Class 12 Physics Chapter 5: Magnetism and Matter
CBSE 2026–27 | NCERT-aligned free study resource
This chapter explains magnetic dipoles, bar magnets, magnetic field lines, magnetisation, magnetic intensity, susceptibility, permeability, and the broad behaviour of dia-, para- and ferromagnetic materials.
1. Bar Magnet as a Magnetic Dipole
A bar magnet behaves approximately like a magnetic dipole. Its field pattern resembles that of a magnetic dipole. Magnetic field lines outside a magnet run from north to south, while inside they continue from south to north, forming closed loops.
Outside: N → → → → S
Inside: S → → → → N
Magnetic field lines form closed loops.
2. Magnetic Dipole in a Uniform Field
A magnetic dipole of magnetic moment m in field B experiences torque τ = mB sinθ. Potential energy U = −m·B. Stable equilibrium occurs when the dipole moment aligns with the field.
3. Magnetisation
Magnetisation M is magnetic dipole moment per unit volume. Magnetic susceptibility χ describes how strongly a material responds to an applied magnetic field. In a simple linear material, M = χH.
4. Magnetic Materials
| Type | Response | General behaviour |
|---|---|---|
| Diamagnetic | Weakly opposed to applied field | χ small and negative |
| Paramagnetic | Weakly attracted | χ small and positive |
| Ferromagnetic | Strongly attracted | Can retain magnetisation |
5. Earth’s Magnetism
Earth behaves approximately like a giant magnetic dipole. Important terms include magnetic declination, dip (inclination) and horizontal component of Earth’s field.
Horizontal component H + vertical component
Dip angle is the angle made by Earth’s magnetic field with the horizontal.
6. Magnetic Field Lines
- They are continuous closed curves.
- The tangent gives field direction.
- They do not intersect.
- Greater density represents stronger field.
7. Permanent Magnets and Electromagnets
Permanent magnets are designed to retain magnetisation. Electromagnets use current to create a magnetic field and can be switched or controlled by changing current.
Worked Example
A magnetic dipole with moment 0.50 A m² is placed in a 0.20 T field at 30°. Torque τ = mB sin30° = 0.50×0.20×0.5 = 0.05 N m.
Common Exam Traps
- Do not treat magnetic field lines as starting or ending at isolated magnetic poles.
- Magnetic moment direction for a current loop follows the right-hand thumb rule.
- Do not confuse susceptibility, permeability and magnetisation.
Practice Questions
- Explain magnetic dipole moment and torque on a magnetic dipole.
- Compare diamagnetic, paramagnetic and ferromagnetic substances.
- Explain declination and angle of dip.
- Why are magnetic field lines closed loops?
- Differentiate a permanent magnet and an electromagnet.
Magnetism Visual Learning
Magnetic Field Lines
↗ ↖
N [ MAGNET ] S
↘ ↙
↘ ↓ ↙
Magnetic field lines form closed loops. Outside a bar magnet they are conventionally drawn from north to south, while inside the magnet they return from south to north.
Magnetic Materials
| Type | Key behaviour |
|---|---|
| Diamagnetic | Weakly repelled by magnetic field |
| Paramagnetic | Weakly attracted |
| Ferromagnetic | Strong magnetic response; domain alignment important |
Worked Concept
A magnetic dipole in an external magnetic field experiences torque tending to align its magnetic moment with the field. The potential energy depends on orientation.
CBSE-Style Practice
- Compare diamagnetic, paramagnetic and ferromagnetic materials.
- Explain magnetic field lines around a bar magnet.
- Analyse torque on a magnetic dipole.
- Explain magnetisation and magnetic susceptibility.
- Interpret a magnetic-field/material response graph.
Common Mistakes to Avoid
- Assuming all magnetic materials behave identically.
- Confusing magnetic moment with magnetic field.
- Drawing magnetic field lines as open curves.
Worked Numerical — Magnetic Dipole
A magnetic dipole of moment 0.50 A m² is placed in a uniform magnetic field of 0.20 T at 60°. The torque is τ=MB sinθ=0.50×0.20×sin60°≈0.0866 N m.