NCERT Class 12 Physics Chapter 5 Notes, Magnetism and Matter Class 12 Chapter 5 Notes

Ever wondered how a compass always points to the North or why some materials stick to magnets but others don't? These properties are fundamental characteristics of these materials and are used in a variety of applications. Such magnetic phenomena and many others are clearly explained in the chater 5 of the NCERT Notes of Class 12 Physics.

Topics such as the origin and history of magnetism, magnetic field lines, magnetic dipole, solenoids, and the magnetic properties of materials are just a few of the key topics covered in these NCERT Notes of Magnetism and Matter. Students can find step-by-step explanations of complex topics, which have been prepared by experts at Careers360. The notes are structured as per the latest CBSE syllabus, making them perfect for board exam preparation. The Class 12 NCERT Notes are crucial for additional learning and in-depth understanding of the chapter.

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

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NCERT Notes for Class 12 Physics Chapter 5

The NCERT Notes of Class 12 Physics chapter Magnetism and Matter covers all concepts in the NCERT textbook and explains them in a concise and easy-to-understand manner. These notes are useful for last minute revision as well as exam preparation.

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 properties of the magnetic field lines are as follows-

• 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.

Bar Magnet as Equivalent Solenoid

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

$B=\frac{n{\mu }_{0}Il{a}^2}{(r{)}^3}$

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

$n\times 2l\times \pi a^2$

$B=\frac{{\mu }_{0}2m}{4\pi r^3}$

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

Dipole Moment of Circular Current Loop

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

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

$\tau =mB\times 2l\sin \theta =m2lB\sin \theta$

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

$\tau =MB\sin \theta$
If $\mathrm{B}=1$ and $\theta ={90}^{\circ }$ i.e.

$\sin \theta =1,\text{ then }$
$\tau =M\times 1\times 1$
$M=\tau$

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

$Nm{T}^{-1}=J{T}^{-1}$

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

$\tau =MB\sin \theta$

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,

$dW=\tau d\theta =MB\sin \theta d\theta$

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

$\begin{array}{rl}& W={\int }_{{\theta }_1}^{{\theta }_2}MB\sin \theta d\theta =MB{\int }_{{\theta }_1}^{{\theta }_2}\sin \theta d\theta =MB|-\cos \theta {|}_{{\theta }_1}^{{\theta }_2}\\ & =-MB(\cos {\theta }_2-\cos {\theta }_1)=MB(\cos {\theta }_1-\cos {\theta }_2)\end{array}$

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.

$U=MB(\cos {\theta }_1-\cos {\theta }_2)$

Suppose that the magnet is initially perpendicular to the direction of the magnetic field, i.e. θ1=90o. 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=90o and θ2=θ in the equation

$U=MB(\cos {90}^{\circ }-\cos \theta )$

$U=-MB\cos \theta$

Gauss’ Law in Magnetism

Gauss's Law of magnetism states that the flux of the magnetic field through any closed surface is zero (as shown in the below figure). It is because, inside the closed surface, the simplest magnetic element is a magnetic dipole with both poles (since a magnet with a monopole does not exist). So several magnetic field lines entering the surface equal the number of magnetic field lines leaving the surface. So the net magnetic flux through any closed surface is zero.

Magnetic Flux

Magnetic flux is a measure of the quantity of magnetism, considering the strength and the extent of a magnetic field. It is defined as the product of the magnetic field (B) and the area (A) through which the field lines pass, and it also depends on the angle (θ) between the magnetic field and the normal (perpendicular) to the surface. It is defined as the magnetic lines of force passing normally through a surface called magnetic flux.

The Gauss Law for a closed surface is,

$\int \overrightarrow{B}\cdot \overrightarrow{ds}=0$

Magnetisation and Magnetic Intensity

Magnetisation and Magentic Intensity are concepts of magnetism that allows us to explain the behaviour of different materials and their magnetic properties.

Magnetisation

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.

$M=\frac{m_{net}}{V}$

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.

$B_0={\mu }_{0}nI$

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

$B=B_0+B_m=B_0+{\mu }_{0}M$

Dividing by μ0

$\frac{B}{{\mu }_0}=\frac{B_0}{{\mu }_0}+M$

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

$B={\mu }_0(H+M)$

H depends on external factors like current flowing etc.

M depends on the material kept inside the solenoid

Susceptibility

In the expression,

$B={\mu }_0(H+M)$

M depends on external factors as well.

Hence,

$M=\chi H$

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

$B={\mu }_0(H+M)$

$B={\mu }_0(H+\chi H)$

$B={\mu }_0(1+\chi )H$

μ is called the magnetic permeability of the substance.

µr is called the relative magnetic permeability of the substance.

Its unit is Tm/A.

Magnetic Properties of Materials

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

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 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,s bismuth is an exception.

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.

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 Xm

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 magnetizatio,n is termed hysteresis phenomenon.

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

Magnetism and Matter Previous year Question and Answer

Q. 1 The magnetic dipole moment of a current-carrying coil does not depend upon-
(1) number of turns of the coil.
(2) cross-sectional area of the coil.
(3) current flowing in the coil.
(4) material of the coil.

Answer:

Magnetic dipole moment M=NIA
N is the number of turns
I am the current
A is the cross-sectional area

The magnetic dipole moment of a current-carrying coil does not depend upon the material of the turns of the coil.

Hence, the answer is the option (4).

Q. 2 Identify the following magnetic materials:
(i) A material having susceptibility $\left({\chi }_m\right)=-0.00015$.
(ii) A material having susceptibility $\left({\chi }_m\right)={10}^{-5}$.

Answer:

(i) $x_m=-0.00015$

Negative sign indicates that the material repels magnetic fields. Since the value is very small, the material is diamagnetic.

(ii) $x_m={10}^{-5}>0$

The material is weakly attracted to magnetic fields. Hence, it is paramagnetic.

Q. 3 Two identical bars, one of paramagnetic material and other of diamagnetic material are kept in a uniform external magnetic field parallel to it. Draw diagrammatically the modifications in the magnetic field pattern in each case.

Answer:

According to the properties of paramagnetic and diamagnetic materials, we can draw the following.

Diamagnetic

Paramagnetic

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