Home › Free notes › Class 12 Physics › Magnetism and Matter

Magnetism and Matter – Class 12 Physics notes, important questions and MCQs

NCERT Class 12 Physics, chapter 5. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Materials respond to magnetic fields differently — dia-, para- and ferromagnetic. Earth itself behaves like a giant magnet.

Bar magnet and Earth

Magnetic dipole moment m; field on axis B = (µ₀/4π)(2m/r³); on equator (µ₀/4π)(m/r³).
Torque τ = mB sin θ; potential energy U = −mB cos θ.
Earth's magnetic elements: declination, inclination (dip), horizontal component.

Magnetic materials

Diamagnetic (bismuth, copper, water): weakly repelled, χ small negative.
Paramagnetic (aluminium, oxygen): weakly attracted, χ small positive.
Ferromagnetic (iron, cobalt, nickel): strongly attracted; lose it above the Curie temperature.

Key terms

Magnetic susceptibility: χ, measure of how a material is magnetised in a field
Curie temperature: temperature above which a ferromagnet becomes paramagnetic
Magnetic dipole moment: m = NIA for a current loop; a bar magnet behaves like a magnetic dipole

Formulas

τ = mB sin θ
B_axial = µ₀2m/(4πr³)

Important questions with answers

Why are field lines of a bar magnet closed loops?2 marks

Magnetic monopoles do not exist. Field lines go from N to S outside the magnet and from S to N inside, so they form continuous closed loops (Gauss's law for magnetism: net flux through a closed surface is zero).

Distinguish dia-, para- and ferromagnetic substances with one example each.3 marks

Diamagnetic: weakly repelled, χ slightly negative, e.g. bismuth, copper, water. Paramagnetic: weakly attracted, χ small positive and falls with temperature (Curie's law), e.g. aluminium, oxygen. Ferromagnetic: strongly attracted, large χ, have domains, become paramagnetic above Curie temperature, e.g. iron, nickel, cobalt.

Explain hysteresis using a B–H loop and state which material is preferred for permanent magnets and for transformer cores, with reasons.5 marks

When a ferromagnet is magnetised and the field is reduced to zero, B does not return to zero: the remaining value is retentivity (remanence). A reverse field (coercivity) is needed to make B zero. Repeating the cycle traces a closed B–H loop; its area is the energy lost as heat per cycle per unit volume. Permanent magnets need high retentivity and high coercivity (steel, alnico) so they keep magnetism. Transformer cores and electromagnets need low coercivity and a thin loop (soft iron) so magnetisation reverses easily with little energy loss, and high permeability.

Quick revision

Dia: repelled; para: weakly attracted; ferro: strongly attracted.
Axial field is twice the equatorial field at the same distance.
Steel is used for permanent magnets.

Exam tips

Soft iron (low retentivity, low coercivity) is used for electromagnets and transformer cores.
Diamagnetic: χ small negative; paramagnetic: χ small positive; ferromagnetic: χ large positive.

Practice MCQs

Which is ferromagnetic?
1. copper
2. aluminium
3. iron
4. bismuth
Show answer
Answer: (c) iron. Iron.
Diamagnetic substances are
1. strongly attracted
2. weakly attracted
3. weakly repelled
4. not affected at all
Show answer
Answer: (c) weakly repelled. Weakly repelled.
Above the Curie temperature, a ferromagnet becomes
1. diamagnetic
2. paramagnetic
3. superconducting
4. more magnetic
Show answer
Answer: (b) paramagnetic. Paramagnetic.
The angle between Earth's field and the horizontal is the
1. dip (inclination)
2. declination
3. latitude
4. meridian
Show answer
Answer: (a) dip (inclination). Dip.
A bar magnet of moment 2 A m² is placed at 30° to a field of 0.5 T. Torque is
1. 0.87 N m
2. 1 N m
3. 0.5 N m
4. 2 N m
Show answer
Answer: (c) 0.5 N m. mB sin 30°.
Material for electromagnet cores should have
1. high retentivity and high coercivity
2. zero permeability
3. high coercivity only
4. low retentivity and low coercivity
Show answer
Answer: (d) low retentivity and low coercivity. Soft iron.
At the same distance, axial field to equatorial field ratio of a short magnet is
1. 1 : 1
2. 2 : 1
3. 1 : 2
4. 4 : 1
Show answer
Answer: (b) 2 : 1. 2m/r³ vs m/r³.
Above the Curie temperature, a ferromagnetic material becomes
1. diamagnetic
2. superconducting
3. paramagnetic
4. more ferromagnetic
Show answer
Answer: (c) paramagnetic. Domains break down.

All chapters of Class 12 Physics

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.