Question 131 markSection A

Assertion (A): All atoms have a net magnetic moment.

Reason (R): A current loop does not always behave as a magnetic dipole.

Magnetism and Matter
Atomic magnetic moment and current loops

Options

(A)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
(B)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
(C)Assertion (A) is true, but Reason (R) is false.
(D)Both Assertion (A) and Reason (R) are false.

Official answer

Correct option: (D) — Both Assertion (A) and Reason (R) are false.


Why A is false: Not every atom has a net magnetic moment. In diamagnetic atoms (e.g. He, Ne, N₂, Cu⁺) the electrons are paired so their orbital and spin magnetic moments cancel, giving a zero net atomic moment.


Why R is false: A current loop always behaves as a magnetic dipole of moment m=NIAm = NIA; the field of a small loop at distant points is exactly that of a magnetic dipole.

Answer keywords:magnetic momentdiamagneticpaired electronscurrent loopmagnetic dipolem = NIAassertion reasonnet moment zero

Marking scheme

  1. 1.1 mark: correct option (D).
  2. 2.Full credit requires only the letter; reasoning not demanded for a 1-mark A-R item but both statements must be judged false.

Hint

Think diamagnetic atoms (paired electrons → zero moment) and recall m=NIAm = NIA for any loop.

Quick oral answer

Both statements are false: diamagnetic atoms have zero net moment because their electron moments pair off, and a current loop always acts as a magnetic dipole with moment m equal to N times I times A.

Common mistakes

  1. 1.Assuming every atom is magnetic because electrons orbit — paired electrons cancel, giving zero net moment in diamagnetic atoms.
  2. 2.Believing a current loop only 'sometimes' acts as a dipole; a small current loop is the archetypal magnetic dipole with m=NIAm = NIA.
  3. 3.Choosing option (C) by treating the assertion as true.

Analysis and explanation

Concept:

An atom's magnetism arises from electron orbital motion and electron spin. Whether an atom has a net moment depends on how these individual moments add up.


Assertion analysis:

  • In diamagnetic substances the electronic moments are fully paired and cancel, so the net atomic moment is zero.
  • Only paramagnetic and ferromagnetic atoms carry a permanent net moment.
  • Hence "all atoms have a net magnetic moment" is false.

Reason analysis:

  • A planar current loop is the fundamental magnetic dipole; its dipole moment is m=NIAm = NIA and its far-field is a pure dipole field.
  • So "a current loop does not always behave as a magnetic dipole" is also false.

Exam trap:

Students often mark A true because "electrons revolve, so there must be a moment." Revolution creates moments, but paired electrons make them cancel.


Real-world:

Diamagnetism (net-zero atomic moment) is why water and graphite are weakly repelled by strong magnets — the basis of frog-levitation demonstrations.

Interesting facts

A single Bohr magneton, μB=eh4πme≈9.27×10−24 J/T\mu_B = \frac{eh}{4\pi m_e} \approx 9.27 \times 10^{-24}\text{ J/T}, is the natural unit of atomic magnetic moment.

Diamagnetism is universal — present in every material — but is so weak it is masked whenever para- or ferromagnetism exists.

In 1997 physicists at Nijmegen levitated a live frog in a 16 T field purely because of the diamagnetism of its body water.

Question details

Q13 of 39 questions

Board
CBSE
Class
Class 12
Subject
Physics
Year
2026
Section
Section A
Marks
1
Chapter
Magnetism and Matter
Topic
Atomic magnetic moment and current loops
Bloom's level
analyze
Syllabus topic
Unit III: Magnetic Effects of Current and Magnetism — Magnetic dipole moment of a current loop; magnetism of atoms
Author
CBSE
Reviewed by
Padhantu Academic Team

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Frequently asked questions

Why don't diamagnetic atoms have a net magnetic moment?

Their electrons occupy fully filled orbitals in spin-paired pairs, so every orbital and spin magnetic moment is cancelled by an equal and opposite one, leaving zero net atomic moment.

Does a current loop always behave as a magnetic dipole?

Yes. A current-carrying loop has a magnetic dipole moment m=NIAm = NIA, and at points far from the loop its magnetic field is exactly that of a dipole, so the Reason statement is false.

Why is the answer D and not C?

Option C would require the Assertion to be true, but it is false because diamagnetic atoms carry no net moment; since both statements are false, D is correct.