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Exam level3 past questions

Force and motion in magnetic fields

Understand the forces experienced by current-carrying conductors and moving charges in magnetic fields, including the Lorentz force, force between parallel conductors, and the motion of charged particles.

Why this shows up in the exam

You will be asked to predict forces, calculate trajectories, and solve problems involving the interaction of currents and magnetic fields.

How NEET tests this

Numerical · 2 QsApplication · 2 QsStatement analysis

Learn the idea

When a charge moves, it feels a force q(E+v×B); the magnetic part only acts perpendicular to its velocity, so if v is parallel to B the magnetic force vanishes and the motion follows the electric field alone.

🧠 Memory hook: If E and B march together (parallel), the charge walks straight – magnetic force takes a back seat when velocity follows the field line.

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • Lorentz force: F = q(E + v × B) (NCERT §3.2)
  • Force on a current‑carrying conductor: F = I L × B, magnitude = I L B sinθ
  • Direction of magnetic force given by right‑hand rule for cross product
  • Force per unit length between two parallel conductors: f = μ₀ I₁ I₂ / (2π r)
  • A charge moving only in a uniform B follows a circular path of radius r = mv/(qB)

How to approach it

  1. 1Read the question and decide which formula (Lorentz or I L×B) is relevant
  2. 2Write the vector expression, resolve the cross product using geometry or right‑hand rule
  3. 3If only magnitude is needed, use |F| = qvB sinθ or |F| = I L B sinθ
  4. 4Insert the given values and compute; check units
  5. 5For direction, apply the right‑hand rule and note sign of charge
  6. 6For closed loops, add forces vectorially; opposite sides often cancel

Worked example — watch it click

In a region, steady and uniform electric and magnetic fields are present. These two fields are parallel to each other. A charged particle is released from rest in this region. The path of the particle will be a:

  • A)helix
  • ✅straight line
  • C)ellipse
  • D)circle

The concept behind this problem

The example checks whether you realise that with v initially zero, the particle accelerates along E, stays parallel to B, so v×B stays zero and the trajectory is a straight line.

Step by step

  1. 1When E and B are parallel, the Lorentz force F = q(E + v × B).
  2. 2Since v × B is perpendicular to B, and E is parallel to B, the magnetic force is perpendicular to the electric force.
  3. 3Starting from rest, the particle accelerates along E.
  4. 4As it gains velocity along E (parallel to B), v × B = 0 (since v ∥ B).
  5. 5Thus only the electric force acts, producing straight-line motion along the field direction.

Watch out

Thinking the magnetic field will curve the path even though the particle’s velocity is always parallel to B, leading to a helix instead of a straight line.

Common slip-ups that cost marks

  • •Treating v×B as non‑zero when v is parallel to B
  • •Forgetting the sine factor; using I L B instead of I L B sinθ when the wire is not perpendicular to B
  • •Assuming forces on opposite sides of a rectangular loop add instead of cancel

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 3NEET 2006

In a region, steady and uniform electric and magnetic fields are present. These two fields are parallel to each other. A charged particle is released from rest in this region. The path of the particle will be a:

Push further

More challenging

10 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 10

A positively charged particle is released from rest in a region where a uniform electric field E and a uniform magnetic field B are present. Both fields are directed along the positive z-axis. A student incorrectly assumes that the magnetic force will cause the particle to move in a circle. What is the actual path of the particle?

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