Moving Charges and Magnetism Class 12 Physics Notes 2026-27 | Chapter 4
Moving Charges and Magnetism is Class 12 Physics Chapter 4. It explains the magnetic field produced by moving charges and electric currents, the magnetic force on moving charges and current-carrying conductors, the torque on a current loop, magnetic dipole moment, and the working of the moving-coil galvanometer.
For board revision: Focus especially on direction rules, force relations, magnetic fields due to current distributions, torque and magnetic dipole moment, and galvanometer sensitivity/conversion.
CBSE Physics Curriculum 2026–27 · CBSE Class XII 2026–27 SQP & Marking Scheme
Moving Charges and Magnetism Class 12 Physics is a concept-and-numerical-heavy chapter. The most useful revision path is to connect the magnetic field, Lorentz force, Biot–Savart law, Ampere’s circuital law, force on current-carrying conductors, torque on a current loop, magnetic dipole moment and moving-coil galvanometer instead of memorising formulas in isolation.
Chapter 4 Study Resources
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1. What This Chapter Connects
Current Electricity taught how charge moves through conductors. This chapter asks the next question: what magnetic effects arise from moving charge and electric current? It also explains how a magnetic field acts on moving charges and current-carrying conductors.
A charge moving through a magnetic field can experience a force perpendicular to its motion.
Electric current produces a magnetic field around the conductor.
A current-carrying conductor in an external magnetic field can experience a force.
A current loop in a uniform magnetic field can experience a torque and behaves as a magnetic dipole.
2. Magnetic Field and Oersted's Experiment
A magnetic field is a vector field that describes the magnetic influence at a point; moving charges and current-carrying conductors can experience magnetic forces in it, and magnetic dipoles can experience torque.
Oersted's experiment showed that a current-carrying conductor can deflect a nearby compass needle. The observation established the connection between electric current and magnetism.
At this level, the important source is moving electric charge or electric current. Oersted's experiment provides the direct experimental connection between electric current and magnetic effects.
3. Magnetic Force on a Moving Charge
Magnitude of magnetic force on a charge q moving with speed v in magnetic field B, where θ is the angle between v and B.
- If v is parallel or antiparallel to B, θ = 0° or 180°, so FB = 0.
- If v is perpendicular to B, θ = 90°, so FB = |q|vB.
- The magnetic force is perpendicular to both v and B.
- For a negative charge, the force direction is opposite to the v × B direction obtained for a positive charge.
Magnetic Force Does No Work
Because the magnetic force is perpendicular to instantaneous velocity, it does no work on an ideal moving charge:
Therefore a magnetic field can change the direction of velocity without changing the kinetic energy of a charged particle when magnetic force is the only force doing work.
4. Motion of a Charged Particle in a Uniform Magnetic Field
When a charged particle enters a uniform magnetic field with velocity perpendicular to B, the magnetic force acts as the centripetal force.
The particle follows a circular path. The angular speed and time period are:
T = 2πm/(|q|B)
If the velocity has both perpendicular and parallel components relative to B, the perpendicular component produces circular motion while the parallel component remains unchanged, giving a helical path.
5. Motion in Combined Electric and Magnetic Fields
The electric force is FE = qE, while the magnetic part is FB = q(v × B). The Lorentz force is their vector sum.
6. Force on a Current-Carrying Conductor
A current-carrying conductor placed in an external magnetic field experiences a magnetic force because its moving charge carriers experience magnetic forces.
Here I is current, L is the length of the conductor in the field, B is magnetic field magnitude and θ is the angle between the current direction and B.
- θ = 0° → F = 0.
- θ = 90° → F = BIL.
- Direction is given by the vector relation F = I(L × B) for conventional current.
7. Force Between Two Parallel Current-Carrying Conductors
Each current-carrying conductor creates a magnetic field that acts on the other conductor.
Here d is the separation between long parallel conductors.
| Current directions | Interaction |
|---|---|
| Same direction | Attraction |
| Opposite directions | Repulsion |
8. Definition of Ampere — Exam Point
The 2026–27 CBSE curriculum explicitly includes the definition of ampere with the force between two parallel current-carrying conductors. For school-level board preparation, learn the wording used in your current NCERT/CBSE-aligned material and understand the physical idea: parallel currents exert forces on one another, with the force depending on the currents and their separation.
9. Biot–Savart Law
The Biot–Savart law gives the magnetic field contribution produced by a small current element.
Vector form:
The direction of dB is perpendicular to the plane containing dℓ and r, determined by the cross product/right-hand rule.
Application: Magnetic Field at the Centre of a Circular Current Loop
For N closely wound turns carrying the same current:
10. Ampere's Circuital Law
Ampere's circuital law relates the circulation of magnetic field around a closed path to the current enclosed by that path.
Application: Infinitely Long Straight Current-Carrying Wire
Choose a circular Amperian path of radius r centred on the wire. By symmetry, B is constant on the path and tangential to it:
Thus the magnetic field decreases as 1/r, and its direction follows the right-hand thumb rule.
11. Straight Solenoid — Qualitative Treatment
A solenoid is a long cylindrical coil containing many closely spaced turns of wire. Its magnetic field is strong and approximately uniform inside an ideal long solenoid and much weaker outside.
Enrichment relation: This standard relation is useful for understanding the ideal long-solenoid field, but the current CBSE syllabus specifies the straight-solenoid treatment as qualitative.
Here n is the number of turns per unit length. For a long ideal solenoid, the field inside is approximately uniform and directed along the solenoid axis; the outside field is much weaker. Keep this section at the qualitative level required by the current syllabus.
12. Torque on a Current Loop
A current loop placed in a uniform magnetic field can experience a torque that tends to rotate the loop.
For a loop of N turns, area A, current I and magnetic field B, θ is the angle between the loop's area vector and B.
Maximum torque occurs when the area vector is perpendicular to B.
13. Current Loop as a Magnetic Dipole
A current-carrying loop behaves like a magnetic dipole. Its magnetic dipole moment is:
Vector direction is perpendicular to the plane of the loop, given by the right-hand grip rule. The SI unit of magnetic dipole moment is A m².
The torque tends to align the magnetic dipole moment with the magnetic field.
14. Moving-Coil Galvanometer
A moving-coil galvanometer detects and measures small electric currents by using the torque on a current-carrying coil placed in a magnetic field.
Principle
When current passes through the coil, the magnetic field exerts a torque. A restoring torque develops in the suspension/spring. At equilibrium, magnetic torque equals restoring torque.
Therefore deflection is directly proportional to current under the instrument's operating conditions.
Current Sensitivity
Higher N, A or B increases current sensitivity, while a larger torsional constant k decreases it.
Conversion of Galvanometer into Ammeter
A low resistance called a shunt is connected in parallel with the galvanometer so that most of the current passes through the shunt.
If galvanometer resistance is G, full-scale galvanometer current is Ig, and desired ammeter range is I, then:
The equivalent resistance of the ammeter should be very small so that it causes minimal disturbance to the circuit.
Conversion of Galvanometer into Voltmeter
A high resistance R is connected in series with the galvanometer so that only a small current flows.
The equivalent resistance of the voltmeter should be very large so that it draws minimal current from the circuit.
15. Direction Rules You Must Master
| Situation | Direction method |
|---|---|
| Field around straight current-carrying wire | Right-hand thumb/grip rule |
| Force on positive moving charge | v × B |
| Force on negative moving charge | Opposite to v × B |
| Force on current-carrying wire | L × B |
| Magnetic moment of current loop | Right-hand grip rule |
16. Which Formula Should You Use?
| If the question gives these data | Start with this relation |
|---|---|
| Charge q, speed v, field B and angle θ | F = |q|vB sinθ |
| Charge moving perpendicular to a uniform B | r = mv/(|q|B) for circular motion |
| Long straight current-carrying wire | B = μ₀I/(2πr) |
| Circular current loop at its centre | B = μ₀NI/(2R) |
| Long ideal solenoid | B ≈ μ₀nI — qualitative-scope relation |
| Current-carrying conductor in external B | F = BIL sinθ |
| Current loop in uniform B | τ = NIAB sinθ |
| Galvanometer to ammeter | S = IgG/(I − Ig) |
| Galvanometer to voltmeter | R = V/Ig − G |
17. High-Value Formula Map
| Concept | Formula |
|---|---|
| Magnetic force on charge | F = |q|vB sin θ |
| Lorentz force | F = q(E + v × B) |
| Circular radius | r = mv/(|q|B) |
| Angular speed | ω = |q|B/m |
| Time period | T = 2πm/(|q|B) |
| Force on conductor | F = BIL sin θ |
| Parallel-wire force per length | F/L = μ₀I₁I₂/(2πd) |
| Biot–Savart law | dB = (μ₀/4π)(I dl sin θ/r²) |
| Long straight wire | B = μ₀I/(2πr) |
| Circular loop centre | B = μ₀NI/(2R) |
| Ampere's law | ∮B·dl = μ₀Ienclosed |
| Long solenoid | B ≈ μ₀nI |
| Loop torque | τ = NIAB sin θ |
| Magnetic dipole moment | m = NIA |
| Galvanometer sensitivity | θ/I = NAB/k |
| Galvanometer → ammeter shunt | S = IgG/(I − Ig) |
| Galvanometer → voltmeter series resistance | R = V/Ig − G |
Formula-Use Checklist
| Before substituting | Check |
|---|---|
| Geometry | Straight wire, circular loop, solenoid, charge path or current loop? |
| Angle | Is θ measured between v and B, L and B, or m and B? |
| Charge | Use |q| for radius/magnitude expressions and account for sign for direction. |
| Units | Convert cm to m, mA to A and use SI units consistently. |
| Final step | State magnitude, unit and direction where the question asks for them. |
18. Common Exam Traps
19. Frequently Asked Questions
The chapter links current/moving charge → magnetic field → magnetic force → torque. Understanding this chain makes the formulas easier to select and apply.
What are the main topics in Moving Charges and Magnetism Class 12?
The core topics are magnetic field and Oersted's experiment, Biot–Savart law and circular loop, Ampere's law and long straight wire, qualitative solenoid treatment, Lorentz force, force on current-carrying conductors, parallel-current force, torque on a current loop, magnetic dipole moment, and moving-coil galvanometer with current sensitivity and instrument conversion.
Which formulas should I revise first?
Start with F = q(v × B), r = mv/(|q|B), T = 2πm/(|q|B), B = μ₀I/(2πr), B = μ₀NI/(2R), F = BIL sinθ, τ = NIAB sinθ, and the galvanometer conversion formulas. Then practise selecting the correct formula from the physical situation.
Is there a fixed Chapter 4 weightage in CBSE 2026–27?
No separate fixed Chapter 4-only allocation is published in the official curriculum. Chapter 4 and Chapter 5 are grouped within Unit III, which carries 17 marks at unit level.
Should I study old Chapter 4 topics found on other websites?
Do not use a third-party topic list as the final syllabus authority. Check the current CBSE curriculum first, then use this page and the linked practice resources for the retained Chapter 4 scope.
20. How to Study This Chapter
- First: master the force direction rules.
- Second: learn magnetic field due to current using Biot–Savart and Ampere's law.
- Third: practise charged-particle motion and current-conductor force numericals.
- Fourth: learn torque, magnetic dipole moment and galvanometer.
- Fifth: revise formulas and solve mixed application questions.
- Sixth: practise writing derivations in a clear sequence: principle → equation → substitution/derivation → final result.
- Seventh: use the marking scheme/SQP to understand the current CBSE question style, then use PYQs and the chapter test to check readiness.
If you have finished the free notes and want one guided system for practice, numericals, PYQs, revision and testing, explore the premium Chapter 4 Study Pack.
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