Case: Read the passage and answer the question.
In terms of the susceptibility, we can classify materials as diamagnetic, paramagnetic or ferromagnetic. Diamagnetic substances are those which have a tendency to move from the stronger to the weaker part of an external magnetic field. In other words, unlike the way a magnet attracts metals like iron, it would repel a diamagnetic substance. Paramagnetic substances are those which get weakly magnetised when placed in an external magnetic field. They have a tendency to move from a region of weak magnetic field to a region of strong magnetic field, i.e., they get weakly attracted to a magnet. Ferromagnetic substances are those which get strongly magnetised when placed in an external magnetic field. They have a strong tendency to move from a region of weak magnetic field to a region of strong magnetic field, i.e., they get strongly attracted to a magnet.
Consider the following statements about magnetic materials:
(A) In an atom of a diamagnetic substance, the resultant magnetic moment is zero.
(B) The atoms of a paramagnetic material have permanent magnetic dipole moments, but with no external field there is no net magnetisation.
(C) A typical domain in a ferromagnetic material contains about 1011 atoms.
(D) In an unmagnetised ferromagnetic sample, the magnetisation of every domain points in one common direction.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (B) and (C) only
The magnetic behaviour of a material comes from the magnetic moments of its atoms. The three classes differ in these atomic moments and in the way they line up.
(A) In an atom of a diamagnetic substance, the resultant magnetic moment is zero. — True. The orbital moments of the electrons in such an atom add up to zero.
(B) The atoms of a paramagnetic material have permanent magnetic dipole moments, but with no external field there is no net magnetisation. — True. Random thermal motion keeps the atomic moments pointing in all directions. So they add up to zero over the sample.
(C) A typical domain in a ferromagnetic material contains about 1011 atoms. — True. A domain is typically about 1 mm in size. All the atoms in it have their moments lined up together.
(D) In an unmagnetised ferromagnetic sample, the magnetisation of every domain points in one common direction. — False. In an unmagnetised sample the magnetisation changes at random from one domain to the next. So the sample has no bulk magnetisation until a field is applied.
NCERT (2026-27):
Diamagnetism – “… Diamagnetic substances are the ones in which resultant magnetic moment in an atom is zero. …”
Paramagnetism – “The individual atoms (or ions or molecules) of a paramagnetic material possess a permanent magnetic dipole moment of their own. On account of the ceaseless random thermal motion of the atoms, no net magnetisation is seen. …”
Ferromagnetism – “… Typical domain size is 1mm and the domain contains about 1011 atoms. In the first instant, the magnetisation varies randomly from domain to domain and there is no bulk magnetisation. …”