Physics Magnetism & Matter Notes Class 12 PDF

If you’re finding Magnetism and Matter Class 12 Notes a bit tricky to understand, don’t worry, most students feel the same, especially before exams. This chapter from Class 12 Physics Syllabus mixes theory with formulas and that’s exactly where the confusion begins. 

In this blog, we are going to have detailed Notes of Magnetism and Matter and analysis of the chapter,  highlighting important concepts, formulas and applications.  

At the end we will also discuss some important practice questions from the chapter that will help students in increasing their understanding of the chapter.

Magnetism and Matter Class 12 Notes

Magnetism and Matter is one of the most interesting chapters in Class 12 Physics because it explains why materials behave differently in a magnetic field. 

You get to understand why iron sticks strongly to a magnet, why some materials are weakly attracted and why others are repelled. 

The chapter introduces simple but fascinating ideas like magnetic dipoles, Earth behaving like a giant magnet and how everyday materials become magnetised. 

Concepts such as diamagnetism, paramagnetism and ferromagnetism are easy to relate to real life, making the chapter less theoretical and more practical. 

What is Magnetism?

Magnetism is a force that can attract or repel objects without touching them. It mostly comes from the motion of electric charges, especially electrons inside atoms. When the magnetic effects of many electrons line up in the same direction, the material becomes magnetized.

Every magnet has a magnetic field around it, which we can represent using magnetic field lines. These lines tell us two things:

• Direction: Which way the magnetic field is pointing
• Strength: How strong the field is (closer lines mean a stronger field)

Magnetic fields aren’t just around permanent magnets. They also exist around moving charges and current-carrying wires, which is why electricity and magnetism are so closely connected.

Magnetism is all about moving charges and how their motion creates forces that act over a distance.

1. Magnetic Materials

Not all materials behave the same in a magnetic field - some barely react, while others stick strongly. That’s why scientists divide materials into three types:

2. Diamagnetic Materials

These guys are kind of “meh” when it comes to magnetism - they’re weakly pushed away by magnets. Their magnetic response is very small and negative. Think of bismuth, copper, gold, or even water - they don’t really care about magnets.

3. Paramagnetic Materials

Paramagnetic materials are a bit more friendly - they’re weakly attracted to magnets. They get magnetized when a field is applied but lose it as soon as the field is gone. Examples include aluminium, platinum, and oxygen.

4. Ferromagnetic Materials

These are the “super fans” of magnets. They’re strongly attracted and can even stay magnetized, which is why we can make permanent magnets from them. Common examples are iron, cobalt, and nickel.

Magnetic Field and Magnetic Field Lines

A magnetic field is the region around a magnet where magnetic forces can be felt. Imagine it as an invisible space where the magnet can pull or push other magnetic materials. The strength and direction of this field are shown using magnetic field lines.

Some key points about magnetic field lines:

• Direction: Field lines emerge from the north pole and enter the south pole of a magnet.
• Density: The closer the lines are, the stronger the magnetic field at that spot.
• No Crossing: Magnetic lines never cross each other. If they did, it would mean the magnetic field points in two directions at once, which is impossible.
• Closed Loops: Field lines always form complete loops - from the north pole to the south pole outside the magnet and back through the magnet itself.

In simple terms, these lines give a visual idea of where the magnetic force is strongest and which way it’s pointing.

Magnetic Force on a Moving Charge

When a charged particle moves through a magnetic field, it experiences a force called the Lorentz force. This force is always perpendicular to both the particle’s velocity and the magnetic field.

The formula for this force is: F = q (v × B), where:

• F = magnetic force
• q = charge of the particle
• v = velocity of the particle
• B = magnetic field

If you want the magnitude of this force, it can be calculated using: F = q × v × B × sin θ

Here, θ is the angle between the velocity vector of the particle and the magnetic field.

Important points to remember:

• The force is maximum when the particle moves perpendicular to the magnetic field (θ = 90°).
• The force is zero if the particle moves parallel or anti-parallel to the field (θ = 0° or 180°).
• Since the force is always perpendicular to the velocity, it doesn’t change the particle’s speed, only its direction.

This perpendicular action is why moving charges often follow circular or helical paths in a magnetic field.

Magnetic Moment of a Magnet

The magnetic moment is a vector quantity that tells us how strong a magnet is and the direction of its magnetic field. It depends on the size of the magnet and how the poles are oriented.

You can calculate it using: M = m × l

Where:

• M = magnetic moment
• m = strength of each magnetic pole
• l = distance between the north and south poles

The direction of the magnetic moment is from the south pole to the north pole of the magnet.

Key idea: A larger pole strength or a bigger distance between poles means a stronger magnetic moment, which produces a stronger magnetic field.

Magnetic Intensity and Magnetic Field Strength

When we talk about a magnetic field, two important quantities come into play: magnetic field strength (H) and magnetic induction (B).

• Magnetic field strength (H) tells us how strong the external magnetic field is that tries to magnetize a material.
• Magnetic induction (B), also called magnetic flux density, tells us the total magnetic effect inside the material, including how the material itself responds.

These two are related by: B = μ₀ (H + M)

Where:

• B = magnetic induction (in tesla, T)
• H = magnetic field strength (in amperes per meter, A/m)
• M = magnetization of the material (in A/m)
• μ₀ = permeability of free space

Key points:

• In a vacuum, B and H are proportional, since there’s no material to magnetize.
• When a material is present, its magnetization (M) adds to the total magnetic induction.

Remember, H is the “applied field” and B is the “total field” inside the material.

Earth’s Magnetic Field

Think of the Earth as one giant magnet! The planet has a magnetic field all around it, and this field is created by the flow of molten iron in the Earth’s outer core. These moving charges produce electric currents, which in turn generate the Earth’s magnetic field.

1. The magnetic poles of the Earth are close to the geographic poles, but not exactly the same - there are slight differences in their positions.

2. The strength of the magnetic field isn’t the same everywhere: it’s strongest near the poles and weakest near the equator.

Why does this matter?

• This magnetic field is what makes compasses work, as the needle aligns itself along the Earth’s magnetic field lines.
• It also protects us by deflecting charged particles from the Sun, forming part of the Earth’s natural shield.

Magnetic Effects of Current

Electric current doesn’t just power devices - it also creates a magnetic field around the conductor it flows through. This is why current-carrying wires can behave like tiny magnets.

Ampère's Circuital Law explains this relationship:

The line integral of the magnetic field (B) along a closed loop around a conductor is equal to the product of the permeability of free space (μ₀) and the current (I) passing through the loop. 

In simple terms:

• B → magnetic field produced
• dl → a small segment of the path around the conductor
• I → current flowing through the conductor
• μ₀ → permeability of free space

Direction of the Magnetic Field: Use the Right-Hand Thumb Rule

• Point your thumb in the direction of the current.
• Curl your fingers around the wire.
• Your fingers now show the direction of the magnetic field lines around the conductor.

Current produces a circular magnetic field around a wire, and its direction can be easily visualized using your right hand. This principle is the foundation for electromagnets, motors, and other devices.

Bar Magnet as an Equivalent Solenoid

A bar magnet can be thought of as a current-carrying solenoid because the magnetic field patterns of both are very similar. In both cases, the magnetic field lines form closed loops:

• Inside the magnet or solenoid: The field lines are straight, parallel, and strong, giving a uniform magnetic field.
• Outside the magnet or solenoid: The field lines spread out and curve back from one end to the other.

One end of the magnet behaves like the North Pole, and the other like the South Pole. Because of this similarity, a bar magnet can be treated as an equivalent solenoid in physics problems.

Magnetic Field at Axial Point

The axial point is a point along the length of the bar magnet, on its axis. Key points:

• The magnetic field is strongest at the axial point.
• The direction of the field is along the axis of the magnet, from South to North outside the magnet.

Formula for axial point (doc-friendly version): Magnetic field BBB = (μ₀ / 4π) × (2 × Magnetic Moment / r³)

Where:

• μ₀ = permeability of free space
• Magnetic Moment (M) = strength × length of the magnet
• r = distance from the center of the magnet

Important points:

• The field strength decreases rapidly as you move away from the magnet (proportional to 1/r³).
• The axial field is stronger than the field at the equatorial point.

Magnetic Field at Equatorial Point

The equatorial point is located on the perpendicular bisector of the magnet, meaning it lies sideways, at a right angle to the magnet’s length.

1. The magnetic field here is weaker compared to the axial point.

2. The direction of the field is opposite to that at the axial point, going from North to South inside the magnet.

Formula for equatorial point (do: Magnetic field BBB = (μ₀ / 4π) × (Magnetic Moment / r³)

Where:

• μ₀ = permeability of free space
• Magnetic Moment (M) = strength × length of the magnet
• r = distance from the center of the magnet

The equatorial field is roughly half the strength of the axial field at the same distance. Like the axial field, the strength decreases rapidly with distance (proportional to 1/r³).

Magnetisation and Magnetic Intensity

Learning magnetisation is crucial for Class 12 Physics because it explains how materials respond to a magnetic field. Let’s break it down in a way that’s easy to grasp.

1. What is Magnetisation?

Magnetisation tells us how strongly a material is magnetised when placed in a magnetic field. It essentially measures the alignment of tiny magnetic dipoles inside the material along the direction of the applied field.

Formula: M = Magnetic dipole moment ÷ Volume

• SI Unit: Ampere per metre (A/m)
• Key Point: A higher value of M means the material is more strongly magnetised.

Magnetisation depends on:

• The nature of the material (some materials are easier to magnetise)
• The strength of the applied magnetic field

2. Magnetic Intensity (H)

Magnetic intensity represents the magnetising force applied to a material. It tells us how strong the external magnetic field is that tries to align the magnetic dipoles.

• Unlike the magnetic field B, H depends only on external sources, not on the material itself.

Formula: H = (B / μ₀) – M
Where:

• B = magnetic field inside the material
• μ₀ = permeability of free space
• M = magnetisation

Important Points About H:

• SI Unit: Ampere per metre (A/m)
• H exists even in vacuum
• H is responsible for producing magnetisation M in materials
• Alternative relation: H = (B / μ₀) + M

3. Magnetic Susceptibility (χ)

Magnetic susceptibility measures how easily a material gets magnetised when placed in a magnetic field.

• Formula: χ = M / H
• No unit (dimensionless)
• Higher χ → material is more easily magnetised
• χ depends on the nature of the material and temperature

Magnetic susceptibility is especially useful when comparing diamagnetic, paramagnetic, and ferromagnetic materials, which we will cover next.

Types of Magnetic Materials Permeability

Magnetic Materials behave differently in a magnetic field. This depends on how their tiny magnetic dipoles (caused by electrons) respond to the field. Based on this, materials are divided into three main types:

1. Diamagnetic Materials

These materials are weakly repelled by a magnetic field. Their magnetic dipoles oppose the external field.

• They do not retain magnetism when the field is removed.
• Examples: Bismuth, Copper, Gold, Water

2. Paramagnetic Materials

Paramagnetic materials are weakly attracted to a magnetic field. Their dipoles align slightly with the field.

• They lose their magnetization when the external field is removed.
• Examples: Aluminium, Platinum, Oxygen

3. Ferromagnetic Materials

Ferromagnetic materials are strongly attracted by a magnetic field.

• Their dipoles tend to align permanently, so these materials can become permanent magnets.
• Examples: Iron, Cobalt, Nickel
• Uses: Transformer cores, electromagnets, permanent magnets

Tip: Think of it like this - diamagnetic materials push away, paramagnetic materials follow a little, and ferromagnetic materials “stick strongly.”

Permeability of Materials

Permeability basically tells us how easily a magnetic field can pass through a material. Think of it like water flowing through pipes - some materials let the field “flow” easily, while others resist it.

1. Absolute Permeability (μ)

This is the actual ability of a material to carry a magnetic field. You calculate it using: μ = B / H

Where B is the magnetic field inside the material and H is the applied field.

• If μ is high → field passes easily
• If μ is low → material resists the field
• Unit: Henry per meter (H/m)
• For vacuum, μ₀ = 4π × 10⁻⁷ H/m

So basically, a material with high absolute permeability is “magnet-friendly.”

2. Relative Permeability (μᵣ)

Relative permeability is how good a material is compared to vacuum:

μᵣ = μ / μ₀

• μᵣ = 1 → behaves like vacuum (field passes as if no material)
• μᵣ > 1 → boosts the magnetic field (like iron, nickel)-+
• μᵣ < 1 → weakens the field (like bismuth, copper)

3. Relation with Magnetic Susceptibility (χ)

Magnetic susceptibility tells us how easily a material gets magnetized. The connection is:

χ = μᵣ − 1

• χ < 0 → diamagnetic → weakly repels magnetic field
• χ > 0 (small) → paramagnetic → weakly attracted
• χ >> 1 → ferromagnetic → strongly attracted

Quick tip: Ferromagnetic materials (iron, cobalt, nickel) have very high μᵣ and χ. That’s why they’re used in transformers, motors, and electromagnets.

FAQs

Q1. What is the magnetic moment of a bar magnet?

Ans. A magnetic moment basically tells you how strong a magnet is and which direction it points. Think of it as the “strength arrow” of the magnet.

Q2. What are Earth’s magnetic elements?

Ans. Earth has its own magnetic field, and we measure it using three main elements:

• Magnetic inclination  the angle the field makes with the horizontal
• Magnetic declination – the angle between geographic north and magnetic north
• Horizontal component – the strength of the field along the surface

Q3. What is the difference between B and H?

Ans. Both are magnetic fields but slightly different:

• B (magnetic induction) = total magnetic field inside a material (includes material’s response)
• H (magnetic intensity) = the external field applied, depends only on the source, not the material

Q4. Which materials are ferromagnetic?

Ans. Materials that get strongly magnetized in a magnetic field. Common ones are iron, cobalt, and nickel.

Q5. What are magnetic field lines?

Ans. Imaginary lines that show the direction and strength of a magnetic field. The closer the lines, the stronger the field; the lines never cross and always form closed loops from north to south pole.