Class 12 Physics - Magnetism and Matter
1. The Bar Magnet
- Magnetic field lines form continuous closed loops.
- Tangent to the field line gives direction of B.
- Magnetic Dipole Moment ($M$): $M = m \times 2l$ (Direction: South to North).
2. Earth's Magnetism
- Magnetic Declination ($\theta$): Angle between magnetic meridian and geographic meridian.
- Angle of Dip ($\delta$): Angle that the total magnetic field of earth makes with the surface.
- At Poles: $\delta = 90^\circ$
- At Equator: $\delta = 0^\circ$
3. Magnetic Materials
| Property | Diamagnetic | Paramagnetic | Ferromagnetic |
|---|---|---|---|
| Susceptibility ($\chi$) | Small & Negative ($-1 \le \chi < 0$) | Small & Positive ($\chi > 0$) | Large & Positive ($\chi \gg 1$) |
| Permeability ($\mu_r$) | $\mu_r < 1$ | $\mu_r > 1$ | $\mu_r \gg 1$ |
| Effect of Temp | Independent | Inversely prop. ($\chi \propto 1/T$) | Decreases with Temp |
| Example | Water, Copper | Aluminum, Sodium | Iron, Nickel, Cobalt |
4. Hysteresis Loop
The cycle of magnetisation and demagnetisation.
- Retentivity: Magnetism remaining after removing external field.
- Coercivity: Reverse field needed to demagnetise.
- Soft Iron: High Retentivity, Low Coercivity (Used in Electromagnets).
- Steel: High Retentivity, High Coercivity (Used in Permanent Magnets).
Magnetism is invisible magic! 🧲🌏
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