NEET Magnetic Effects of Current and Magnetism PYQs with Solutions | 2013–2025

Magnetic Effects of Current and Magnetism — Solutions (2025 → 2013)

2025

Q1. A long straight wire carries a current I. Derive an expression for the magnetic field at a point at distance r from the wire.

Solution:

• Use Ampere’s Law: $\oint \vec{B} \cdot d\vec{l} = \mu_0 I_{\text{enclosed}}$∮B⋅dl=μ0​Ienclosed​
• For a straight wire, B is tangential and constant on a circular path of radius r. So:

$$B (2 \pi r) = \mu_0 I \implies B = \frac{\mu_0 I}{2 \pi r}$$B(2πr)=μ0​I⟹B=2πrμ0​I​

✅ Answer: $B = \frac{\mu_0 I}{2 \pi r}$B=2πrμ0​I​

Q2. Circular loop of radius R carrying current I — magnetic field at center.

Solution:

• Biot–Savart law:

$$d\vec{B} = \frac{\mu_0}{4\pi} \frac{I d\vec{l} \times \hat{r}}{r^2}$$dB=4πμ0​​r2Idl×r^​

• Integrating over a circular loop:

$$B = \frac{\mu_0 I}{2R} \quad \text{(perpendicular to plane of loop)}$$B=2Rμ0​I​(perpendicular to plane of loop)

Q3. Principle of moving-coil galvanometer & conversion into ammeter.

Solution:

• Principle: Torque on current-carrying coil in magnetic field:
  $\tau = n I A B$τ=nIAB
• Conversion: Add low resistance in parallel (shunt) → splits current, protecting galvanometer → becomes ammeter.

2024

Q1. Two long parallel conductors separated by d carry currents I₁ and I₂. Force per unit length.

Solution:$$F/L = \frac{\mu_0 I_1 I_2}{2 \pi d}$$F/L=2πdμ0​I1​I2​​

• Attraction if currents same, repulsion if opposite.

Q2. Ampere’s circuital law:
$\oint \vec{B} \cdot d\vec{l} = \mu_0 I_{\text{enclosed}}$∮B⋅dl=μ0​Ienclosed​

Q3. Solenoid of n turns/unit length, current I:$$B = \mu_0 n I$$B=μ0​nI

2023

Q1. Torque on current-carrying loop:$$\tau = \vec{\mu} \times \vec{B}, \quad \mu = nIA$$τ=μ​×B,μ=nIA

Q2. Force on conductor in magnetic field (Lorentz force):$$\vec{F} = I (\vec{L} \times \vec{B})$$F=I(L×B)

Q3. Bar magnet suspended freely → aligns along horizontal component of Earth’s magnetic field.

2022

Q1. Two coaxial circular coils — midpoint field:

• Each coil: $B = \frac{\mu_0 I R^2}{2 (R^2 + x^2)^{3/2}}$B=2(R2+x2)3/2μ0​IR2​
• At midpoint: sum of fields of both coils.

Q2. Tangent galvanometer: $B_H = B \tan \theta$BH​=Btanθ

Q3. Magnetic moment of current loop: $\mu = n I A$μ=nIA

2021

Q1. Biot–Savart law: $d\vec{B} = \frac{\mu_0}{4\pi} \frac{I d\vec{l} \times \hat{r}}{r^2}$dB=4πμ0​​r2Idl×r^​

Q2. Moving coil galvanometer sensitivity:$$\text{Sensitivity} = \frac{\text{deflection}}{\text{current}}$$Sensitivity=currentdeflection​

Q3. Current perpendicular to uniform B → electrons experience circular motion (Lorentz force).

2020

Q1. Force on straight conductor: $F = I L B \sin \theta$F=ILBsinθ

Q2. Two parallel wires:$$F/L = \frac{\mu_0 I_1 I_2}{2\pi d}$$F/L=2πdμ0​I1​I2​​

Q3. Magnetic moment: $\mu = I A$μ=IA

2019

Q1. Bar magnet disturbed in horizontal plane → oscillates → Damped harmonic motion, aligns along Earth’s magnetic field.

Q2. Axis of circular loop:$$B = \frac{\mu_0 I R^2}{2 (R^2 + x^2)^{3/2}}$$B=2(R2+x2)3/2μ0​IR2​

Q3. Parallel wires 50 cm apart, I = 2 A:$$F/L = \frac{\mu_0 I^2}{2\pi d} = \frac{4\pi ×10^{-7} ×4}{2π×0.5} = 1.6 × 10^{-6} N/m$$F/L=2πdμ0​I2​=2π×0.54π×10−7×4​=1.6×10−6N/m

2018

Q1. Tangent galvanometer: $\theta = \tan^{-1}(B_H/B)$θ=tan−1(BH​/B)

Q2. Fleming’s left-hand rule: Thumb → motion, Forefinger → B, Middle → current.

Q3. Rectangular loop torque: $\tau = n I A B \sin \theta$τ=nIABsinθ

2017

Q1. Cyclotron: Charged particle moves in circular path under B, accelerated by electric field → principle: Lorentz force provides centripetal acceleration.

Q2. Solenoid field:

• Inside: $B = \mu_0 n I$B=μ0​nI
• Outside ≈ 0

Q3. Two parallel wires, opposite currents: $F/L = \frac{\mu_0 I^2}{2 \pi d}$F/L=2πdμ0​I2​, repulsive.

2016

Q1. Circular coil — Biot–Savart law: $B = \frac{\mu_0 I}{2R}$B=2Rμ0​I​ at center

Q2. Magnetic dipole moment: $\mu = I A$μ=IA → Torque: $\tau = \mu B \sin\theta$τ=μBsinθ

Q3. Moving coil galvanometer → voltmeter: Add series high resistance.

2015

Q1. Charged particle perpendicular to B → circular motion: $r = \frac{mv}{qB}$r=qBmv​

Q2. Ampere’s law applied to straight wire: $B = \frac{\mu_0 I}{2\pi r}$B=2πrμ0​I​

Q3. Two parallel wires, same direction → attraction: $F/L = \frac{\mu_0 I_1 I_2}{2 \pi d}$F/L=2πdμ0​I1​I2​​

2014

Q1. Circular loop field: $B = \frac{\mu_0 I}{2R}$B=2Rμ0​I​

Q2. Torque on rectangular loop: $\tau = n I A B \sin \theta$τ=nIABsinθ

Q3. Tangent galvanometer: $B_H = B \tan \theta$BH​=Btanθ

2013

Q1. Two parallel wires 1 m apart, I₁ = 3 A, I₂ = 5 A:$$F/L = \frac{\mu_0 I_1 I_2}{2\pi d} = \frac{4\pi×10^{-7} × 15}{2π ×1} = 3 × 10^{-6} N/m$$F/L=2πdμ0​I1​I2​​=2π×14π×10−7×15​=3×10−6N/m

Q2. Coil N turns, area A, current I → torque in B: $\tau = n I A B \sin \theta$τ=nIABsinθ

Q3. Moving coil galvanometer → ammeter: Add parallel shunt resistance.