CBSE Class 12 Physics Revision Notes Chapter 5: Magnetism and Matter

A magnet produces a magnetic field and behaves as a dipole with inseparable north and south poles.
This chapter explains magnetic dipoles, magnetic field lines and the response of different materials to external magnetic fields.

Magnetism and Matter examines the basic properties of magnets and magnetic materials. It explains how a bar magnet behaves in a magnetic field and why its field resembles that of a current-carrying solenoid.

Use these CBSE Class 12 Physics Revision Notes Chapter 5 for the 2026–27 session. Begin with bar magnets and magnetic field lines. Then revise dipole formulas, Gauss’s law, magnetisation, susceptibility, permeability and the classification of magnetic materials.

Key Takeaways

• Magnetic poles: Isolated north or south magnetic poles have not been observed.
• Dipole torque: τ = mB sin θ acts on a magnetic dipole in a uniform field.
• Gauss’s law: The net magnetic flux through any closed surface is zero.
• Magnetic materials: Susceptibility determines whether a material is diamagnetic, paramagnetic or ferromagnetic.

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Bar Magnet and Magnetic Field Lines in Class 12 Physics Chapter 5 Notes

A bar magnet is a magnetic dipole with a north pole and a south pole. When suspended freely, it points approximately in the geographic north-south direction.

Basic Properties of Magnets

The main properties of magnets are:

• A freely suspended magnet aligns approximately north-south.
• Like magnetic poles repel each other.
• Unlike magnetic poles attract each other.
• Magnets attract magnetic substances such as iron.
• Magnetic poles always occur in pairs.
• An isolated magnetic north or south pole has not been observed.
• Cutting a magnet produces smaller magnets with both poles.

A bar magnet may be cut across its length or along its length. In both cases, every piece behaves as a complete magnet.

Magnetic Field Lines

Magnetic field lines visually represent the magnetic field around a magnet.

Their main properties are:

• They form continuous closed loops.
• Outside a bar magnet, they move from the north pole to the south pole.
• Inside the magnet, they move from the south pole to the north pole.
• The tangent at any point gives the magnetic-field direction.
• Closely spaced field lines indicate a stronger field.
• Widely spaced field lines indicate a weaker field.
• Two magnetic field lines never intersect.

If two field lines intersected, the magnetic field would have two different directions at the same point.

Magnetic and Electric Field Lines

Bar Magnet as an Equivalent Solenoid

The field pattern around a finite current-carrying solenoid resembles the field pattern around a bar magnet.

This explains the idea of a bar magnet as an equivalent solenoid.

A solenoid has:

• One end behaving like a north pole
• One end behaving like a south pole
• Closed magnetic field lines
• A magnetic dipole moment

Ampere proposed that magnetic behaviour inside matter can be explained through microscopic circulating currents.

Cutting a bar magnet is similar to cutting a solenoid. Each resulting part continues to have a north and a south pole.

Magnetic Dipole in a Uniform Field Revision Notes

A magnetic dipole placed in a uniform magnetic field experiences a torque. The net force on it remains zero.

Magnetic Dipole Moment

The magnetic dipole moment is represented by m.

Its direction is from the south pole to the north pole through the magnet.

Its SI unit is:

A m²

A current loop also behaves as a magnetic dipole.

For a current loop:

m = NIA

Here:

• N is the number of turns.
• I is the current.
• A is the area of the loop.

Torque on a Magnetic Dipole

For a magnetic dipole in a uniform magnetic field:

τ = m × B

Its magnitude is:

τ = mB sin θ

Here, θ is the angle between magnetic moment m and magnetic field B.

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

A magnetic dipole in a uniform field experiences torque but no net force.

In a non-uniform field, it may experience both force and torque.

Magnetic Potential Energy

The magnetic potential energy of a dipole in a uniform magnetic field is:

U = −m·B

Therefore:

U = −mB cos θ

The zero of potential energy is usually taken at θ = 90°.

The dipole has minimum potential energy when it is parallel to the field.

It has maximum potential energy when it is antiparallel to the field.

Axial and Equatorial Magnetic Field in Magnetism and Matter Class 12 Notes

At points far from a short bar magnet, its magnetic field depends on its magnetic moment and the distance from its centre.

Let:

• m be the magnetic dipole moment.
• r be the distance from the centre.
• r be much greater than the length of the magnet.

Magnetic Field on the Axial Line

The magnetic field on the axial line is:

BA = (μ₀/4π) × 2m/r³

Its direction is along the magnetic dipole moment.

The field varies as:

BA ∝ 1/r³

Magnetic Field on the Equatorial Line

The magnetic field on the equatorial line is:

BE = −(μ₀/4π) × m/r³

The negative sign shows that the field is opposite to the magnetic dipole moment.

Its magnitude is:

BE = (μ₀/4π) × m/r³

Axial and Equatorial Magnetic Field Comparison

At the same distance:

BA = 2BE

This relation compares the magnitudes of the axial and equatorial magnetic field.

Electric and Magnetic Dipole Analogy

The formulas are mathematically similar. However, isolated electric charges exist, while isolated magnetic poles have not been observed.

Gauss’s Law for Magnetism in CBSE Class 12 Physics Chapter 5 Notes

Magnetic field lines form closed loops. Every field line entering a closed surface also leaves it.

Magnetic Flux

Magnetic flux through a small area element dS is:

dΦB = B·dS

The total magnetic flux through a surface is:

ΦB = ∫B·dS

The SI unit of magnetic flux is weber.

1 Wb = 1 T m²

Gauss’s Law for Magnetism

Gauss’s law for magnetism states that the net magnetic flux through any closed surface is zero.

Mathematically:

∮B·dS = 0

This law applies to every closed surface, regardless of its shape or size.

Meaning of Zero Net Magnetic Flux

Zero net flux does not mean that the magnetic field is zero.

It means:

• The number of field lines entering a closed surface equals the number leaving it.
• Magnetic field lines have no beginning or end.
• Magnetic fields have no sources or sinks.
• Isolated magnetic monopoles have not been observed.

Electric and Magnetic Gauss’s Laws

Magnetisation and Magnetic Intensity in Class 12 Physics Magnetism Notes

Atoms and molecules may possess magnetic moments. Their combined effect determines the response of a material to an external magnetic field.

Magnetisation

Magnetisation is the net magnetic dipole moment per unit volume.

M = mnet/V

Here:

• M is magnetisation.
• mnet is the net magnetic moment.
• V is the volume of the sample.

Magnetisation is a vector quantity.

Its SI unit is:

A/m

Its dimensional formula is:

[M] = [A L⁻¹]

Magnetic Intensity

Magnetic intensity H represents the magnetic effect produced by external sources such as a current-carrying solenoid.

It is defined as:

H = B/μ₀ − M

Therefore:

B = μ₀(H + M)

Both H and M are measured in A/m.

For an empty long solenoid:

H = nI

and:

B₀ = μ₀H

Magnetisation and Magnetic Intensity Comparison

The relationship between magnetisation and magnetic intensity helps describe the behaviour of magnetic materials.

Magnetic Susceptibility and Permeability Chapter 5 Physics Notes

Magnetic susceptibility and permeability measure how a material responds to an applied magnetic field.

Magnetic Susceptibility

For a linear magnetic material:

M = χmH

Here, χm is the magnetic susceptibility.

It is a dimensionless quantity.

Its sign and magnitude indicate the material type:

• Negative for diamagnetic substances
• Small and positive for paramagnetic substances
• Large and positive for ferromagnetic substances

Relative Magnetic Permeability

Using:

B = μ₀(H + M)

and:

M = χmH

we get:

B = μ₀(1 + χm)H

Therefore:

B = μ₀μrH

Here, μr is the relative magnetic permeability.

The relation is:

μr = 1 + χm

Relative permeability is dimensionless.

Magnetic Permeability

The magnetic permeability of a material is:

μ = μ₀μr

Therefore:

μ = μ₀(1 + χm)

The magnetic field inside a linear material is:

B = μH

Magnetic permeability has the same SI unit as μ₀:

T m A⁻¹

Diamagnetic Materials in Magnetism and Matter Revision Notes

Diamagnetic materials develop a weak magnetic moment opposite to the applied field.

They are weakly repelled by a magnet.

Properties of Diamagnetic Materials

• Magnetic susceptibility is negative.
• Relative permeability is less than 1.
• Permeability is less than μ₀.
• Magnetisation is opposite to the applied field.
• Magnetic field lines are slightly expelled.
• They move from stronger magnetic-field regions to weaker regions.
• Their resultant atomic magnetic moment is zero without an applied field.

Mathematically:

−1 ≤ χm < 0

0 ≤ μr < 1

μ < μ₀

Examples of Diamagnetic Materials

Examples include:

• Bismuth
• Copper
• Lead
• Silicon
• Water
• Sodium chloride
• Nitrogen at standard conditions

Diamagnetism is present in all substances, but stronger magnetic effects may hide it.

Perfect Diamagnetism

A superconductor may behave as a perfect diamagnet.

For a perfect diamagnet:

χm = −1

μr = 0

Magnetic field lines are completely expelled. This phenomenon is called the Meissner effect.

Paramagnetic Materials in Class 12 Magnetism and Matter Revision Notes

Paramagnetic materials become weakly magnetised in the direction of an external magnetic field.

They are weakly attracted by a magnet.

Properties of Paramagnetic Materials

• Magnetic susceptibility is small and positive.
• Relative permeability is slightly greater than 1.
• Permeability is slightly greater than μ₀.
• Atoms or molecules have permanent magnetic dipole moments.
• Thermal motion keeps these dipoles randomly oriented without a field.
• An external field causes partial alignment.
• They move from weaker field regions to stronger field regions.

Mathematically:

χm > 0 but small

μr > 1

μ > μ₀

Examples of Paramagnetic Materials

Examples include:

• Aluminium
• Sodium
• Calcium
• Oxygen at standard conditions
• Copper chloride

Effect of Field and Temperature

Magnetisation increases when:

• The applied magnetic field becomes stronger.
• The temperature decreases.

At strong fields and low temperatures, the magnetic dipoles may approach complete alignment.

Ferromagnetic Materials in CBSE Class 12 Magnetism and Matter Notes

Ferromagnetic materials become strongly magnetised in an external magnetic field.

They are strongly attracted towards regions of stronger magnetic field.

Properties of Ferromagnetic Materials

• Magnetic susceptibility is large and positive.
• Relative permeability is much greater than 1.
• Permeability is much greater than μ₀.
• Atomic magnetic moments interact strongly.
• The material contains magnetic domains.
• External fields align and enlarge domains oriented along the field.

Mathematically:

χm >> 1

μr >> 1

μ >> μ₀

Magnetic Domains

A magnetic domain is a region in which many atomic dipoles are aligned in one direction.

In an unmagnetised ferromagnetic substance:

• Individual domains are magnetised.
• Domain directions are random.
• Their magnetic moments cancel.
• The net magnetisation is nearly zero.

When an external field is applied:

• Domains aligned with the field grow.
• Other domains rotate towards the field.
• Strong magnetisation develops.

Examples of Ferromagnetic Materials

Examples include:

• Iron
• Cobalt
• Nickel
• Gadolinium

Hard Ferromagnetic Materials

Hard ferromagnetic materials retain their magnetisation after the external field is removed.

They are used to make permanent magnets.

Examples include:

• Alnico
• Lodestone
• Certain steel alloys

They resist demagnetisation.

Soft Ferromagnetic Materials

Soft ferromagnetic materials lose most of their magnetisation after the external field is removed.

They are easily magnetised and demagnetised.

Soft iron is a common example.

These materials are useful where repeated magnetisation and demagnetisation are required.

Effect of Temperature on Ferromagnetism

Ferromagnetic behaviour weakens as temperature rises.

At a sufficiently high temperature:

• Domain alignment becomes disordered.
• Ferromagnetic behaviour disappears.
• The substance becomes paramagnetic.

The temperature above which a ferromagnetic material becomes paramagnetic is called the Curie temperature.

Magnetic Materials Comparison

Magnetism and Matter Formula Notes

Important Terms in Class 12 Physics Chapter 5

Access Class 12 Physics Chapter 5 Magnetism and Matter Notes in 30 Minutes

Divide the chapter into three revision blocks:

• First 10 minutes: Bar magnets, magnetic field lines and dipole formulas
• Next 10 minutes: Gauss’s law, magnetisation, magnetic intensity and permeability
• Final 10 minutes: Diamagnetic, paramagnetic and ferromagnetic materials

During formula revision, remember that axial magnetic field is twice the equatorial field at the same distance. Also distinguish between susceptibility, which is dimensionless, and magnetisation and magnetic intensity, which are measured in A/m.

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