Chapter 5 - Magnetism and Matter

Magnets have been known for centuries, but their behaviour is closely connected with moving electric charges and microscopic currents.

Magnetism and Matter Class 12 studies bar magnets, magnetic dipoles, field lines, magnetisation and the magnetic properties of materials.

Bar magnet and magnetic poles

A bar magnet has a north-seeking pole and a south-seeking pole.

Like poles repel and unlike poles attract. However, individual magnetic poles have not been isolated in ordinary magnetism.

If a bar magnet is cut into smaller pieces, each piece becomes a smaller complete magnet with both poles.

Bar magnet as a magnetic dipole

A bar magnet behaves as a magnetic dipole.

Its strength and orientation are described using magnetic dipole moment. The magnetic dipole moment is directed from the south pole towards the north pole inside the magnet.

Students compare the field pattern of a bar magnet with that of a current-carrying solenoid.

Magnetic field of a dipole

Students study the magnetic field due to a bar magnet:

• Along its axial position

• Along its perpendicular or equatorial position

The current CBSE treatment of these expressions is primarily qualitative where specified, so students should focus on direction, field pattern and comparative behaviour.

Torque on a magnetic dipole

A magnetic dipole placed in a uniform magnetic field experiences torque.

The torque tends to align its magnetic dipole moment with the field.

When the dipole moment is parallel to the field, the orientation is stable. When it is opposite, the orientation is unstable.

Magnetic field lines

Magnetic field lines:

• Form closed loops

• Emerge from the north pole and enter the south pole outside a magnet

• Continue from south to north inside the magnet

• Never intersect

• Are more crowded where the magnetic field is stronger

Unlike electrostatic field lines, magnetic field lines do not begin or end at isolated magnetic charges.

Magnetisation

Magnetisation describes the magnetic dipole moment per unit volume of a material.

Different materials respond differently when placed in an external magnetic field.

Diamagnetic materials

Diamagnetic materials develop a weak magnetic response opposite to the applied field and are weakly repelled.

Their magnetisation disappears when the external field is removed.

Paramagnetic materials

Paramagnetic materials are weakly attracted by an external magnetic field.

Their atomic dipoles tend to align partially with the applied field, but thermal motion prevents complete alignment.

Ferromagnetic materials

Ferromagnetic materials can become strongly magnetised.

They contain regions called domains whose magnetic moments may align under an external field.

Examples include iron, cobalt and nickel.

Effect of temperature

Temperature affects magnetic ordering.

Increased thermal motion can weaken alignment. Above a characteristic temperature, a ferromagnetic material may lose its strong ferromagnetic behaviour and become paramagnetic.

Common student difficulties

Students frequently:

• Treat magnetic poles like isolated electric charges

• Reverse the direction of magnetic dipole moment

• Draw magnetic field lines that start or end abruptly

• Confuse magnetic field with magnetisation

• Mix diamagnetic, paramagnetic and ferromagnetic properties

• Assume every magnetic material retains magnetism permanently

• Ignore the effect of temperature

Learning with Teachoo

Teachoo provides:

• Bar-magnet and solenoid comparisons

• Field-line diagrams

• Dipole orientation explanations

• Material-classification tables

• NCERT solutions

• MCQs and assertion-reason questions

• Application-based questions

Frequently Asked Questions

Can a magnetic north pole exist alone?

An isolated magnetic pole has not been observed in ordinary magnetism. Cutting a magnet produces smaller dipoles.

What is the direction of magnetic dipole moment?

For a bar magnet, it is directed from the south pole towards the north pole inside the magnet.

How do magnetic field lines differ from electric field lines?

Magnetic field lines form closed loops. Electrostatic field lines begin on positive charges and end on negative charges or at infinity.

What is the difference between diamagnetic and paramagnetic materials?

Diamagnetic materials are weakly repelled and respond opposite to the applied field. Paramagnetic materials are weakly attracted and align partially with the field.

Why does temperature affect magnetism?

Thermal motion disrupts the alignment of magnetic dipoles or domains.