetic
field over a small region is known as Helmholtz coils.]

**Question 4.17** A toroid has a core (non-ferromagnetic) of inner radius 25 cm and
outer radius 26 cm, around which 3500 turns of a wire are wound.
If the current in the wire is 11 A, what is the magnetic field
(a) outside the toroid, (b) inside the core of the toroid, and (c) in the
empty space surrounded by the toroid.

**Question 4.18** Answer the following questions:

(a) A magnetic field that varies in magnitude from point to point
but has a constant direction (east to west) is set up in a chamber.
A charged particle enters the chamber and travels undeflectedalong a straight path with constant speed. What can you say
about the initial velocity of the particle?

(b) A charged particle enters an environment of a strong and
non-uniform magnetic field varying from point to point both in
magnitude and direction, and comes out of it following a
complicated trajectory. Would its final speed equal the initial
speed if it suffered no collisions with the environment?

(c) An electron travelling west to east enters a chamber having a
uniform electrostatic field in north to south direction. Specify
the direction in which a uniform magnetic field should be set
up to prevent the electron from deflecting from its straight line
path.

**Question 4.19** An electron emitted by a heated cathode and accelerated through a
potential difference of 2.0 kV, enters a region with uniform magnetic
field of 0.15 T. Determine the trajectory of the electron if the field

(a) is transverse to its initial velocity,

(b) makes an angle of 30º with
the initial velocity.

**Question 4.20** A magnetic field set up using Helmholtz coils (described in Exercise
4.16) is uniform in a small region and has a magnitude of 0.75 T. In
the same region, a uniform electrostatic field is maintained in a
direction normal to the common axis of the coils. A narrow beam of
(single species) charged particles all accelerated through 15 kV
enters this region in a direction perpendicular to both the axis of
the coils and the electrostatic field. If the beam remains undeflected
when the electrostatic field is 9.0 × 10–5 V m–1, make a simple guess
as to what the beam contains. Why is the answer not unique?

**Question 4.21** A straight horizontal conducting rod of length 0.45 m and mass
60 g is suspended by two vertical wires at its ends. A current of 5.0 A
is set up in the rod through the wires.

(a) What magnetic field should be set up normal to the conductor
in order that the tension in the wires is zero?

(b) What will be the total tension in the wires if the direction of
current is reversed keeping the magnetic field same as before?
(Ignore the mass of the wires.) g = 9.8 m s–2.

**Question 4.22** The wires which connect the battery of an automobile to its starting
motor carry a current of 300 A (for a short time). What is the force
per unit length between the wires if they are 70 cm long and 1.5 cm
apart? Is the force attractive or repulsive?

**Question 4.23** A uniform magnetic field of 1.5 T exists in a cylindrical region of
radius10.0 cm, its direction parallel to the axis along east to west. A
wire carrying current of 7.0 A in the north to south direction passes
through this region. What is the magnitude and direction of the
force on the wire if,

(a) the wire intersects the axis,

(b) the wire is turned from N-S to northeast-northwest direction,

(c) the wire in the N-S direction is lowered from the axis by a distance
of 6.0 cm?

**Question 4.24** A uniform magnetic field of 3000 G is established along the positive
z-direction. A rectangular loop of sides 10 cm and 5 cm carries a
current of 12 A. What is the torque on the loop in the different cases
shown in Fig. 4.28? What is the force on each case? Which case
corresponds to stable equilibrium?

**Question 4.25** A circular coil of 20 turns and radius 10 cm is placed in a uniform
magnetic field of 0.10 T normal to the plane of the coil. If the current
in the coil is 5.0 A, what is the

(a) total torque on the coil,

(b) total force on the coil,

(c) average force on each electron in the coil due to the magnetic
field?
(The coil is made of copper wire of cross-sectional area 10–5 m2, and
the free electron density in copper is given to be about
1029 m–3.)

**Question 4.26** A solenoid 60 cm long and of radius 4.0 cm has 3 layers of windings
of 300 turns each. A 2.0 cm long wire of mass 2.5 g lies inside the
solenoid (near its centre) normal to its axis; both the wire and the
axis of the solenoid are in the horizontal plane. The wire is connected
through two leads parallel to the axis of the solenoid to an external
battery which supplies a current of 6.0 A in the wire. What value of
current (with appropriate sense of circulation) in the windings of
the solenoid can support the weight of the wire? g = 9.8 m s–2.

**Question 4.27** A galvanometer coil has a resistance of 12 Ω and the metre shows
full scale deflection for a current of 3 mA. How will you convert the
metre into a voltmeter of range 0 to 18 V?

**Question 4.28** A galvanometer coil has a resistance of 15 Ω and the metre shows
full scale deflection for a current of 4 mA. How will you convert the
metre into an ammeter of range 0 to 6 A?

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- Chapter 1 Electric Charges and Fields
- Chapter 2 Electrostatic Potential and Capacitance
- Chapter 3 Current Electricity
- Chapter 4 Moving Charges and Magnetism
- Chapter 5 Magnetism and Matter
- Chapter 6 Electromagnetic Induction
- Chapter 7 Alternating Current
- Chapter 8 Electromagnetic Waves
- Chapter 9 Ray Optics and Optical Instruments
- Chapter 10 Wave Optics
- Chapter 11 Dual Nature of Radiation and Matter
- Chapter 12 Atoms
- Chapter 13 Nuclei
- Chapter 14 Semiconductor Electronics: Materials, Devices and Simple Circuits
- Chapter 15 Communication Systems

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