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Exam level5 past questions

Magnetic moment and properties of magnets

Explore the concept of magnetic moment for current loops and bar magnets, properties of divided magnets, and effective length of magnets.

Why this shows up in the exam

You need to understand how magnets behave, how their moments are calculated, and how their properties change when divided or arranged differently.

How NEET tests this

Numerical · 3 QsApplication · 3 Qs

Learn the idea

Magnetic moment of a magnet equals its pole strength multiplied by the effective length (distance between its poles). For a current loop it equals the current times the area of the loop.

🧠 Memory hook: M equals m times l – "Magnet's Moment = pole strength × length"

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

Formulas & facts to keep ready

  • Magnetic moment (M) = pole strength (m) × effective length
  • Effective length is the straight line joining the two poles
  • When a magnet is divided, pole strength stays unchanged while effective length changes
  • For a moving charge, M = current × area = qvR/2
  • Direction of M follows the right‑hand rule for current loops

How to approach it

  1. 1Identify whether the problem involves a bar magnet or a current loop
  2. 2Write the appropriate expression for M (m×effective length or I×area)
  3. 3Find the effective length or area using the given geometry and substitute

Worked example — watch it click

An iron rod of length L and magnetic moment M is bent in the form of a semicircle. Now its magnetic moment will be:

  • A)M
  • ✅2M/π
  • C)M/π
  • D)Mπ

The concept behind this problem

The example forces you to replace the straight‑line length of a rod by the chord length after bending, showing that magnetic moment depends on the distance between poles, not on the shape of the magnet

Step by step

  1. 1Magnetic moment M = m × 2l where m is pole strength and 2l is the effective length (distance between poles).
  2. 2For a straight rod of length L: M = m × L.
  3. 3When bent into a semicircle, the effective length becomes the chord = 2R = 2(L/π) = 2L/π (since arc length L = πR).
  4. 4New magnetic moment M' = m × (2L/π) = (m × L)/π = M/π.

Watch out

Treating the curved part’s arc as the effective length leads to a wrong magnetic moment

Common slip-ups that cost marks

  • •Do not assume pole strength changes when a magnet is cut
  • •Do not use the arc length of a curved magnet as its effective length; use the chord

🌟 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 5

An iron rod of length L and magnetic moment M is bent in the form of a semicircle. Now its magnetic moment will be:

Push further

More challenging

3 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 3

An electron (charge e) orbits a proton in a circular path of radius 'r' with a constant angular speed 'ω'. What is the magnitude of the magnetic moment associated with this orbital motion?

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