Moving Charges and Magnetism is Chapter 4 of NCERT Class 12 Physics Part I and part of Unit 13, Magnetic Effects of Current and Magnetism, in both the NEET UG 2026 and JEE Main 2026 syllabi. These notes cover the Lorentz force, circular and helical motion, the Biot-Savart and Ampere’s laws, parallel currents, torque on a loop and the galvanometer.
Moving Charges and Magnetism Notes for NEET and JEE Main
Section-wise key points from NCERT Chapter 4, formula tables, most-asked facts and common mistakes for 2027 revision.
Moving Charges and Magnetism at a Glance
| Item | Details |
|---|---|
| Syllabus unit | NEET UG 2026: Unit 13, Magnetic Effects of Current and Magnetism. JEE Main 2026 (Paper 1 Physics): Unit 13, same title |
| Exams that cover it | Both NEET UG and JEE Main |
| NCERT textbook | Physics Part I, Class 12, Chapter 4 (rationalised book, reprint 2026-27) |
| Other chapter in the unit | Magnetism and Matter (Chapter 5) |
| Syllabus topics | Biot-Savart law and the circular loop; Ampere’s law for a long straight wire and a solenoid; force on a moving charge in uniform magnetic and electric fields; force on a current-carrying conductor; force between parallel currents and the definition of the ampere; torque on a current loop; moving coil galvanometer, its sensitivity and conversion to ammeter and voltmeter; current loop as a magnetic dipole |
| Official chapter | NCERT Chapter 4 |
| Notes PDF | View PDF (unit notes: Magnetic Effects of Current and Magnetism, EMI and Alternating Current) |
Topics follow the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025 and the JEE Main 2026 syllabus released by the National Testing Agency (NTA); both stay the reference until the 2027 syllabi are out. Full lists: NEET syllabus and JEE Main syllabus.
Magnetic Force and the Lorentz Force
A charge q moving with velocity v in electric and magnetic fields feels the Lorentz force F = q[E + v × B].
- In 1820 Hans Christian Oersted noticed that a current in a straight wire deflected a nearby compass needle, which showed that electricity and magnetism are linked.
- The magnetic force qv × B is zero for a charge at rest or moving parallel (or antiparallel) to B, and largest when v is perpendicular to B.
- The magnetic force is always perpendicular to v, so it does no work and cannot change the speed or kinetic energy.
- SI unit of B: tesla (T). The gauss (1 G = 10⁻⁴ T) is a smaller non-SI unit.
- Force on a straight conductor of length l carrying current I: F = I l × B. For a wire of any shape, add the forces on its small elements.
- Convention: a dot means a field or current coming out of the page; a cross means going into it.
Motion of a Charge in a Uniform Magnetic Field
A charge moving perpendicular to a uniform field moves in a circle; if it also has a velocity component along the field, it moves in a helix.
| Quantity | Formula | Note |
|---|---|---|
| Radius of the circle | r = mv/(qB) | v is the component perpendicular to B |
| Angular frequency | ω = qB/m | Independent of speed and radius |
| Frequency (cyclotron frequency) | ν = qB/(2πm) | Independent of energy; this is why a cyclotron works |
| Time period | T = 2πm/(qB) | Same for slow and fast particles |
| Pitch of the helix | p = v<sub>∥</sub>T = 2πmv<sub>∥</sub>/(qB) | Distance moved along B in one rotation |
Biot-Savart Law
The Biot-Savart law gives the magnetic field of a small current element: dB = (μ₀/4π) I dl × r/r³, with magnitude (μ₀/4π) I dl sin θ/r².
- μ₀ is the permeability of free space; μ₀/4π = 10⁻⁷ T m A⁻¹ (μ₀ = 4π × 10⁻⁷ T m A⁻¹).
- dB is perpendicular to the plane containing dl and r, and is zero along the line of the current element (θ = 0).
| Feature | Coulomb’s law (electric field) | Biot-Savart law (magnetic field) |
|---|---|---|
| Source | Scalar source: charge q | Vector source: current element I dl |
| Distance dependence | 1/r² (long range) | 1/r² (long range) |
| Direction of field | Along the displacement vector | Perpendicular to the plane of dl and r |
| Angle dependence | None | Depends on sin θ between dl and r |
| Superposition | Applies | Applies |
Magnetic Field Formulas: Loop, Wire and Solenoid
These standard results come from the Biot-Savart and Ampere’s laws.
| Current configuration | Magnetic field B | Direction |
|---|---|---|
| Circular loop, on its axis at distance x | μ₀IR²/[2(x² + R²)^(3/2)] | Along the axis, by the right-hand thumb rule |
| Circular loop, at the centre | μ₀I/(2R) (multiply by N for N turns) | Perpendicular to the plane of the loop |
| Long straight wire, distance R | μ₀I/(2πR) | Circles centred on the wire (right-hand rule) |
| Inside a long solenoid | μ₀nI (n = turns per unit length) | Along the axis; uniform inside, nearly zero outside |
- Right-hand thumb rule for a loop: curl the fingers along the current and the thumb gives the direction of the field.
Ampere’s Circuital Law
Ampere’s circuital law states that the line integral of B around a closed loop equals μ₀ times the net current through the surface bounded by the loop: ∮B·dl = μ₀I.
- Simplified form: when B is tangential and constant along a loop of length L, BL = μ₀I<sub>e</sub>.
- Applications in the syllabus: the field of an infinitely long straight wire and of a long solenoid.
- Ampere’s law is not independent of the Biot-Savart law; it is related to it the way Gauss’s law is related to Coulomb’s law.
- The results in this chapter hold for steady currents only.
Force Between Two Parallel Currents and the Ampere
Two long parallel conductors carrying currents I<sub>a</sub> and I<sub>b</sub> at distance d exert a force per unit length f = μ₀I<sub>a</sub>I<sub>b</sub>/(2πd) on each other.
- Parallel currents attract and antiparallel currents repel, the opposite of the rule for like and unlike charges.
- Definition of the ampere (adopted in 1946, as given in NCERT): the steady current that, in two very long straight parallel conductors of negligible cross-section placed 1 m apart in vacuum, produces a force of 2 × 10⁻⁷ N per metre of length on each.
- One coulomb is the charge that flows through a cross-section in 1 s when a steady current of 1 A flows.
Torque on a Current Loop: the Magnetic Dipole
In a uniform magnetic field, the net force on a current loop is zero, but it experiences a torque τ = m × B.
- Magnetic moment of a planar loop: m = NIA. Unit: A m² (also J T⁻¹).
- Direction of m: curl the right-hand fingers along the current; the thumb gives m.
- Torque magnitude: τ = mB sin θ, where θ is the angle between m and B.
- A current loop behaves as a magnetic dipole. Its field on the axis at a large distance x is B = (μ₀/4π)(2m/x³), like the field of an electric dipole.
Moving Coil Galvanometer, Ammeter and Voltmeter
A moving coil galvanometer (MCG) is a coil that turns in a radial magnetic field; the magnetic torque NIAB is balanced by a spring’s restoring torque kφ.
- A cylindrical soft iron core makes the field radial and also increases its strength, so sin θ = 1 at every position of the coil.
- At equilibrium kφ = NIAB, so the deflection φ = (NAB/k)I, where k is the torsion constant of the spring.
- Current sensitivity φ/I = NAB/k. Voltage sensitivity φ/V = NAB/(kR), where R is the galvanometer’s resistance.
- Doubling the number of turns doubles the current sensitivity, but since the resistance also doubles, the voltage sensitivity is unchanged.
- A galvanometer gives full-scale deflection for currents of the order of microamperes, so it cannot be used directly as an ammeter.
| Feature | Ammeter | Voltmeter |
|---|---|---|
| Conversion | Small shunt resistance r<sub>s</sub> in parallel with the galvanometer | Large resistance R in series with the galvanometer |
| Connected in the circuit | In series | In parallel with the element |
| Resistance of the meter | Very low (≈ r<sub>s</sub>) | Very high (≈ R) |
| Value needed for full-scale current I<sub>g</sub> in a galvanometer of resistance G | r<sub>s</sub> = I<sub>g</sub>G/(I − I<sub>g</sub>) to read up to I | R = V/I<sub>g</sub> − G to read up to V |
| Ideal value | Zero resistance | Infinite resistance |
Most-asked Facts for NEET and JEE Main: Moving Charges and Magnetism
Revise these one-line facts from the NCERT chapter just before the exam.
- Oersted (1820) first linked electricity and magnetism.
- Lorentz force F = q(E + v × B); the magnetic part does no work.
- r = mv/(qB); the time period 2πm/(qB) does not depend on speed.
- 1 T = 10⁴ G; μ₀ = 4π × 10⁻⁷ T m A⁻¹.
- Field at the centre of a loop is μ₀I/(2R); of a long straight wire μ₀I/(2πR); inside a solenoid μ₀nI.
- Biot-Savart: field is zero along the axis of the current element itself.
- Ampere’s law ∮B·dl = μ₀I is to Biot-Savart what Gauss’s law is to Coulomb’s law.
- Parallel currents attract; force per unit length μ₀I₁I₂/(2πd).
- Ampere defined by a force of 2 × 10⁻⁷ N m⁻¹ between wires 1 m apart.
- Net force on a current loop in a uniform field is zero; torque τ = m × B with m = NIA.
- Galvanometer: kφ = NIAB; radial field from a soft iron core.
- Ammeter: low shunt in parallel; voltmeter: high resistance in series.
Common Mistakes to Avoid
Watch for these mix-ups when you answer questions on this chapter.
- Saying a magnetic field can speed up a charge. The magnetic force is perpendicular to the velocity and does no work.
- Thinking a faster particle takes longer to complete a circle. The radius grows with speed, but the period 2πm/(qB) stays the same.
- Using the wire formula μ₀I/(2πR) at the centre of a loop. The loop’s centre field is μ₀I/(2R).
- Applying the rule for charges to currents. Parallel currents attract, unlike like charges, which repel.
- Expecting a net force on a closed loop in a uniform field. Only a torque acts.
- Putting the shunt in series. An ammeter uses a small shunt in parallel; a voltmeter uses a large resistance in series.
- Assuming more turns always improve a voltmeter. Voltage sensitivity stays the same because resistance rises with the turns.
Moving Charges and Magnetism Notes PDF and Related Pages
Download the AglaSem Class 12 Physics unit notes (Magnetic Effects of Current and Magnetism, and EMI and Alternating Current; key notes and important questions with answers) free: View PDF
- All chapters: Physics notes for NEET and JEE Main
- Full topic list: NEET syllabus and JEE Main syllabus
- Practice: NEET question papers and JEE Main question papers
- Source text: NCERT Chapter 4 (NCERT, Class 12 Physics Part I)
Physics Notes for NEET and JEE Main: All Chapters
- Units and Measurement
- Motion in a Straight Line
- Motion in a Plane
- Laws of Motion
- Work, Energy and Power
- Systems of Particles and Rotational Motion
- Gravitation
- Mechanical Properties of Solids
- Mechanical Properties of Fluids
- Thermodynamics
- Kinetic Theory of Gases
- Oscillations
- Waves
- Thermal Properties of Matter
- Electric Charges and Fields
- Electrostatic Potential and Capacitance
- Current Electricity
- Moving Charges and Magnetism
- Magnetism and Matter
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- Wave Optics
- Dual Nature of Radiation and Matter
- Atoms
- Nuclei
- Semiconductor Electronics
All chapters with key points and notes PDFs are on the Physics notes for NEET and JEE Main page.
Moving Charges and Magnetism Notes for NEET and JEE Main FAQs
Which NCERT chapter is Moving Charges and Magnetism?
It is Chapter 4 of the NCERT Class 12 Physics Part I textbook (rationalised book). Older notes call it Moving Charges and Magnetic Field. With Magnetism and Matter (Chapter 5), it makes up the Magnetic Effects of Current and Magnetism unit.
Is Moving Charges and Magnetism in the NEET and JEE Main 2027 syllabus?
Yes. It is in Unit 13, Magnetic Effects of Current and Magnetism, of both the NEET UG 2026 syllabus (NMC, 22 December 2025) and the JEE Main 2026 syllabus (NTA), which remain the reference until the 2027 syllabi are released.
Why does a magnetic field not change the speed of a charged particle?
The magnetic force qv × B is always perpendicular to the velocity, so it does no work. It changes only the direction of motion, which is why a charge moving perpendicular to a uniform field goes round in a circle at constant speed.
How is a galvanometer converted into an ammeter and a voltmeter?
For an ammeter, a small shunt resistance is connected in parallel so that most of the current bypasses the coil and the meter has very low resistance. For a voltmeter, a large resistance is connected in series so that the meter draws very little current.
Are these notes enough for NEET and JEE Main questions on this chapter?
No. Read NCERT Chapter 4 and solve its examples and exercises first, then use these notes and the PDF for quick revision. Practise previous year questions on fields of wires and loops and on charged-particle motion.