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 continuous curves drawn in a magnetic field such that the tangent at any point on the curve gives the direction of the net magnetic field \[\vec B\] at that point.

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.

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

\[\vec H\] represents the magnetising influence of the applied external field. In practice, it can be controlled by changing the current in a coil or solenoid.

Intensity of magnetisation is the magnetic moment per unit volume of a magnetised substance.

Magnetic induction \[\vec B\] is the number of magnetic lines of induction crossing a unit area normally inside a magnetised substance.

Magnetic permeability is the ratio of magnetic induction inside a substance to the magnetic intensity of the magnetising field.

Magnetic susceptibility measures how easily a substance becomes magnetised in an applied magnetic field.

Relative permeability is the ratio of the permeability of a substance to the permeability of free space.

m = IA (or m = NIA)

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

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

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

\[B=\mu_0nI\]

n = turns per unit length

\[{\vec{H}=\frac{\vec{B}}{\mu_0}-\vec{M}}\]

Equivalently, \[{\vec{B}=\mu_0(\vec{H}+\vec{M})}\]

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

Alternatively, \[{\mu_r=\frac{B}{B_0}}\]

where B_0​ is the magnetic flux density in vacuum for the same magnetising field.

\[\vec M\] = χ_m​\[\vec H\]

\[\therefore\] χ_m = \[\frac {M}{H}\]

μ = \[\frac {B}{H}\]

SI Units: T m A⁻¹ = N A⁻² = Wb A⁻¹m⁻¹

\[\vec{M}=\frac{\vec{m}}{V}\]

where:

• \[\vec m\] = magnetic moment of the substance
• V = volume of the substance

SI Unit: A m⁻¹

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.

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.

• 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

• Diamagnetic substances are weakly repelled and have negative susceptibility.
• Paramagnetic substances are weakly attracted and obey Curie law.
• Ferromagnetic substances are strongly attracted and contain domains.
• Ferromagnets become paramagnetic above the Curie temperature.
• The comparison table is the most important revision tool for board preparation.