NCERT Solutions Class 12 Physics Chapter 4 Moving Charges And Magnetism

Question : 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?
Answer : Given:
Number of turns, n = 100
Radius of coil, r = 8 cm
Current through the coil, I = 0.40 A
Magnitude of magnetic field at centre of coil, B = ?

Question : 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?
Answer : Current in the wire, I = 35 A
Distance of a point from the wire, r = 20 cm = 0.2 m
Magnitude of the magnetic field at this point is given as:

Hence, the magnitude of the magnetic field at a point 20 cm from the wire is 3.5 × 10^–5 T.

Question : 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.
Answer : Given:
Current through the wire, I = 50 A    (North to South)
Distance of point P East of the wire, d = 2.5 m

Direction of magnetic field,
The point is in a plane normal to the wire and the wire carries current in north to south. Using Right hand thumb rule we can conclude that the direction of magnetic field is vertically upwards, or out of the paper.
The magnitude of the magnetic field is 4 × 10^-6T and its direction is upwards or out of paper.

Question : 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?
Answer : Given:
Current through the wire, I = 90 A   (East to West)
Distance of point P below the wire, d = 1.5 m

Using Biot Savart law we can find magnetic field at a distance d from a current carrying conductor. It is given by:

Where, :
B = Magnetic field strength
I = current through the coil
μ₀ is the permeability of free space.
μ₀ = 4n x 10^-7 Tm A^-1
d = distance of point P
By plugging the values in the equation (1), we get

⇒ │B│= 1.2 x 10-5T
Direction of magnetic field,
We know that wire carries current in east to west direction. Using Right hand thumb rule, we can conclude that the direction of magnetic field is from north to south as indicated in the figure.
The magnitude of the magnetic field is 1.2 × 10^-5T and its direction is from north to south.

Question : What is the magnitude of magnetic force per unit length on a wire carrying a current of 8 A and making an angle of 30º with the direction of a uniform magnetic field of 0.15 T?
Answer : Current in the wire, I = 8 A
Magnitude of the uniform magnetic field, B = 0.15 T
Angle between the wire and magnetic field, θ = 30°.
Magnetic force per unit length on the wire is given as:
f = BI sinθ
= 0.15 × 8 ×1 × sin30°
= 0.6 N m^–1
Hence, the magnetic force per unit length on the wire is 0.6 N m^–1.

Question : 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?
Answer : Length of the wire, l = 3 cm = 0.03 m
Current flowing in the wire, I = 10 A
Magnetic field, B = 0.27 T
Angle between the current and magnetic field, θ = 90°
Magnetic force exerted on the wire is given as:
F = BIlsinθ
= 0.27 × 10 × 0.03 sin90°
= 8.1 × 10^–2 N
Hence, the magnetic force on the wire is 8.1 × 10^–2 N. The direction of the force can be obtained from Fleming’s left hand rule.

Question : 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.
Answer : Given:
Current in wire A, I_A = 8.0 A
Current in wire B, I_B = 5.0 A
Distance between the conductors A and B, d = 4 cm
Length of conductor on which we have to calculate force, L = 10cm

Intuitively we can break the problem into two parts,
1) the magnetic field due to wire A, at a distance d.
2) Then we introduce a current carrying conductor B, at distance d and find the force on it due to field created by wire A.
1) Field at distance d due to current I_A in conductor A is given by,

Where, :
B = Magnetic field strength
I = current through the coil
μ₀ is the permeability of free space.
μ₀ = 4n x 10^-7 Tm A^-1
d = distance of point P
By plugging the values in the equation (1), we get

⇒ │B│= 3.18 x 10^-6T
2) Force on a current carrying conductor due to a magnetic field is given by
F = B x I_B x L ......(2)
Now by putting the values in equation (2) we get,
F = 3.18 x 10^-6 T x 5A x 0.1m
F = 2 x 10^-5  N
So, the force on the 10 cm section on wire A is 2 × 10^-5N. Since the current is flowing in the same direction the force will be attractive in nature.
Note: The force will be same on both the wires, we can use Newton’s third law of motion to such conclusion.

Question : 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.
Answer : Length of the solenoid, l = 80 cm = 0.8 m
There are five layers of windings of 400 turns each on the solenoid.
Total number of turns on the solenoid, N = 5 × 400 = 2000
Diameter of the solenoid, D = 1.8 cm = 0.018 m
Current carried by the solenoid, I = 8.0 A
Magnitude of the magnetic field inside the solenoid near its centre is given by the relation,

Hence, the magnitude of the magnetic field inside the solenoid near its centre is 2.512 × 10^–2 T.

Question : 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?
Answer : Length of a side of the square coil, l = 10 cm = 0.1 m
Current flowing in the coil, I = 12 A
Number of turns on the coil, n = 20
Angle made by the plane of the coil with magnetic field, θ = 30°
Strength of magnetic field, B = 0.80 T
Magnitude of the magnetic torque experienced by the coil in the magnetic field is given by the relation,
τ = n BIA sinθ
Where,
A = Area of the square coil
⇒ l × l = 0.1 × 0.1 = 0.01 m²
∴ τ = 20 × 0.8 × 12 × 0.01 × sin30°
= 0.96 N m
Hence, the magnitude of the torque experienced by the coil is 0.96 N m.

Question : Two moving coil meters, M₁ and M₂ have the following particulars:
R₁ = 10 B, N1 = 30,
A₁ = 3.6 × 10^–3 m², B1 = 0.25 T
R₂ = 14 B, 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.
Answer : For moving coil meter M₁:
Resistance, R₁ = 10 B
Number of turns, N₁ = 30
Area of cross-section, A₁ = 3.6 × 10^–3 m²
Magnetic field strength, B₁ = 0.25 T
Spring constant K₁ = K
For moving coil meter M_2:
Resistance, R₂ = 14 B
Number of turns, N₂ = 42
Area of cross-section, A₂ = 1.8 × 10^–3 m²
Magnetic field strength, B₂ = 0.50 T
Spring constant, K₂ = K
(a) Current sensitivity of M₁ is given as:

Question : 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)
Answer : Magnetic field strength, B = 6.5 G = 6.5 × 10^–4 T
Speed of the electron, v = 4.8 × 10⁶ m/s
Charge on the electron, e = 1.6 × 10^–19 C
Mass of the electron, me = 9.1 × 10^–31 kg
Angle between the shot electron and magnetic field, θ = 90°
Magnetic force exerted on the electron in the magnetic field is given as:
F = evB sinθ
This force provides centripetal force to the moving electron. Hence, the electron starts moving in a circular path of radius r.
Hence, centripetal force exerted on the electron,

Question : 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.
Answer : Magnetic field strength, B = 6.5 × 10⁻⁴ T
Charge of the electron, e = 1.6 × 10⁻¹⁹ C
Mass of the electron, me = 9.1 × 10⁻³¹ kg
Velocity of the electron, v = 4.8 × 10⁶ m/s
Radius of the orbit, r = 4.2 cm = 0.042 m
Frequency of revolution of the electron = ν
Angular frequency of the electron = ω = 2πν
Velocity of the electron is related to the angular frequency as: 
v = rω
In the circular orbit, the magnetic force on the electron is balanced by the centripetal force. Hence, we can write:

Hence, the frequency of the electron is around 18 MHz and is independent of the speed of the electron.

Question : (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.)
Answer : (a) Number of turns on the circular coil, n = 30
Radius of the coil, r = 8.0 cm = 0.08 m
Area of the coil = πr2 = π (0.08)2 = 0.0201 m²
Current flowing in the coil, I = 6.0 A
Magnetic field strength, B = 1 T
Angle between the field lines and normal with the coil surface,
θ = 60°
The coil experiences a torque in the magnetic field. Hence, it turns. The counter torque applied to prevent the coil from turning is given by the relation,
τ = n IBA sinθ … (i)
= 30 × 6 × 1 × 0.0201 × sin60°
= 3.133 N m
(b) It can be inferred from relation (i) that the magnitude of the applied torque is not dependent on the shape of the coil. It depends on the area of the coil. Hence, the answer would not change if the circular coil in the above case is replaced by a planar coil of some irregular shape that encloses the same area.

Question : 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.
Answer : Radius of coil X, r₁ = 16 cm = 0.16 m
Radius of coil Y, r₂ = 10 cm = 0.1 m
Number of turns of on coil X, n₁ = 20
Number of turns of on coil Y, n₂ = 25
Current in coil X, I₁ = 16 A
Current in coil Y, I₂ = 18 A
Magnetic field due to coil X at their centre is given by the relation,

Question : 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 currentcarrying 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
Answer : Magnetic field strength, B = 100 G = 100 × 10⁻⁴ T
Number of turns per unit length, n = 1000 turns m⁻¹
Current flowing in the coil, I = 15 A

If the length of the coil is taken as 50 cm, radius 4 cm, number of turns 400, and current 10 A, then these values are not unique for the given purpose. There is always a possibility of some adjustments with limits.'

Question : 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,\\

(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, B = 0.72 - μ_oBNI / R​​​​​​​ , approximately.
[Such an arrangement to produce a nearly uniform magnetic field over a small region is known as Helmholtz coils.]
Answer : Radius of circular coil = R
Number of turns on the coil = N
Current in the coil = I
Magnetic field at a point on its axis at distance x is given by the relation

This is the familiar result for magnetic field at the centre of the coil.
(b) Radii of two parallel co-axial circular coils = R
Number of turns on each coil = N
Current in both coils = I
Distance between both the coils = R
Let us consider point Q at distance d from the centre.

Then, one coil is at a distance of R / 2 + d from point Q.
∴ Magnetic field at point Q is given as:

Hence, it is proved that the field on the axis around the mid-point between the coils is uniform.

Question : 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.
Answer : Inner radius of the toroid, r₁ = 25 cm = 0.25 m
Outer radius of the toroid, r₂ = 26 cm = 0.26 m
Number of turns on the coil, N = 3500
Current in the coil, I = 11 A
(a) Magnetic field outside a toroid is zero. It is non-zero only inside the core of a toroid.
(b) Magnetic field inside the core of a toroid is given by the relation,

(c) Magnetic field in the empty space surrounded by the toroid is zero.

Question : 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 undeflected along 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.
Answer : (a) The initial velocity of the particle is either parallel or anti-parallel to the magnetic field. Hence, it travels along a straight path without suffering any deflection in the field.
(b) Yes, the final speed of the charged particle will be equal to its initial speed. This is because magnetic force can change the direction of velocity, but not its magnitude.
(c) An electron travelling from West to East enters a chamber having a uniform electrostatic field in the North-South direction. This moving electron can remain undeflected if the electric force acting on it is equal and opposite of magnetic field.
Magnetic force is directed towards the South. According to Fleming’s left hand rule, magnetic field should be applied in a vertically downward direction.

Question : 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.
Answer : Magnetic field strength, B = 0.15 T
Charge on the electron, e = 1.6 × 10⁻¹⁹ C
Mass of the electron, m = 9.1 × 10⁻³¹ kg
Potential difference, V = 2.0 kV = 2 × 10³ V
Thus, kinetic energy of the electron = eV

Where,
v = velocity of the electron
(a) Magnetic force on the electron provides the required centripetal force of the electron.
Hence, the electron traces a circular path of radius r.
Magnetic force on the electron is given by the relation, B ev
Centripetal force = mv²/r

Hence, the electron has a circular trajectory of radius 1.0 mm normal to the magnetic field.
(b) When the field makes an angle θ of 30° with initial velocity, the initial velocity will be, V₁ = vsin θ
From equation (2), we can write the expression for new radius as:

= 0.5 x 10^-3 m
= 0.5mm
Hence, the electron has a helical trajectory of radius 0.5 mm along the magnetic field direction.

Question : 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?
Answer : Magnetic field, B = 0.75 T
Accelerating voltage, V = 15 kV = 15 × 10³ V
Electrostatic field, E = 9 × 10⁵ V m⁻¹
Mass of the electron = m
Charge of the electron = e
Velocity of the electron = v
Kinetic energy of the electron = eV

This value of specific charge e/m is equal to the value of deuteron or deuterium ions.
This is not a unique answer. Other possible answers are He⁺⁺, Li⁺⁺, etc.

Question : 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.
Answer : Length of the rod, l = 0.45 m
Mass suspended by the wires, m = 60 g = 60 × 10⁻³ kg
Acceleration due to gravity, g = 9.8 m/s²
Current in the rod flowing through the wire, I = 5 A
(a) Magnetic field (B) is equal and opposite to the weight of the wire i.e.,

A horizontal magnetic field of 0.26 T normal to the length of the conductor should be set up in order to get zero tension in the wire. The magnetic field should be such that
Fleming’s left hand rule gives an upward magnetic force.
(b) If the direction of the current is revered, then the force due to magnetic field and the weight of the wire acts in a vertically downward direction.
∴Total tension in the wire = BIl + mg
= 0.26 x 5 x 0.45 + (60 x 10^-3) x 9.8
= 1.176 N

Question : 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?
Answer : Current in both wires, I = 300 A
Distance between the wires, r = 1.5 cm = 0.015 m
Length of the two wires, l = 70 cm = 0.7 m
Force between the two wires is given by the relation

Since the direction of the current in the wires is opposite, a repulsive force exists between them.

Question : 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?
Answer : Magnetic field strength, B = 1.5 T
Radius of the cylindrical region, r = 10 cm = 0.1 m
Current in the wire passing through the cylindrical region, I = 7 A
(a) If the wire intersects the axis, then the length of the wire is the diameter of the cylindrical region.
Thus, l = 2r = 0.2 m
Angle between magnetic field and current, θ = 90°
Magnetic force acting on the wire is given by the relation,
F = BIl sin θ
= 1.5 × 7 × 0.2 × sin 90°
= 2.1 N
Hence, a force of 2.1 N acts on the wire in a vertically downward direction.
(b) New length of the wire after turning it to the Northeast-Northwest direction can be given as: :

Magnetic force exerted on the wire,
F₂ = BH₂
= 1.5 x 7 x 0.16
= 1.68 N
Hence, a force of 1.68 N acts in a vertically downward direction on the wire.

Question : 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?

Answer : Magnetic field strength, B = 3000 G = 3000 × 10⁻⁴ T = 0.3 T
Length of the rectangular loop, l = 10 cm
Width of the rectangular loop, b = 5 cm
Area of the loop,
A = l × b = 10 × 5 = 50 cm² = 50 × 10⁻⁴ m²
Current in the loop, I = 12 A
Now, taking the anti-clockwise direction of the current as positive and vise-versa:

In case (e), the direction of I¯A and B is the same and the angle between them is zero. If displaced, they come back to an equilibrium. Hence, its equilibrium is stable.
Whereas, in case (f), the direction of I¯A and B is opposite. The angle between them is 180°. If disturbed, it does not come back to its original position. Hence, its equilibrium is unstable.

Question : 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.)
Answer : Number of turns on the circular coil, n = 20
Radius of the coil, r = 10 cm = 0.1 m
Magnetic field strength, B = 0.10 T
Current in the coil, I = 5.0 A
(a) The total torque on the coil is zero because the field is uniform.
(b) The total force on the coil is zero because the field is uniform.
(c) Cross-sectional area of copper coil, A = 10⁻⁵ m²
Number of free electrons per cubic meter in copper, N = 10²⁹ /m³
Charge on the electron, e = 1.6 × 10⁻¹⁹ C
Magnetic force, F = Bevd
Where,
vd = Drift velocity of electrons

Question : 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
Answer : Length of the solenoid, L = 60 cm = 0.6 m
Radius of the solenoid, r = 4.0 cm = 0.04 m
It is given that there are 3 layers of windings of 300 turns each.
Total number of turns, n = 3 × 300 = 900
Length of the wire, l = 2 cm = 0.02 m
Mass of the wire, m = 2.5 g = 2.5 × 10⁻³ kg
Current flowing through the wire, i = 6 A
Acceleration due to gravity, g = 9.8 m/s²

I = Current flowing through the windings of the solenoid
Magnetic force is given by the relation,

Question : A galvanometer coil has a resistance of 12 B 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?
Answer : Resistance of the galvanometer coil, G = 12 B
Current for which there is full scale deflection, I_g = 3 mA = 3 × 10⁻³ A
Range of the voltmeter is 0, which needs to be converted to 18 V.
V = 18 V
Let a resistor of resistance R be connected in series with the galvanometer to convert it into a voltmeter. This resistance is given as:

Question : A galvanometer coil has a resistance of 15 B 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?
Answer : Resistance of the galvanometer coil, G = 15 B
Current for which the galvanometer shows full scale deflection,
I_g = 4 mA = 4 × 10⁻³ A
Range of the ammeter is 0, which needs to be converted to 6 A.
∴ Current, I = 6 A
A shunt resistor of resistance S is to be connected in parallel with the galvanometer to convert it into an ammeter. The value of S is given as: