Detailed Magnetism and Matter Class 12 notes

Magnetism and Matter Class 12 Notes

The example of magnetism in universal phenomena are as follows:

  • The earth also acts as a magnet with the magnetic subject pointing about from the geographic south to the north.
  • If you suspend a bar magnet, it will automatically align itself according to the geographical poles as the bar itself has the north-south direction. The tip which factors to the geographic north is referred to as the north pole and the end which factors to the geographic south is referred to as the south pole of the magnet.
  • There is a repulsive pressure while the north poles ( or south poles ) of magnets are introduced near together. Conversely, there’s an appealing pressure between the north pole of 1 magnet and the south pole of the other.
  • We can not isolate the north or south pole of a magnet. If a bar magnet is damaged into halves, we get comparable bar magnets with fairly weaker properties. Unlike electric powered charges, remoted magnetic north and south poles called magnetic monopoles do now no longer exist.
  • It is possible to make magnets out of iron and its alloys.

The Bar Magnet

The magnet has poles just like the wonderful and terrible rate of an electric powered dipole. One pole is distinctive the North pole and the other, the South pole. When suspended freely, those poles factor about in the direction of the geographic north and south poles, respectively. A comparable sample of iron filings is found around a modern-day sporting solenoid.

Magnetic Field Lines

Magnetic Field Lines

These lines going inside the bar and coming out from another end and forming a sideways loop are called magnetic field lines. The magnetic field lines are defined as a visual and intuitive realisation of the magnetic field. Their common properties are listed below:

  • The magnetic area traces a magnet (or a solenoid) shape non-stop closed loops. This is in contrast to the electrical dipole in which those area traces start from a fantastic price and give up at a poor price or break out to infinity.
  • The tangent to the field line at a given point represents the direction of the internet magnetic area at that factor.
  • The large the variety of area traces crossing in line with the unit area, the more potent the importance of the magnetic area.
  • The magnetic area traces do now no longer intersect, for in the event that they did, the path of the magnetic area could now no longer be specific on the factor of intersection.

One can plot the magnetic area traces in numerous ways. One manner is to locate a small magnetic compass needle at numerous positions and word its orientation. This offers us a concept of the magnetic area path at numerous factors in space.

Bar Magnet as Equivalent Solenoid

  • The solenoid is an electromagnet used to generate managed magnetic fields via electric-powered modern passing via a coil with a period extra than diameter.
  • In magnetism, a bar magnet is made from poles, north and south, in a square or rectangular item comprised of iron or steel.
  • The resemblance of magnetic area strains for a bar magnet and a solenoid advocate that a bar magnet can be an idea of a massive wide variety of circulating currents in analogy with a solenoid.
  • Cutting a bar magnet in 1/2 is like reducing a solenoid. We get smaller solenoids with weaker magnetic properties.
  • The area strains are continuous, rising from one face of the solenoid and moving into the alternative face.

A bar magnet of magnetic moment m and size at a distance from its mid-point, >>l, the formula for magnetic field B due to this bar along the axis is:

A bar magnet of magnetic moment m and size at a distance from its mid-point, >>l, the formula for magnetic field B due to this bar along the equator is given by – μ0m/4πr3.

Dipole in a Uniform Magnetic Field

The formula for Torque in a needle having magnetic moment “m” and Magnetic field “B” will be as follows:

In magnitude τ = mB sinθ

In case of simple harmonic motion, formula for magnetic field B would be :

magnetism and matter class 12 notes

formula for finding out” t ” goes like

magnetisn and matter class 12 notes

Magnetic Potential Energy

magnetisn and matter class 12 notes

when theta is zero degrees, it has minimum potential energy with the most stable position.

theta is 90 degrees, Potential energy becomes zero.

when theta is 180 degrees, the potential energy is maximum with the most unstable position.

The Electrostatic Analog

The magnetic field of a bar magnet with magnetic moment m can be derived from the equation of electric field of an electric dipole of dipole moment p, which is given by –p/4πε0r3 along the equatorial axis and by 2p/4πε0r3 along. the axial axis. Torque is given by p * E.

Gauss Law

Gauss’s law for magnetism as the magnetic flux across any closed surface is zero; provided the isolated magnetic poles (monopoles) do not exist. The formula for Gauss law in magnetism is given by:

Gauss Law

Earth’s Magnetism

The reason for the magnetic field is so evident on the reason is thought to be due to electrical currents generated by the motion of metal such as molten iron & nickel in the outer core of the earth. This states the dynamo effect.

Important Point

  • The energy of the earth’s magnetic area varies from one area to another at the earth’s surface, the value however stands in the order 10–5T.
  • The vicinity of the north magnetic pole is at a range of 79.74° N and a longitude of 71.8° W, an area someplace in north Canada.
  • The magnetic south pole is at 79.74° S and 108.22° E within side Antarctica.
  • The pole close to the geographic north pole of the earth is known as the north magnetic pole.
  • Likewise, the pole close to the geographic south pole is known as the south pole.

Magnetic Declination and Dip

Magnetic Declination refers to the angle between the north shown by a compass needle and the true geographic north. The declination is always more at a higher latitude and lowers towards the equator.

The magnetic declination in India is small, it being 0°41′ E at Delhi and 0°58′ W at Mumbai which means it shows the true north at both the places quite accurately.

The dip is the angle that the total magnetic field BE of the earth makes with the surface of the earth. In most regions of the northern hemisphere, the north pole of the dip needle often tilts downwards while in the southern hemisphere, the south pole of the dip needle tends to tilt downwards.

ZE BE sin I (Horizontal component of Dip)

HBE cos I (Vertical Component of Dip)

tan I= ZE/HE. where I is the angle of Inclination

Magnetisation And Magnetic Intensity

The magnitude of magnetisation is often denoted by the symbol M is equal to its net magnetic moment per unit volume. Magnetisation is a vector quantity and it is measured in the units A m–1.

M = mnet/ V

The magnetic field is directly proportional to magnetisation and is expressed as Bm= μ0M; where μ0 is the permivity of vaccum that appears in Biot-savart law.

Another formula is M= χH where χ is called magnetic susceptibility and is a dimensionless quantity. magnetic Susceptibility is small and positive for materials. Such materials are called paramagnetic. Some materials are called Diamagnetic in which magnetic susceptibility is small but negative.

μ = μ0μr = μ0 (1+χ).

There is one more term that is called the magnetic permeability of the substance. It isdenoted by μ and it has the same dimensions and units as μ

There is one more term called magnetic permeability of the substance. It is denoted by μ and it has the same dimensions and units as μ0;

μ = μ0μr = μ0 (1+χ).

Magnetic Properties of Materials

In Magnetism and matter class 12 notes, materials are classified into three categories: Diamagnetic, Paramagnetic, and Ferromgnetic.


  • Diamagnetic materials are the ones that have a tendency to transport from the more potent to the weaker party in the magnetic field.
  • Some diamagnetic substances are bismuth, copper, lead, silicon, nitrogen (at STP), water and sodium chloride.
  • The maximum extraordinary diamagnetic substances are superconductors.
  • These are metals, cooled to very low temperatures which reveal each best conductivity and best diamagnetism.
  • Here the sector traces are absolutely expelled! χ = –1 and μr = 0.
  • The phenomenon of best diamagnetism in superconductors is referred to as the Meissner effect.


  • Ferromagnetic substances are substances that have the potential to get strongly magnetised if placed in an external magnetic field. Example includes iron, cobalt, nickel, gadolinium, etc.
  • They have a tendency to move from a region of a weak magnetic field to a strong magnetic field. Thus, in a ferromagnetic material, the field lines are highly concentrated. 
  • There are materials in which magnetism persists, they are called hard ferromagnets. The example includes Alnico and copper, Naturally occurring loadstones.

A soft ferromagnetic material is a class of ferromagnetic material in which the magnetisation totally disappears on the removal of the external field. An example is a Soft iron. The relative magnetic permeability is >1000!


  • Paramagnetic materials are the ones that get weakly magnetised whilst located in an outside magnetic area.
  • They have a tendency to transport from a place of vulnerable magnetic area to a robust magnetic area, i.e., they get weakly interested in a magnet. Some paramagnetic materials are aluminum, sodium, calcium, oxygen (at STP) and copper chloride.

χ = /T where C is Curie constant.

The above mentioned equation was experimentally confirmed by Pierre Curie. It was proved that the magnetisation of a paramagnetic material is inversely proportional to the absolute temperature T.

Permanent Magnets and Electromagnets

Permanent magnets are those substances that pertain to the magnetic property for a long period of time within them. How can we make it?

  1. By continuously hammering an iron rod in the north-south direction.
  2. by placing a ferromagnetic rod in a Solenoid and passing a current through it.

What materials can be used to make a permanent magnet?

Those retaining high retentivity as well as higher permeability. Such as Steel, Alnico, Cobalt steel, and ticonal and can be used.

The core of electromagnets is synthesised from ferromagnetic substances that have excessive permeability and low retentivity. Soft iron is an appropriate option for making electromagnets.

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