Moving Charges and Magnetism is a conceptual and high-weightage chapter in Class 12 Physics that explains the relationship between electric current and magnetic fields. This chapter covers important topics such as magnetic force, Biot–Savart law, Ampere’s circuital law, force on a current-carrying conductor, motion of a charged particle in a magnetic field, cyclotron, and torque on a current loop, which are frequently asked in CBSE board exams and competitive exams like JEE and NEET.
NCERT Solutions for Class 12 Physics Chapter 4 – Moving Charges and Magnetism are prepared strictly according to the latest CBSE syllabus and exam pattern. The solutions are written in simple, step-by-step language with clear derivations, vector diagrams, and solved numericals, helping students build strong conceptual understanding and score well in board examinations.
NCERT Solutions For Class 12 Physics Chapter 4 Moving Charges and Magnetism
Q.
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?
Q.
(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.)
Q.
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?
Q.
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.
Q.
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.
Q.
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.
Q.
Q.
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
Q.
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.
Q.
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.
Q.
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?
Q.
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)
Q.
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.
Q.
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?
Q.
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.
Q.
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?
Q.
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?
Q.
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?
Q.
Q.
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?
NCERT Solutions For Class 12 Physics Chapter 4 Moving Charges and Magnetism
Q. 1) 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.
Ans:
(a) Since the initial velocity of the particle is either parallel or anti-parallel to the magnetic field, therefore, it travels along a straight path without suffering any deflection in the field.
(b) The final speed of the charged particle will be equal to its initial speed. This is because the magnetic force acting on the charged particle which is perpendicular to direction of its
velocity can change the direction of velocity, but not its magnitude.
(c) When an electron travelling from West to East will enter a chamber having a uniform electrostatic field in the North-South direction, it will be deflected by the electric field towards north. The moving electron can remain undeflected only if the electric force acting on it is equal and opposite of the magnetic field.
Since the magnetic force is directed towards the South. Therefore, according to Fleming’s left hand rule, the magnetic field should be applied in a vertically downward direction to nullify the deflection.
Note: Q&A containing MathML or Latex or Katex code cannot be rendered in pdf document.
FAQs: Class 12 Physics Chapter 4 – Moving Charges and Magnetism
Q1. Is Moving Charges and Magnetism important for exams?
Yes, it is a high-weightage chapter for CBSE, JEE, and NEET.
Q2. Which topics are most important in this chapter?
Biot–Savart law, Ampere’s law, cyclotron, and motion of charged particles.
Q3. Are numericals asked from this chapter?
Yes, force and motion-based numericals are very common.
Q4. Are derivations important here?
Yes, derivations of magnetic field expressions and cyclotron are frequently asked.
Q5. How do NCERT Solutions help?
They provide NCERT-based, exam-ready explanations with solved numericals and diagrams.