NCERT Class 12 Physics Chapter 5 Notes Magnetism and Matter - Download PDF

The NCERT Class 12 Physics textbook's chapter 'Magnetism and Matter' explores a variety of fundamental concepts and phenomena related to magnetism. This Magnetism And Matter class 12 notes cover a wide range of topics, including the history of magnetism, magnet properties, magnetic field lines, the behaviour of circular and current loops in magnetic fields, the properties of solenoids and bar magnets, dipole interactions in magnetic fields, potential energy considerations, exploration of Earth's magnetism, magnetic materials, and hysteresis. While the class 12 physics chapter 5 notes provide detailed explanations of these topics and basic equations, they lack important derivations. These physics class 12 chapter 5 notes pdf versions are available for free of charge.

Also, students can refer,

• NCERT Notes Class 12 Physics
• NCERT Solutions for Class 12 Physics Chapter 5 Magnetism and Matter
• NCERT Exemplar Class 12 Physics Chapter 5 Solutions Magnetism and Matter

NCERT Class 12 Physics Chapter 5 Notes

Magnetism and Matter:

• Magnetism can be defined as the phenomenon due to which certain substances attract pieces of steel, iron, nickel etc.

• We find the use of magnets in many devices like an electric bell, telephone, radio, loudspeaker, motors, fans, screwdrivers, lifting heavy iron loads, super-fast trains especially in foreign countries, refrigerators etc.

• Magnetite is considered to be the world’s first magnet. It is also known as a natural magnet. Magnets occur naturally, but we can impart magnetic property on a substance as well. Doing so will create an artificial magnet.

History of magnets

• During 600 BC in Greece, it was observed by the shepherds that their wooden shoes which had iron nails used to strike at some places on the ground.

• This was due to an island in Greece called magnesia which had magnetic ore deposits. The word magnet was taken from there.

• The technological use of magnets began around 400 BC by the Chinese. When a thin piece of magnet was suspended freely it always used to point towards the North-South direction. This phenomenon was used by the Chinese emperor Huang-ti to win a war.

Magnet Properties

• The Earth behaves like a magnet.

• When a bar magnet is freely suspended, it points towards the geographical North-South direction.

• Like poles repel one another and in contrast to unlike poles attract one another..

• There is no existence of Magnetic monopoles which suggests we tend to not have a magnet with the North pole alone or South pole alone.

• If a bar magnet is broken into two halves, we will get two bar magnets that are similar with weaker properties.

• Using iron and its alloys, magnets can be made.

Magnetic Field Lines

• When we sprinkle iron filings on a sheet of glass that is placed over a short bar magnet, then a pattern is observed. The pattern below shows that the magnet has two poles.

• The magnetic field lines inside and around the magnet are imaginary lines.

Properties of the Magnetic Field Lines

• The field lines are continuous outside the magnet. They are considered to originate from the North pole and terminate at the South pole.

• They are kind of closed loops traversing within the magnet, however, the lines appear to originate from the South Pole and terminate at the North pole to create closed loops.

• More range of shut lines indicates stronger flux of the magnet.

• The field lines never intersect each other.

• The tangent drawn at the sector line provides the direction of the flux at that time.

These are the properties of the magnetic field lines according to Class 12 Magnetism and Matter notes.

Circular Current Loop in the Uniform Magnetic Field

In the chapter, moving charges and magnetism, we discussed the current-carrying circular conductor.

The current-carrying circular loop of N turns is similar to a magnetic dipole. In a current-carrying circular loop, if we see from one side, say the right side, the current appears to move in a clockwise direction. This is like South polarity. If we see it from the opposite face, say left face, this seems to manoeuvre within the anticlockwise direction that is like North polarity.

The dipole moment of a current-carrying loop is given by

M = IA

where

I is the current

A is the area of cross-section of the coil

If there are N such turns of the coil, then the Magnetic dipole moment will be, M = NIA

The expression for the moment in the case of the current-carrying loop with N turns is quite similar to the rectangular loop placed in a uniform magnetic field with area vector A. In both cases, m = NIA

Solenoid and Bar Magnet

The magnetic field at a far axial point of a solenoid:

Let us consider

• 2l to be the length of the solenoid

• a to be the radius of the solenoid

• n to be the number of turns / unit length

• r to be the distance of the point P from the centre of the solenoid O

The magnetic moment of the solenoid is given by (total number of turns X current X cross-sectional area) which is

A bar magnet is considered to be as a large number of circulating currents analogous to a solenoid.

Dipole in a Uniform Magnetic Field

Let a bar magnet NS having pole strength ‘m’ and of length 2lbe placed in a uniform magnetic field of strength B making an angle with the direction of the magnetic field.

Force on N-pole of the magnet =mB(along the direction of magnetic field B)

Force on S-pole of the magnet =mB(opposite to the direction of magnetic field B)

Thus, the bar magnet is acted upon by two equal, parallel and opposite forces. The two forces constitute a torque and it tends to rotate the magnet in the clockwise direction. The magnitude of the torque is given by

Torque= either force x perpendicular distance between the two forces

Since, m2l=M, the magnetic dipole moment of the bar magnet, we have

If B=1and θ=90^o i.e.

Hence, the magnetic dipole moment of a magnetic dipole is numerically equal to the torque acting on the dipole, when placed perpendicular to the direction of a uniform magnetic field of unit strength.

Unit of magnetic dipole moment

Unit of magnetic dipole moment is

.

Potential Energy of a Bar Magnet Placed in a Magnetic Field

Let a bar magnet of dipole moment M be placed in a uniform magnetic field of strength B, such that the magnet makes an angle with the direction of the field. Then, the magnitude of the torque acting on the dipole is given by

This torque tends to align the magnet along the direction of the field. If the magnet is rotated against the action of this torque, work has to be done. Suppose that the magnet is rotated through an infinitesimally small angle dθ,

If the magnet is rotated from the initial position θ= θ₁ to the final position θ= θ₂, then the total work done is given by

The work done in rotating the magnet is stored inside the magnet as its potential energy (U). Thus, the potential energy of the magnet inside the magnetic field.

Suppose that the magnet is initially perpendicular to the direction of the magnetic field, i.e. θ₁=90^o. Then, the potential energy of the magnet in any position making angle θ with the direction of the field can be obtained by setting θ₁=90^o and θ₂=θ in the equation

Gauss’ Law in Magnetism

According to Gauss’ law in electrostatic, the surface integral of electric field Earound a surface S which is closed is equal to 1/ϵ₀times the total charge q which is enclosed by the surface i.e.

An isolated magnetic pole does not exist. The magnetic analogue of Gauss’ law in electrostatic may be stated as

Earth’s Magnetism

The strength of the earth’s magnetic field, on the surface of the earth, varies from place to place and is of order 10^-5 Tesla.

Dynamo Effect

There are many theories on earth’s magnetic field, but out of them, the dynamo effect seems to be the most accepted one.

• The earth has three parts: core, mantle and crust.

• In the outer core, there's liquefied iron and nickel

• The convective motion of those bronze fluids ends up in electrical currents.

• The magnetic field occurs due to these electrical currents.

Magnetic lines of the earth

Let us consider a magnetic dipole present in the centre of the earth. Now, when we are drawing magnetic field lines, then we will observe that the magnetic lines of the earth resemble the same as of a magnet.

Magnetic poles:

The dipole does not coincide with the axis of rotation of earth and is tilted by approximately 11.3^oHence, apart from the geographic North Pole N_g and South Pole S_g, there is a magnetic North pole N_m and magnetic South Pole S_m.

Elements of Earth’s Magnetic Field

The 3 elements of the earth’s magnetic field are –

(1) Angle of declination (α)

(2) Angle of dip (δ) and

(3) Horizontal component of earth’s magnetic field (H_e)

The angle of declination (α):

The angle of declination is the angle between the geographical meridian and magnetic meridian.

This angle is smaller at the equator and greater at higher latitudes. At the equator, we find the magnetic meridian closer to the geographic meridian.

The angle of dip/inclination (δ):

The angle between the total of earth’s magnetic field (B_e) making with the surface of the earth in the magnetic meridian is known as the angle of dip.

The horizontal component of earth’s magnetic field (H_e)

The total magnetic field on the surface of the earth can be divided into two components i.e. horizontal component He and vertical component Ve. The angle between (B_e) and (H_e) is known as the angle of dip δ

Hence,

Some Important Terms:

Magnetism and Matter Class 12 notes include some of the important terms of the chapter. They are as follows:

Magnetization M:

As we know, the nucleus of the atom consists of Neutrons and charged protons, the electrons that are charged revolve around the nucleus. Thus, the circulating electron in an atom will have a magnetic moment.

Material is made of many atoms and it will have multiple magnetic moments. These magnetic moments add up in vector form and give a net magnetic moment which is non-zero.

Magnetisation

which is net magnetic moment per unit volume. Its unit is A/m²

Magnetic Intensity / Magnetising force H

Let us consider a solenoid of n turns per unit length and carrying a current I.

The inner part of the solenoid is filled with a material that has non-zero magnetisation (M).

Dividing by μ₀

This change in the magnetic field with permeability is called Magnetic intensity.

H depends on external factors like current flowing etc.

M depends on the material kept inside the solenoid

Susceptibility :-

In the expression,

M depends on external factors as well.

Hence,

X(khi) which is a dimensionless quantity, is called Susceptibility. It tells the response of magnetic material to an external field. It is a dimensionless quantity.

Permeability:-

μ is called the magnetic permeability of the substance.

µ_r is called the relative magnetic permeability of the substance.

Its unit is Tm/A.

Magnetic Materials

Based on the magnetic behaviour of different magnetic materials, Faraday divided the magnetic materials into three categories:-

1. Diamagnetic:- The substances placed in a magnetic field are feebly magnetized in opposite direction to the magnetizing field are called diamagnetic substances.

When a diamagnetic substance is placed inside an external magnetic field, then the magnetic field inside the diamagnetic field is slightly less than the external magnetic field. When diamagnetic material is kept inside a non-uniform magnetic field, it prefers to move from the stronger magnetic field to the weaker magnetic field. Despite the application of a strong magnetic field, the diamagnetic effects are weak to be detected. Some of the examples of diamagnetic substances include zinc, gold, copper, bismuth, silver, lead, glass, marble, helium, etc

A diamagnetic substance has a small negative value for the magnetic susceptibility

The order is of 10^-6 to 10^-3(negative).

The status of a magnetic force substance doesn't modify with temperature for sensible functions. However, at low temperature bismuth is an exception.

2. Paramagnetic:- The substances, in the direction of the magnetizing field, are weakly magnetized and known as paramagnetic substances.

If the paramagnetic substance is kept inside an external magnetic field, it is observed that the magnetic field inside the substance is slightly greater than the external magnetic field. A paramagnetic substance, when placed in a non-uniform magnetic field, tends to move from the weaker part of the magnetic field to the stronger part. When a strong magnetic field is there, then the paramagnetic effects are perceptible. Examples of paramagnetic substances are aluminium, sodium, antimony, platinum, copper chloride, liquid oxygen etc.

Paramagnetic substances has a small positive value for the magnetic susceptibility

It is of the order of 10^-5 to 10^-3.

The susceptibility of paramagnetic substances is generally inversely proportional to their absolute temperature.

3. Ferromagnetic:- Those substances, once placed in an exceedingly field area unit powerfully magnetic within the direction of the magnetizing field, are area units known as magnetic force substances.

When a magnetic force substance is placed within an associate degree external field, the field within the ferromagnetic field is found to be greatly increased than the external field. As a result, once a magnetic force substance is placed in an exceedingly non-uniform field, it quickly moves from the weaker half to the stronger part of the field. In alternative words, the magnetic force affects an area unit perceptible even within the presence of a weak field. A number of the few samples of magnetic force substances are unit iron, nickel, cobalt, alnico etc.

Ferromagnetic substances have a large positive value for magnetic susceptibility X_m

It is of the order of several thousand. With the rise of temperature the susceptibility of ferromagnetic substances decreases.

Hysteresis

The word hysteresis means lagging behind. The property of insulant intensity of magnetisation (M) behind magnetic flux density (H), once a specimen of magnetic material is subjected to a cycle of magnetization is termed hysteresis phenomenon.

The curve shown would vary for different materials like steel, soft iron etc.

These topics can be seen in Magnetism and Matter class 12 notes pdf download.

Significance of NCERT Class 12 Physics Chapter 5 Notes

Revision Aid: The Magnetism And Matter notes class 12 are useful tools for reviewing the chapter's content and reinforcing students' understanding of important concepts.

Comprehensive Coverage: These CBSE class 12 physics ch 5 notes provide a concise overview of the chapter's main topics, allowing for a clear understanding of magnetism and its various aspects.

Exam Preparation: The Magnetism And Matter class 12 notes are essential for covering the major topics outlined in the Class 12 CBSE Physics Syllabus , making them invaluable to students preparing for board exams.

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