Magnetism and Matter – Class 12 Physics Notes | CBSE NCERT Based

 Chapter: Magnetism and Matter – Class 12 Physics Notes


πŸ”Έ Introduction:


Magnetism is the phenomenon associated with the motion of electric charges, which gives rise to magnetic fields. This chapter explores bar magnets, Earth's magnetism, magnetic materials, and related laws.



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πŸ”Ή 1. The Bar Magnet


A bar magnet has two poles: North (N) and South (S).


Magnetic field lines emerge from the North pole and enter the South pole.


Magnetic field strength is strongest at the poles.



Magnetic Moment (M):


\vec{M} = m \times 2l


 = magnetic pole strength

 = length of the magnet


Torque on a Bar Magnet in Magnetic Field:


\vec{\tau} = \vec{M} \times \vec{B}


Potential Energy:


U = -\vec{M} \cdot \vec{B}



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πŸ”Ή 2. Earth’s Magnetism


Earth behaves like a giant bar magnet.


The magnetic field of Earth is called geomagnetic field.



Important terms:


Magnetic Declination (D): Angle between geographic meridian and magnetic meridian.


Magnetic Inclination or Dip (I): Angle between Earth's magnetic field and horizontal line.


Horizontal component of Earth’s Magnetic Field (Bh):



B_h = B \cos I



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πŸ”Ή 3. Magnetic Properties of Materials


Types of Magnetic Materials:


Type Properties


Diamagnetic Weakly repelled by magnetic field (e.g., Bismuth, Copper)

Paramagnetic Weakly attracted (e.g., Aluminium, Platinum)

Ferromagnetic Strongly attracted, can retain magnetism (e.g., Iron, Nickel, Cobalt)




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πŸ”Ή 4. Magnetic Field Due to a Magnetic Dipole (Short Bar Magnet)


On Axial Line:


B = \frac{\mu_0}{4\pi} \cdot \frac{2M}{r^3}


On Equatorial Line:


B = \frac{\mu_0}{4\pi} \cdot \frac{M}{r^3}



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πŸ”Ή 5. Magnetization and Magnetic Intensity


Magnetization (M): Magnetic moment per unit volume



M = \frac{m}{V}


Magnetic Intensity (H):



B = \mu_0(H + M)


Where  = magnetic field,

 = permeability of free space



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πŸ”Ή 6. Magnetic Susceptibility and Permeability


Susceptibility (Ο‡):



\chi = \frac{M}{H}


Relative Permeability (ΞΌr):



\mu_r = \frac{\mu}{\mu_0}



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πŸ”Ή 7. Hysteresis Loop


It shows the relationship between magnetization (M) and magnetic field intensity (H).


Ferromagnetic materials exhibit hysteresis.


Area of loop = energy loss per cycle.




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πŸ”Ή 8. Comparison Table


Property Diamagnetic Paramagnetic Ferromagnetic


Susceptibility Negative Small Positive Large Positive

Field Behavior Repelled Weakly Attracted Strongly Attracted

Examples Bismuth Aluminium Iron




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πŸ“Œ Written by: Shahid (Study with Shahid)


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