Magnetism and Matter – Class 12 Physics Notes | CBSE NCERT Based
📚 Chapter: Magnetism and Matter – Class 12 Physics Notes
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🔸 1. Introduction
This chapter deals with the magnetic properties of materials, Earth's magnetism, and how magnetism and matter are related.
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🔹 2. Bar Magnet
Magnetic dipole has two poles: North and South
Like poles repel, unlike poles attract
Magnetic field lines emerge from North and enter South
Magnetic dipole moment (M):
M = m × 2l
where, m = pole strength, 2l = length of magnet
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🔹 3. Magnetic Field Due to a Bar Magnet
At a point on the axial line:
B_{\text{axial}} = \frac{\mu_0}{4\pi} \cdot \frac{2M}{r^3}
At a point on the equatorial line:
B_{\text{equatorial}} = \frac{\mu_0}{4\pi} \cdot \frac{M}{r^3}
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🔹 4. Torque on a Magnetic Dipole in Uniform Magnetic Field
\vec{\tau} = \vec{M} \times \vec{B}
Where:
M = Magnetic dipole moment
B = External magnetic field
Direction by right-hand rule
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🔹 5. Magnetic Properties of Materials
Type Magnetic Susceptibility (χ) Relative Permeability (μr) Example
Diamagnetic χ < 0 μr < 1 Bismuth, Copper
Paramagnetic χ > 0 (small) μr > 1 Aluminum, Platinum
Ferromagnetic χ ≫ 0 μr ≫ 1 Iron, Cobalt, Nickel
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🔹 6. Hysteresis Loop (Ferromagnets)
It shows how magnetization changes with applied magnetic field.
Retentivity: Residual magnetism
Coercivity: Reverse field needed to demagnetize
Area of loop = energy loss per cycle
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🔹 7. Earth's Magnetism
Earth behaves like a giant bar magnet
Magnetic field lines emerge from near geographic south and enter geographic north
Terms:
Magnetic Declination: Angle between geographic and magnetic meridian
Magnetic Inclination (Dip): Angle between magnetic field and horizontal
Horizontal Component (Bh):
Bh = B cos(θ)
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📌 Conclusion:
This chapter builds your understanding of how materials respond to
magnetic fields and helps in concepts related to Earth's magnetism, useful in navigation and technology.
📊 Important Diagrams – Magnetism and Matter (Class 12 Physics)
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🔹 1. Magnetic Field Lines of a Bar Magnet
Show how field lines emerge from the north pole and enter the south pole
Very common 2-mark diagram
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🔹 2. Earth as a Magnet
Earth’s magnetic field lines
Magnetic axis and geographic axis
Magnetic equator and poles
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🔹 3. Magnetic Declination and Inclination
Diagram showing:
Magnetic meridian
Geographic meridian
Angle of declination (D)
Angle of dip/inclination (I)
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🔹 4. Hysteresis Loop
Magnetization (M or B) vs Magnetic field (H) graph
Shows:
Retentivity
Coercivity
Area = energy loss
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🔹 5. Magnetisation Curve
Shows behavior of diamagnetic, paramagnetic, and ferromagnetic materials
Often asked to compare magnetic behavior
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🔹 6. Bar Magnet as Equivalent Solenoid
Explains how a bar magnet behaves like a
current-carrying solenoid
Magnetic field lines inside and outside shown
✍️ Written by: Shahid – Study with Shahid
🔗 Visit: https://shahidnotes12.blogspot.co
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