NEET · Physics

NEET Magnetism: Magnetic Forces, Fields, Dipoles and Galvanometer

Merge the official Unit 13 content into one defensible route without inventing removed topics. The page covers fields produced by currents, force on moving charges/current conductors, current loops/dipoles, bar-magnet equivalence, magnetic materials and moving-coil galvanometer. Earth magnetism is deliberately excluded from the current core because it is not explicitly listed in the verified 2026 unit.

Subject: Physics
Official syllabus unit: Unit 13: Magnetic Effects of Current and Magnetism

In short

NEET UG 2026 Unit 13 combines magnetic effects of current with magnetism. The current scope includes Biot-Savart and Ampere-law applications, forces on charges and conductors, torque on current loops, moving-coil galvanometer, current-loop and bar-magnet dipoles, and para/diamagnetic/ferromagnetic substances. Earth magnetism is not listed as a current Unit 13 bullet.

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Ashwini M.

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Also in Physics: Kumar Gandharva Saxena

Syllabus mapping

Syllabus mapping
Unit	Topics
Editorial route owning the full current content of NEET UG 2026 Physics Unit 13: Magnetic Effects of Current and Magnetism	Direction first, algebra second, Force on a moving charge, Fields due to current, Current loops and galvanometer, Bar magnets and materials, Current-scope exclusion

Unit
Editorial route owning the full current content of NEET UG 2026 Physics Unit 13: Magnetic Effects of Current and Magnetism
Topics
Direction first, algebra second, Force on a moving charge, Fields due to current, Current loops and galvanometer, Bar magnets and materials, Current-scope exclusion

Before this chapter

Concepts in this chapter

1. Direction first, algebra second

Magnetic-force questions fail when the learner substitutes magnitudes before fixing the direction of v, I, B and the relevant cross product. Every diagram should carry coordinate arrows or a right-hand-rule cue.

2. Force on a moving charge

The magnetic component of Lorentz force is perpendicular to velocity and magnetic field. A magnetic field alone does no work on a point charge, so it can change direction of motion without changing speed. Circular motion occurs for the perpendicular component in a uniform magnetic field; an oblique velocity produces a helix.

3. Fields due to current

Use Biot-Savart for current elements/finite high-symmetry arrangements and Ampere's law as a practical shortcut when symmetry supports a simple constant-field path. The official named Ampere applications are an infinitely long straight current-carrying wire and solenoid.

4. Current loops and galvanometer

A current loop behaves as a magnetic dipole with moment normal to the loop according to the right-hand rule. The moving-coil galvanometer should connect torque balance to current sensitivity and then to idealized conversion into ammeter/voltmeter, with added resistance chosen for the required measurement role.

5. Bar magnets and materials

The current syllabus explicitly includes bar magnet as equivalent solenoid, magnetic dipole fields along axis/perpendicular direction, torque on a magnetic dipole, and para-/dia-/ferromagnetic substances with temperature effects. Keep this at NCERT/NEET depth.

6. Current-scope exclusion

Do not import Earth magnetism merely because older chapter structures or coaching notes include it. It is not an explicit 2026 Unit 13 bullet in the verified source.

7. Decision aids for this chapter

Magnetic direction board: positive-charge force, negative-charge force, current-element field and loop magnetic moment.
Method chooser: Lorentz/centripetal vs Biot-Savart vs Ampere.
Current-scope boundary card: magnetic materials included; Earth magnetism not explicitly listed for 2026.

Formula sheet

Lorentz force: F = q(E + v x B). Uniform/local fields as applied; charge sign changes force direction. Unit N.
Lorentz force
Use when — Uniform/local fields as applied; charge sign changes force direction. Unit N.
Magnetic-force magnitude: . F_B
Magnetic-force magnitude
Use when — F_B
Circular radius: r = m v_perp/(. q
Circular radius
Use when — q
Period: T = 2πm/(. q
Period
Use when — q
Conductor force: F = I L x B. Straight segment in uniform B; L points with conventional current.
Conductor force
Use when — Straight segment in uniform B; L points with conventional current.
Biot-Savart: dB = (mu_0/4π) I dl x r_hat / r^2. Steady current element.
Biot-Savart
Use when — Steady current element.
Circular loop centre: B = mu_0 N I/(2R). At centre of N-turn circular loop in air/vacuum.
Circular loop centre
Use when — At centre of N-turn circular loop in air/vacuum.
Ampere law: closed integral B.dl = mu_0 I_enclosed. Magnetostatic current; practical simplification needs symmetry.
Ampere law
Use when — Magnetostatic current; practical simplification needs symmetry.
Long straight wire: B = mu_0 I/(2πr). Infinitely long/very long straight wire approximation.
Long straight wire
Use when — Infinitely long/very long straight wire approximation.
Long solenoid: B approx mu_0 n I. Ideal long solenoid, interior away from ends.
Long solenoid
Use when — Ideal long solenoid, interior away from ends.
Magnetic dipole moment: m = N I A. Area vector by right-hand rule. Unit A m^2.
Magnetic dipole moment
Use when — Area vector by right-hand rule. Unit A m^2.
Loop torque: tau = m x B;. tau
Loop torque
Use when — tau
Dipole axial field: B = (mu_0/4π)(2m/r^3). Dipole/far-field axis model.
Dipole axial field
Use when — Dipole/far-field axis model.
Dipole equatorial field: B = (mu_0/4π)(m/r^3) opposite m. Dipole/far-field equatorial line.
Dipole equatorial field
Use when — Dipole/far-field equatorial line.
Galvanometer current: I = (k/(NAB)) theta. Linear torsion k theta, radial/uniform field idealization.
Galvanometer current
Use when — Linear torsion k theta, radial/uniform field idealization.

Worked examples

a positive charge enters a uniform magnetic field with speed v at angle 30 degrees to B.

Target concept: charged-particle motion in a uniform magnetic field.

Condition: a positive charge enters a uniform magnetic field with speed v at angle 30 degrees to B.

Method selection: split velocity into parallel and perpendicular components.

Reasoning/calculation: v_parallel = v cos30 remains unchanged; v_perp = v sin30 = v/2 produces circular motion with r = m(v/2)/(qB). Combined motion is helical.

Units/sign check: m v/(qB) has unit metres; helix handedness depends on positive charge and B direction.

PI category: Decision / Selection Error if total v is used for radius; Execution Error if vector direction is mishandled.

Common mistakes and what they actually indicate

Uses right-hand rule for an electron without reversing sign
Execution error
Why it happens
Preparation Intelligence classification: Execution Error.
How it is corrected
Find force for positive q, then reverse for negative charge.
Assumes magnetic field changes particle speed
Knowledge gap
Why it happens
Preparation Intelligence classification: Knowledge Gap.
How it is corrected
Check that magnetic force is perpendicular to v.
Uses Ampere shortcut without symmetry
Decision / selection error
Why it happens
Preparation Intelligence classification: Decision / Selection Error.
How it is corrected
Switch to Biot-Savart or another model.
Mixes loop plane angle with magnetic-moment angle
Execution error
Why it happens
Preparation Intelligence classification: Execution Error.
How it is corrected
Draw area normal first.
Adds Earth magnetism as current 2026 content
Needs review
Why it happens
Preparation Intelligence classification: Needs Review.
How it is corrected
Remove current-scope badge; official Unit 13 does not list it.

FAQ