Magnetism And Matter Class 12 Notes Physics Chapter 5 - CBSE

Chapter : 5

What are Magnetism and Matter ?

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Magnetic Field Intensity Due To A Magnetic Dipole (Bar Magnet) Along Its Axis And Perpendicular To Its Axis

The magnetic field intensity due to a bar magnet at any point on the axial line is B₁(µ₀/4π)(2md/(d²-l²)²) where d is the distance of the point from the centre of the magnet.
Magnetic field intensity due to a bar magnet at any point on the equatorial line of the bar magnet is
B2= (µ₀/4π)(2md/(d²+l²)^3/2)
Direction of B₁ is along SN
Direction of B₂ is along a line parallel to NS.

Torque On A Magnetic Dipole (Bar Magnet) In A Uniform Magnetic Field

$$\text{When a magnetic dipole of dipole moment}\space \vec{\text{M}} \text{is placed in a uniform magnetic field}\space \vec{\text{B}},\\ \text{it will experience a torque and is given by}\space \vec{\text{τ}}=\vec{\text{M}}×\vec{\text{B}} \space\text{or}\space\vec{\text{τ}}=\text{MB\space sin}\space\theta, \\ \text{where}\space\theta\space \text{is the angle between} \space\vec{\text{M}} \space\text{and} \space\vec{\text{B}} $$

Bar Magnet As An Equivalent Solenoid

A bar magnet and a solenoid (for a far axial point) produces similar magnetic fields.

B =(µ₀/4π)(2M/r³),

where M is the magnetic moment of the solenoid and is given by, M = n (2l) × I × (πa²)

n = number of turns per unit length of solenoid

2l = length of solenoid

I = current passed through the solenoid

r = distance between the point and centre of the solenoid

a = radius of solenoid

Magnetic Field Lines

Magnetic field line is an imaginary curve, the tangent to which at any point gives the direction of magnetic field $$\vec{\text{B}}\space \text{at that point.} $$

Larger the number of field lines crossing per unit area, stronger is the magnitude of magnetic field.
Magnetic field lines do not intersect.

Para-dia And Ferro-magnetic Substances With Examples

Paramagnetic substances are those, which when placed in a magnetic field are feebly magnetised in the direction of the magnetising field. Aluminium, platinum, and chromium are some examples of paramagnetic substances.
Paramagnetic substances has small and positive value of magnetic susceptibility (Χ_m).
Diamagnetic substances are those which when placed in a magnetising field are feebly magnetised in a direction opposite to that of the magnetising field. Copper, zinc, bismuth, gold, silver, etc., are some of the examples of diamagnetic substances.
Ferromagnetic substances are those, which when placed in a magnetic field are strongly magnetised in the direction of magnetising field. Iron, nickel and cobalt are the examples of ferromagnetic substances.
Diamagnetic substances has a small and negative value of magnetic susceptibility (χ _m).
Ferromagnetic substances has large and positive value of magnetic susceptibility (χ_m).

Electromagnets And Factors Affecting Their Strengths

Electromagnets are made of ferromagnetic materials which have high permeability and low retentivity, such as soft iron.
Factors affecting the strength of electromagnets are :

(i) Number of loops in the coil

(ii) Strength of current passed

(iii) Nature of core of the electromagnet

(iv) Temperature

Electomagnets are used in electric bells, loudspeakers and telephone diaphragms.
Giant electromagnets are used in cranes to lift machinery and heavy loads.

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