Physics notesElectromagnetismMagnetism
Independent topic page
Pick Biot–Savart (local source) or Ampere (symmetry), get B, then apply F=q v×B or F=I ℓ×B; meter problems are shunt/series multiplier arithmetic.
01. Biot–Savart builds B from current elements; Ampere’s law ∮B·dl=μ₀ I_enc is the symmetry shortcut (infinite straight wire, long solenoid, etc.).
02. Straight long wire: B=μ₀ I/(2π r). Coil axis: B=μ₀ I R²/2(R²+x²)^{3/2}. Long solenoid: B=μ₀ n I inside (ideal).
03. Force: F=q(v×B) on charges; F=I(ℓ×B) on wires. Parallel currents attract if same direction.
04. Loop moment m=IA; τ=m×B, U=−m·B in uniform B. Galvanometer conversions: shunt for ammeter, series multiplier for voltmeter.
05. Magnetic force on a charge is q(v×B); on a wire Idl×B. Force does no work on a charged particle—speed stays constant in pure B; only direction changes.
06. Biot–Savart and Ampère need symmetry honesty (straight wire, loop, solenoid, sheet). Right-hand rules are orientation tools, not substitutes for vector products.
07. Torque on a dipole m×B; energy −m·B in uniform B. Moving charges in crossed E and B: cyclotron ω=qB/m; drift when E and B are perpendicular.
Official · 2026
Official syllabus points for Magnetic Effects of Current only — not the full Electromagnetism portion map. Portion hub: Electromagnetism · Magnetism/EMI/AC analysis.
No topic-specific exclusions flagged — finish the unit points below for Magnetism.
Source: https://jeeadv.ac.in/documents/jee-advanced-2026-syllabus.pdf
Exam focusBiot–Savart/Ampere fields, Lorentz force, and current-carrying geometry.
Syllabus points
2020–2026 paper weight
Topic match inside parent portion Magnetism/EMI/AC (9.8% of Physics tags). Stats below are portion backdrop; matched subtopics show where Magnetic Effects of Current sits.
Portion-wide type/difficulty mix lives on the Magnetism/EMI/AC analysis — not repeated here so this page stays on Magnetism.
01. Magnetism/EMI/AC accounts for ~9.8% of tagged physics questions (2020–2026).
02. Dominant subtopics: EMI/AC (12), Magnetism (11).
Cross-topic stems
Advanced rarely asks a chapter in isolation. These are tagged multi-idea Physics stems that touch Magnetic Effects of Current — practise the knowledge bridge, not only the single-chapter drill.
No tagged mixed stem matched this topic’s keywords yet. Still revise the joins listed under Trends, and scan the mixed joins board for Mechanics / EM / Optics bridges.
Why methods matter
Right-hand directions plus Ampere/Biot choice organise magnetic field and force asks.
Stops fake ∮B·dl = B·2πr on asymmetric currents.
Do thisUse Ampere only when |B| is constant on the Amperian path by symmetry.
Stops fake ∮B·dl = B·2πr on asymmetric currents.
Infinite straight wire: circle centred on wire → B·2πr = μ₀ I.
Force direction errors dominate wrong options.
Do thisDetermine B direction before magnitude; mark × / · on the figure.
Force direction errors dominate wrong options.
Thumb along I, fingers curl in B for a loop; F uses v×B separately.
Prevents claiming translation from uniform B alone.
Do thisNet F=0 on closed loop in uniform B; keep τ=m×B.
Prevents claiming translation from uniform B alone.
Only nonuniform B yields net force on a magnetic dipole.
One current-split picture covers both conversions.
Do thisGalvanometer full-scale I_g through G; excess current through shunt, excess voltage on series R.
One current-split picture covers both conversions.
Ammeter: I_g G = (I−I_g) R_s; voltmeter: I_g(G+R)=V.
Processed examples
Read the impact line first. Then notice which family/model was chosen. The solution is written the way your rough sheet should look in the hall.
Find B at distance a from a long straight wire carrying I.
This stem rewards early recognition of the biot–savart / ampere field model.
Model chosenAmpere: B·2πa=μ₀I.
Answerμ₀I/(2πa)
Takeaway: High symmetry ⇒ Ampere’s law.
A proton moves at 10⁶ m/s perpendicular to B=0.1 T. Find the magnetic force. Take e=1.6×10⁻¹⁹ C.
This stem rewards early recognition of the lorentz force on charge/wire model.
Model chosenF=qvB.
Answer1.6×10⁻¹⁴ N
Takeaway: Maximum force when v⊥B.
A particle with charge q and mass m enters a region of uniform B=Bẑ with velocity v₀î. A uniform electric field E=Eĵ is also present. Find E so that the particle goes undeflected, and the pitch if instead E=0 and a small v_z is added.
This Advanced-level variation couples hidden constraints with the lorentz force on charge/wire model.
Model chosenUndeflected ⇒ E+v×B=0; helical pitch uses parallel velocity.
AnswerE=v₀B (opposite v×B); pitch=2πm v_z/(qB)
Takeaway: Crossed E×B selects a speed; parallel v makes helices.
Reusable models
Advanced stems cluster. Name the family in 10–15s, write the fast model, then run the same path. Do not invent a new plan each time.
Looks likeWire, loop axis, solenoid, or sheet
Fast modelUse Ampere when symmetry is high; Biot–Savart for finite pieces.
Recognition fixes the model before arithmetic begins.
Do not start blind expansion or casework before identifying the governing model.
Looks likeF=q(v×B) or Iℓ×B; cyclotron radius
Fast modelKeep vector directions; r=mv/(qB) for ⊥ motion.
Recognition fixes the model before arithmetic begins.
Do not start blind expansion or casework before identifying the governing model.
Looks likeLoop μ=IA; τ=μ×B; U=−μ·B
Fast modelFind area vector; then μ and torque/energy.
Recognition fixes the model before arithmetic begins.
Do not start blind expansion or casework before identifying the governing model.
Looks likeParallel wires attraction/repulsion
Fast modelF/L = μ₀ I₁ I₂ /(2πd); same direction attracts.
Recognition fixes the model before arithmetic begins.
Do not start blind expansion or casework before identifying the governing model.
Hall timing
Underline charges, geometry, symmetry, source type and the exact ask (E, V, flux, I, B, Φ, energy). Reject any formula whose assumptions are already broken.
Write the governing law symbolically before numbers: Coulomb/Gauss, Kirchhoff, Biot–Savart/Ampere, Faraday/Lenz, or phasor impedance. Fix sign convention and reference once.
Compute with ratios and conserved quantities first. Carry direction, SI units and powers of ten on the rough sheet instead of repairing them at the end.
Check dimensions, limiting cases (κ→1, r→∞, ω→0/∞) and physical sense. For multi-correct, test every option independently.
After the model is locked
dB = (μ₀/4π) I dl×r̂ / r²
General current element
B = μ₀ I/(2π r)
Infinite straight current
B_centre=μ₀ I/(2R); axis B=μ₀ I R²/2(R²+x²)^{3/2}
Circular coil
B = μ₀ n I (inside, ideal)
Dense long solenoid
F=q v×B; F=I ℓ×B
Force on charge or straight segment
r = mv/(qB) (v⊥); T=2πm/(qB)
Uniform B, no E
m = I A; τ = m×B; U=−m·B
Planar loop in uniform B
R_s = I_g G / (I − I_g)
Convert G to ammeter range I
R = V/I_g − G
Convert G to voltmeter range V
Make it automatic