NEET Physics — Magnetism and EMI Complete Chapter Guide

Chapter Overview & Weightage

Magnetism and EMI covers magnetic fields, Biot-Savart law, Ampere’s law, force on moving charges and current-carrying conductors, electromagnetic induction (Faraday’s law), and AC circuits. NEET asks 3-4 questions from this combined unit.

This unit carries 6-8% weightage in NEET Physics with 3-4 questions. Force on a current-carrying conductor, Faraday’s law applications, and basic AC circuits are the most commonly tested.

Key Concepts You Must Know

Tier 1 (Core)

Force on a moving charge: $\vec{F} = q\vec{v} \times \vec{B}$ (magnitude: $F = qvB\sin\theta$ )
Force on current-carrying conductor: $F = BIL\sin\theta$
Biot-Savart law: $dB = \frac{\mu_0}{4\pi}\frac{Idl\sin\theta}{r^2}$
Magnetic field at centre of circular loop: $B = \frac{\mu_0 NI}{2R}$
Faraday’s law: $\text{EMF} = -\frac{d\Phi_B}{dt}$ (Lenz’s law gives the negative sign)
Magnetic flux: $\Phi_B = BA\cos\theta$

Tier 2 (Frequently tested)

Solenoid: $B = \mu_0 nI$ (inside), $B \approx 0$ (outside)
Self-inductance: $\text{EMF} = -L\frac{dI}{dt}$ , energy stored $= \frac{1}{2}LI^2$
Transformer: $V_s/V_p = N_s/N_p$ (ideal), step-up/step-down
AC circuits: impedance $Z$ , power factor $\cos\phi$ , resonance ( $X_L = X_C$ )

Important Formulas

Force on charge: $F = qvB\sin\theta$ (perpendicular to both $v$ and $B$ )

Force on conductor: $F = BIL\sin\theta$

Circular motion in B field: $r = \frac{mv}{qB}$ , time period $T = \frac{2\pi m}{qB}$

B at centre of loop: $B = \frac{\mu_0 NI}{2R}$

B inside solenoid: $B = \mu_0 nI$ ( $n$ = turns per unit length)

Faraday’s law: $|\text{EMF}| = N\frac{d\Phi_B}{dt}$

Motional EMF: $\text{EMF} = Bvl$ (rod of length $l$ moving with velocity $v$ perpendicular to $B$ )

Self-inductance EMF: $\text{EMF} = -L\frac{dI}{dt}$

Energy in inductor: $U = \frac{1}{2}LI^2$

Component	Impedance	Phase
Resistor $R$	$R$	$V$ and $I$ in phase
Inductor $L$	$X_L = \omega L$	$V$ leads $I$ by 90 degrees
Capacitor $C$	$X_C = 1/\omega C$	$I$ leads $V$ by 90 degrees
LCR series	$Z = \sqrt{R^2 + (X_L - X_C)^2}$	Resonance when $X_L = X_C$

Resonance frequency: $\omega_0 = 1/\sqrt{LC}$ , $f_0 = 1/(2\pi\sqrt{LC})$

For NEET, the motional EMF formula ( $EMF = Bvl$ ) is the most commonly tested EMI concept. A rod sliding on rails in a magnetic field — calculate the EMF, then the current, then the force. This problem type appears almost every year.

Solved Previous Year Questions

PYQ 1 — NEET 2024

Problem: A straight conductor of length 0.5 m carries a current of 2 A in a uniform magnetic field of 0.4 T perpendicular to it. Find the force on the conductor.

Solution:

$F = BIL\sin\theta = 0.4 \times 2 \times 0.5 \times \sin 90° = \mathbf{0.4 \text{ N}}$

PYQ 2 — NEET 2023

Problem: A circular coil of 100 turns and radius 10 cm carries a current of 1 A. Find the magnetic field at the centre.

Solution:

B = \frac{\mu_0 NI}{2R} = \frac{4\pi \times 10^{-7} \times 100 \times 1}{2 \times 0.1} = \frac{4\pi \times 10^{-5}}{0.2} = 2\pi \times 10^{-4} \approx \mathbf{6.28 \times 10^{-4} \text{ T}}

PYQ 3 — NEET 2022

Problem: A rod of length 1 m moves with velocity 5 m/s perpendicular to a magnetic field of 0.2 T. Find the motional EMF.

Solution:

$\text{EMF} = Bvl = 0.2 \times 5 \times 1 = \mathbf{1 \text{ V}}$

Expert Strategy

Week 1: Magnetic force — on charges and conductors. Practice circular motion of charged particles in magnetic fields. Know the right-hand rule for direction.

Week 2: EMI — Faraday’s law and motional EMF. Solve rod-on-rails problems. Understand Lenz’s law physically (induced current opposes the change that caused it).

Week 3: AC circuits — impedance of R, L, C individually, then LCR series. Know resonance condition and transformer ratio.

Common Traps

Trap 1 — Magnetic force on a charge is ZERO when $v$ is parallel to $B$ . $F = qvB\sin\theta$ . When $\theta = 0$ (parallel), $F = 0$ . The force is maximum when $v$ is perpendicular to $B$ .

Trap 2 — Lenz’s law gives the DIRECTION, not magnitude, of induced EMF. The induced current opposes the change in flux. If flux is increasing, induced current creates a field to oppose the increase. If decreasing, it supports. This determines the sign of EMF.

Trap 3 — In an ideal transformer, power is conserved, not voltage. $V_p I_p = V_s I_s$ . A step-up transformer increases voltage but decreases current proportionally. Students sometimes think both increase.

Trap 4 — Time period of circular motion in B field doesn’t depend on velocity. $T = 2\pi m/(qB)$ . Changing the speed changes the radius but not the time period. This counterintuitive fact is a NEET favourite.