Magnetism and Matter holds a significant position in the NEET physics syllabus as it has an approximate 1-2% weightage in the NEET exam. Students studying this topic learn about the behaviour of various materials in the presence of magnetic fields. In addition to this, one can also learn the attractive and repulsive forces in magnets and their effects on the earth.

 Magnets are not a laboratory phenomenon but occur everywhere in our daily life. Some common applications of magnets are compass, motors, loudspeakers, and MRI machines. This topic of Magnetism and Matter links natural and artificial magnetism. Natural magnetism is the magnetism caused by the earth’s magnetic field, while artificial magnetism is induced by electric current.

Concepts like magnetic field, magnetic moment, bar magnet as a dipole, magnetic intensity and susceptibility, hysteresis loop and retentivity, magnetic permeability, behaviour of magnetic materials such as diamagnetic, paramagnetic, and ferromagnetic fall under this topic.

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What are Magnetism and Matter?

Magnetism is the ability of the substance or material to attract certain materials. The Matter is known as a variety of materials that produce the magnetic field and magnetic moment or react to the existing magnetic field. Magnetism and Matter explain the properties of different materials in a magnetic field.

 All materials have a small magnetic dipoles, which are created due to the movement of electrons in the atoms. In some materials, all of the dipoles point in the same direction which means a big magnet. In others, they are neutralizing each other.

Also Read: Magnetism and Matter NEET Notes

What is Magnet?

A magnet is an object that attracts iron and steel objects. It has a magnetic field in which another magnet or magnetic material experiences a force. Every magnet has two poles. They are North Pole and South Pole. The region around the magnet where the magnetic force is experienced is called the magnetic field.

Magnets can be natural (lodestone) or artificial (produced by electric currents). If a magnet is cut into two equal parts, each part will have both the North Pole and South Pole. So it means there are no magnetic monopoles (isolated poles).

General Properties of Magnets

The magnets have a few simple properties:

• Two poles: Every magnet has a north pole and a south pole.

• Like poles repel each other and unlike poles attract each other: The north pole of one magnet repels the north pole of the other magnet, and it attracts the south pole.

• Magnetic poles cannot be separated: The poles always come in pairs. If a magnet is divided into two pieces, each piece will have both the North Pole and the South Pole.

• Magnetic field lines are always closed loops: The field lines leave the north pole of a magnet and enter the south pole on the outside of the magnet.

Also Read: Magnetism and Matter MCQs

Bar Magnet

A bar magnet is the simplest form of a magnetic dipole. It consists of two opposite poles separated by a certain distance, which generates a magnetic field around it. The strength of a bar magnet is determined by its magnetic moment.

Bar Magnet as a Dipole 

A bar magnet can be considered as a magnetic dipole because it has two opposite poles separated by a finite distance. The magnetic moment (M) of a bar magnet is given by the following formula:

𝑀 = 𝑚 × 2 𝑙  

where 

𝑚 is the pole strength and 

2𝑙 is the distance between the poles. 

 The magnetic field due to a bar magnet can be likened to the electric field due to an electric dipole.

Magnetic Field Lines

Magnetic field lines are a visual way of representing magnetic fields.

The direction of the magnetic field at any point is tangential to the field line at that point.

Magnetic Field Lines Properties

• They never intersect. 

• The tangent at any point on the field line gives the direction of the magnetic field.

• Density of lines is proportional to field strength.

Magnetic Dipole

A magnetic dipole is a pair of equal and oppositely charged magnetic poles separated by a small distance. All bar magnets are dipoles. The magnetic dipole moment (M) is a vector quantity which specifies the strength and orientation of a dipole. It is directed from its south pole to its north pole.

Also Read: Magnetism and Matter Important Questions

Gauss-Law of Magnetisation

Gauss law of magnetisation is a principle which explains that the net magnetic flux coming out of a closed surface is always zero. In simple terms, the number of field lines entering and leaving a surface is equal because magnetic monopoles do not exist. As per Gauss’s law of magnetism 

The total magnetic flux through a closed surface is zero because there are no magnetic monopoles. It also shows that magnetic field lines always form closed loops but electric field lines start and end on charges.

Magnetic Dipole in Uniform Magnetic Field

A magnetic dipole is something like a small bar magnet. The small bar magnet has two poles, a north pole and a south pole. The distance between the two poles is small.

• If we place a small bar magnet in a uniform magnetic field, then equal and opposite forces will act on the two poles.

• The north pole will experience a force in the direction of the field. The south pole will experience a force in the direction opposite to the direction of the field. Since the forces are equal and opposite, they form a couple.

• The couple does not move the magnet but it rotates the magnet. The reason for rotation is torque. Torque tries to align the magnet in the direction of the field.

Torque on a Magnetic Dipole 

The torque τ on a magnetic dipole placed in a uniform magnetic field is given by:

τ = M B sinθ 

 where 

M = magnetic dipole moment 

B = magnetic field 

θ = angle between M and B 

Explanation: 

For θ = 0° , dipole is parallel to the field and hence, torque is zero and dipole is in stable equilibrium.

For θ = 90° , torque is maximum. 

For θ = 180° , torque is again zero but dipole is in unstable equilibrium.

So, torque always tends to rotate the dipole such that it aligns along the field direction.

Work Done in Rotating a Dipole 

When we rotate a magnetic dipole from one position to another in a uniform magnetic field, we have to do some work against the magnetic torque.

 Work done W in rotating a dipole from angle θ₁ to θ₂ is given by:

 W = M B (cosθ₁ – cosθ₂)  

Explanation: 

If a dipole is rotated from θ₁ = 180° to θ₂ = 0° , then

W = 2 M B 

This is the maximum work done. 

 This work done is equal to the change in potential energy of a dipole as its orientation in a magnetic field is changed.

Potential Energy of a Magnet 

If a magnetic dipole is placed in a uniform magnetic field, it will have a potential energy depending on its orientation in the magnetic field.

The potential energy U of a magnetic dipole in a magnetic field is given by:

U = - M B cosθ 

Explanation: 

When θ = 0°, U = -MB → minimum energy → most stable (dipole is aligned with the field).

When θ = 180°, U = +MB → maximum energy → least stable (dipole is anti-parallel to the field).

Hence, a magnetic dipole always tends to align itself in a magnetic field such that it is in the most stable state (minimum potential energy).

Circular Loop as Magnetic Dipole 

A current carrying circular loop acts like a magnetic dipole.

If a current I flows in a circular loop of radius r, then the magnetic dipole moment M is given by:

M = I × A 

 where, A = πr² = area of the loop 

The direction of M is given by right-hand thumb rule: curl your fingers in the direction of current and your thumb will point in the direction of magnetic moment.

The magnetic field at the center of a circular loop is given by:

B = μ₀ I / (2r) 

 The above equation shows that a current loop also produces a magnetic field similar to that of a bar magnet. Hence, a current loop acts as a magnetic dipole with north and south polarity.

Force Between Two Bar-Magnet

Two bar magnets interact with each other, as do electric charges. A magnet has two poles (north and south). The two magnets placed in proximity to each other experience force of attraction or repulsion.

Like poles (north–north or south–south) repel each other, while unlike poles (north–south) attract each other. The Force between two magnetic poles is given by a law, analogous to the Coulomb’s law of electrostatics.

Force between two magnetic poles of strength m₁ and m₂ at a distance r in air or vacuum is given by:

 F = (μ₀ / 4π) × (m₁ × m₂) / r² 

where 

F = force between the poles 

μ₀ = permeability of free space = 4π × 10⁻⁷ Tm/A 

r = distance between poles 

 Explanation: 

• The force acts along the line joining the two poles.

• When poles have same sign (N and N or S and S) then F is positive → repulsive force

• When the poles are unlike (north–south) F is negative → attractive force

• It shows that the magnetic force decreases rapidly with increase in the distance between the poles.

Analogy with Coulomb’s Law: 

Electric charges interact with force F = (1 / 4πε₀)(q₁q₂ / r²)

Magnetic poles interact with force F = (μ₀ / 4π)(m₁m₂ / r²)

 Hence we see the similarity of inverse-square law nature in both magnetism and electrostatics.

Oscillation Magnetometer

It is a simple magnetometer. It is used to determine the magnetic moment of a magnet, horizontal component of Earth’s magnetic field elements, or for comparing the strengths of two magnets.

It works on the principle of the torque experienced by a magnetic dipole in a uniform magnetic field.

Construction 

It consists of a small mirror attached to the bar magnet. The magnet with the mirror is suspended horizontally by a fine fiber or thread in a glass box.

• The mirror allows us to measure the angular deflection by using a scale and telescope.

• The magnet is placed in a uniform magnetic field, such as Earth’s magnetic field.

If the magnet is slightly displaced from its equilibrium position, then a restoring torque will act on the magnet. This torque will bring the magnet back to its initial position, and it will keep on oscillating to and fro.

Magnetic Materials

Not all materials behave in the same way in a magnetic field. They can be divided in three broad classes:

• Diamagnetic Substances: These substances are weakly repelled by a magnetic field. They have negative susceptibility and very low magnetisation.

Examples: copper, bismuth, silver 

• Paramagnetic Substances: These substances are weakly attracted by a magnetic field. They have small positive susceptibility and magnetisation. 

Examples: aluminum, platinum, sodium 

• Ferromagnetic Substances: These substances are strongly attracted by a magnetic field. They have very high susceptibility and can retain magnetisation and magnetic permeability even after the external field is removed.

Examples: iron, cobalt, nickel

Magnetism and Matter PDF

The Magnetism and Matter PDF is helpful for NEET aspirants to get conceptual clarity and revise the formula related to the torque on a magnetic dipole, magnetisation, magnetic permeability, potential energy of a magnet, etc. The Magnetism and Matter Notes PDF also contains derivations and examples to simplify the complex terms such as diamagnetic, paramagnetic, and ferromagnetic substances.

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Students who need to have a good overview of the Magnetism and Matter topic can download the PDF file to have an easy access to the notes and quick revision. The Magnetism and Matter Notes PDF has been provided in the article for the NEET aspirants to have quick and easy revision of the complete topic. The Magnetism and Matter PDF notes contain a comprehensive explanation of each subtopic including the magnetic field and magnetic moment, bar magnet as dipole, magnetic field lines, earth’s magnetic field elements, magnetic intensity and susceptibility, hysteresis loop and retentivity, etc.

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