JEE / NEET Practice Set

JEE & NEET Practice Set — Moving Charges and Magnetism

You have reached the competitive arena. The quiz below has 30+ hard questions modelled on the latest JEE Main, JEE Advanced, and NEET papers. Before you attempt them, here is the lay of the land.

Why this chapter is high-yield

• JEE Main: 2–3 questions every year. Favourites: cyclotron numericals, Biot–Savart on finite or polygon wires, force between parallel wires, motion in combined $\vec{E}$ and $\vec{B}$, Ampère’s law on coaxial geometry.
• JEE Advanced: Almost always at least one multi-concept question — often combined with Chapter 6 (EMI) or Chapter 13 (atomic physics). Expect symmetry- and vector-heavy problems.
• NEET: 2–4 questions every year. Mostly conceptual + 1 numerical. Heavy on direction reasoning, ammeter/voltmeter, and basic Biot–Savart.

Topics that dominate the question paper

Topic	JEE weight	NEET weight
Force on a moving charge / wire	High	High
Cyclotron (frequency, energy)	Medium	Medium
Biot–Savart law, $B$ for loops & arcs	Very High	High
Ampère’s law (solenoid, toroid)	High	Medium
Force between parallel wires	Medium	High
Torque on a current loop / dipole	High	Medium
Galvanometer & conversion	Medium	High
Helical motion / mass spectrometer	High	Low

If you can confidently answer 25 of the 32 quiz questions in this section in under 50 minutes, you are in the top 5 percentile for this chapter.

How to Approach This Chapter in JEE/NEET

Symmetry tips

• If a closed loop has an axis of symmetry through the field point, the perpendicular components of $d\vec{B}$ cancel — only the axial component survives.
• If $n$ identical wires are arranged symmetrically about a centre with the same current direction, their fields at the centre sum vectorially to zero (e.g. equilateral triangle).
• In Ampère’s-law problems, exploit cylindrical or planar symmetry to assume $\vec{B}$ has only one component.

Time per question

• JEE Main / Advanced: @@GYANGHAR_MATH@@390 s per MCQ; up to 3 min for numericals.
• NEET: @@GYANGHAR_MATH@@460 s per MCQ. If you can't recall the formula in 30 s, skip and return.

Top Pitfalls to Avoid

These ten traps appear every single year in JEE / NEET. Burn them into memory.

1. Forgetting $\sin\theta$ in helical motion. The radius uses $v\sin\theta$, not $v$, and the pitch uses $v\cos\theta\cdot T$.
2. Confusing $n$ vs $N$ in a solenoid/toroid. $n$ is turns per unit length; $N$ is total turns. $B_{\rm solenoid} = \mu_0 nI = \mu_0 NI/L$.
3. Sign of $U = -\vec{m}\cdot\vec{B}$. Equilibrium ($\theta=0$) is minimum energy ($U=-mB$); flipping by 180° costs $2mB$.
4. Treating magnetic force as doing work. It does not. Speed (and KE) is conserved in a pure magnetic field. Any work must come from $\vec{E}$ or external agents.
5. Field lines around an infinite wire are concentric circles, not radial lines. Drawing them radial leads to wrong direction reasoning.
6. Cyclotron limitation on electrons. At relativistic speeds the electron’s mass increases, breaking the resonance condition.
7. Energy of a current loop. $U=-mB\cos\theta$ with $\theta$ between $\vec{m}$ and $\vec{B}$, not the loop plane angle.
8. Angles in the finite-wire formula. $B = \dfrac{\mu_0 I}{4\pi d}(\sin\theta_1+\sin\theta_2)$ with $\theta_i$ measured from the perpendicular at the field point.
9. Force between currents direction. Use the other wire’s field and then the right-hand rule — mixing up wires leads to attraction/repulsion errors.
10. Sensitivity of galvanometer vs voltmeter. Increasing $N$, $A$, or $B$ boosts current sensitivity but does not change voltage sensitivity unless coil resistance is adjusted.