Revision: Class 12 >> Magnetism and Magnetic Matter NEET (UG) Magnetism and Magnetic Matter

The temperature above which a ferromagnetic substance becomes paramagnetic is called curie temperature. 

Magnetic field lines are imaginary lines representing the direction of the magnetic field

Substances which when placed in a magnetic field are feebly magnetised in the direction of the magnetising field are called paramagnetic substances.

Substances which when placed in a magnetising field are strongly magnetised in the direction of the magnetising field are called ferromagnetic substances.

Substances which when placed in a magnetic field are feebly magnetised in a direction opposite to that of the magnetising field are called diamagnetic substances.

m = IA (or m = NIA)

\[B=\frac{\mu_0}{4\pi}\cdot\frac{2m}{d^3}\]

\[B=\frac{\mu_0}{4\pi}\cdot\frac{m}{d^3}\]

\[\tau=MB\sin\theta\]

Vector form: \[\vec{\tau}=\vec{M}\times\vec{B}\]

\[B=\mu_0nI\]

n = turns per unit length

\[B=\frac{\mu_0NI}{2\pi r}\]

The magnetic susceptibility of a paramagnetic material varies inversely with its absolute temperature. Mathematically,

On cooling, paramagnetic substances get converted to ferromagnetic materials at the Curie temperature.

For ferromagnetic substances above the Curie temperature, the magnetic susceptibility is inversely proportional to (T − T_C), where T_C is the Curie temperature. Mathematically,

On heating beyond the Curie temperature (T_C(iron) = 770 °C), ferromagnetic substances get converted into paramagnetic materials.

• A current-carrying loop behaves like a magnetic dipole (bar magnet)
  
  
• Polarity Rule 
  Anticlockwise current → North pole (upper face)
  Clockwise current → South pole (lower face)

• Direction given by right-hand thumb rule; for a loop, B at centre and M are parallel.
• Magnetic moment of a straight current-carrying wire = 0.
• Magnetic moment of a toroid = 0.
• Dipole moment direction: S → N (inside magnet field taken N → S).

• A bar magnet behaves like a solenoid
• Both produce similar magnetic field patterns
• Solenoid Relation: M = NIA

Direction:

• Outside magnet: North → South
• Inside magnet: South → North

Key Properties:

• Closed curves (no start or end)
• Never intersect each other
• The tangent at any point gives the direction of the magnetic field
• Closer lines → stronger field
• The field is strongest at the poles

• Relative permeability ranges:   μ_r ≫ 1, of the order of 10²; μ ≫ μ₀
• Diamagnetic: B ≫ B₀​; B_m ≫ B₀
• Magnetic susceptibility (χ): positive and high, χ ≈ 10²; very large, positive, temperature dependent, χ_m ∝ \[\frac {1}{T−T_C}\]​ (Curie–Weiss law)
• Magnetic moment: very high
• Intensity of magnetisation (I) vs H: I is very large, positive, varies non-linearly with H (I is in the direction of H, value of I is very high)

• Relative permeability ranges: μ_r < 1 (as B is less than μ₀H); also 1 > μ_r > 0, μ < μ₀
• Diamagnetic: B < B₀​; B_m < B₀
• Magnetic susceptibility (χ): low and negative, ∣χ∣ ≈ 1; small, negative and temperature-independent, χ_m ∝ T₀
• Magnetic moment: very low (≈ 0)
• Intensity of magnetisation (I) vs H: I is small, negative, varies linearly with H (I and H in opposite direction, I is negative with respect to H)

• Relative permeability ranges:  μ_r > 1 (as B is slightly greater than μ₀H); (1 + ε) ≥ μ_r > 1, μ > μ₀
• Diamagnetic: B < B₀​; B_m < B₀
• Magnetic susceptibility (χ): low and positive, χ ≈ 1; small, positive, varies inversely with temperature, χ_m ∝ \[\frac {1}{T}\]​ (Curie law)
• Magnetic moment: very low but not zero
• Intensity of magnetisation (I) vs H: I is small, positive, varies linearly with H (I and H in same direction, value of I is low)