JEE · Physics
Understand the official JEE scope of Magnetism, separate field sources from field effects, choose the correct method and diagnose why questions go wrong.
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In short
Electric current and moving charge create magnetic fields. A magnetic field then exerts force on moving charge or current, changing direction of motion without doing work on a point charge. Current loops also behave as magnetic dipoles and experience torque in an external field.
The chapter becomes clearer when every problem is placed in one of two families:
Use this page to answer three questions:
Verified against the current JEE Main 2026 syllabus and JEE Advanced 2026 syllabus on 7 September 2026. This is a wording and scope mapping, not a claim about question difficulty or frequency.
Sources: JEE Main 2026 syllabus and JEE (Advanced) 2026 syllabus, both linked in the sources section below. No frequency or weightage assertion is attached to either scope.
Prerequisite
Current Electricity
Conventional current, drift and circuit reasoning before treating current as a field source.
Prerequisite
Kinematics
Resolving velocity into components before splitting motion parallel and perpendicular to a field.
Prerequisite
Laws of Motion
Force, circular motion and torque before magnetic force and loop torque.
This is a readiness check, not a weightage or scoring-priority list.
Biot-Savart always works; Ampere's law is a shortcut only when symmetry allows it.
Use Biot-Savart for contributions from current elements and Ampere's law when symmetry makes the circulation integral simple.
Ampere's law is always valid in its magnetostatic context, but it is not always useful for extracting B.
The magnetic field describes the environment; force needs charge, velocity, current, length and orientation too.
Magnetic field B describes the magnetic environment. Force also depends on charge, velocity, current, length, and orientation.
For a point charge, F = q v cross B.
The force is perpendicular to instantaneous velocity, so it changes direction but does no work in the ideal magnetic-only case.
The component perpendicular to B produces circular motion. The parallel component remains unchanged. Together they create a helix.
The area vector follows the current right-hand rule. In a uniform field, a loop experiences torque tending to align its moment with the field.
A moving-coil galvanometer uses magnetic torque and restoring torque. Adding a low-resistance shunt creates an ammeter range; adding a high series resistance creates a voltmeter range.
Do not assume Main and Advanced scope are identical without checking both official documents.
Bar-magnet equivalence, dipole fields, para-, dia-, and ferromagnetic materials, and temperature effects are explicit in JEE Main 2026.
Six decisions cover most Magnetism questions. Select the representation before any algebra.
The field contribution d B equals mu nought over four pi, times I d l cross r hat, over r squared.
Field contribution from a current element.
Use when — Steady current element in free space, source-field problems without enough symmetry for Ampere's law.
Common trap — Dropping the vector direction or geometry of the cross product.
The closed-path integral of B dot d l equals mu nought times the enclosed current.
Ampere circuital law relating field circulation to enclosed current.
Use when — Magnetostatics; useful as a field-solving shortcut only with strong symmetry.
Common trap — Assuming B is constant on an arbitrary loop without justifying it from symmetry.
B equals mu nought I over two pi r.
Field of an infinite straight current-carrying wire at perpendicular distance r.
Use when — Ideal infinitely long straight wire.
Common trap — Applying near a finite end without correction.
B equals mu nought N I over two R.
Field at the centre of N circular turns.
Use when — Closely wound coplanar circular turns, field at the centre.
Common trap — Using this centre-only result at any axial point.
B equals mu nought n I.
Field inside an ideal long solenoid.
Use when — Long solenoid, away from ends.
Common trap — Using total turns where turn density is required.
Force equals q times v cross B.
Magnetic force on a moving charge.
Use when — Point charge in a magnetic field.
Common trap — Forgetting that charge sign reverses direction.
r equals m v perpendicular over the magnitude of q times B.
Radius of the circular component of a charged particle's path.
Use when — Uniform B, non-relativistic particle.
Common trap — Using total speed when a parallel component also exists.
Force equals I times L cross B.
Force on a straight current-carrying segment.
Use when — Uniform field over the segment.
Common trap — Losing current direction when forming the cross product.
Magnetic moment equals N I A; torque equals mu cross B.
Magnetic moment of a current loop and the torque it experiences in a uniform field.
Use when — Planar loop in a uniform field.
Common trap — Using the geometric plane instead of the oriented area vector.
Shunt resistance equals I g G over the quantity I minus I g.
Shunt resistance needed to convert a galvanometer to an ammeter of range I.
Use when — Converting a galvanometer to an ammeter.
Common trap — Placing the shunt in series instead of in parallel.
Added series resistance equals V over I g, minus G.
Series resistance needed to convert a galvanometer to a voltmeter of range V.
Use when — Converting a galvanometer to a voltmeter.
Common trap — Using a low parallel resistance instead of a high series resistance.
Answer: The path is a helix: the perpendicular velocity component produces circular motion of radius r = m v_perp / (qB), while the unchanged parallel component carries the circle along the field direction with pitch p = v_parallel (2 pi m / qB).
Treating B as a force
Knowledge gapWhy it happens
Field and force are different physical quantities.
How it is corrected
Compute the field first, then apply F = q v cross B or F = I L cross B separately.
Applying Ampere's law as an algebra shortcut without symmetry
Decision / selection errorWhy it happens
The circulation integral only simplifies when field direction and magnitude are known on the path.
How it is corrected
First prove constant magnitude and known direction on the chosen path before pulling B outside the integral.
Using v instead of v_perp for the circular-motion radius
Execution errorWhy it happens
Only the component perpendicular to B contributes to circular motion.
How it is corrected
Resolve velocity relative to the field before using the radius formula.
Forgetting negative charge reverses force direction
Recall gapWhy it happens
The cross product F = q v cross B depends on the sign of q.
How it is corrected
Apply the cross product for a positive charge, then reverse the direction for a negative charge.
Saying magnetic force changes kinetic energy
Knowledge gapWhy it happens
A pure magnetic force on a point charge is always perpendicular to velocity.
How it is corrected
Remember that a pure magnetic force on a point charge does no work.
Swapping ammeter and voltmeter conversion resistors
Recall gapWhy it happens
The two instrument conversions use opposite resistor placements.
How it is corrected
An ammeter uses a low parallel shunt; a voltmeter uses a high series resistance.
Use the smallest Preparation Intelligence v1.1 label supported by what the student actually did.
The official JEE Advanced archive provides past question papers, including Paper 1 and Paper 2 downloads across years. The official JEE Main site exposes current question papers through its Question Papers section.
Record whether an item asks for source field, force effect, trajectory, dipole behaviour, material response, or instrument conversion, and note the symmetry and direction method used.
FAQ
Straight answers about how Rank Sarthi fits into serious exam preparation.
In the ideal magnetic-only case, no. The force is perpendicular to velocity.
When symmetry makes the field direction known and its magnitude constant over useful parts of the integration path.
Its perpendicular velocity component produces circular motion while its parallel component remains unchanged, and the combination traces a helix.
JEE Main explicitly names bar-magnet equivalence, magnetic dipole fields, magnetic materials and temperature effects. Check the current official JEE Advanced syllabus directly rather than assuming identical scope.
Evidence boundary: the syllabus mapping is tied to the official 2026 JEE Main and JEE Advanced documents. No chapter weightage, question frequency or forecast is asserted.
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