NCERT · Class 12 · Physics · Magnetism and MatterDifferentiate clearly between diamagnetic, paramagnetic, and ferromagnetic substances on the basis of their magnetic properties, susceptibility, behavior in a non-uniform magnetic field, and effect of temperature. Give two examples of each.

Introduction\nSubstances are classified into different magnetic categories based on how they respond to an external magnetic field. This behavior is determined by the alignment and pairing of atomic magnetic moments within the material.

Detailed Comparison

1. Behavior in an External Magnetic Field:

   • Diamagnetic Substances: These substances are weakly repelled by a magnet. When placed in a non-uniform magnetic field, they tend to move from stronger to weaker parts of the field.
   • Paramagnetic Substances: These substances are weakly attracted by a magnet. In a non-uniform magnetic field, they tend to move from weaker to stronger parts of the field.
   • Ferromagnetic Substances: These substances are strongly attracted by a magnet and get strongly magnetized in the direction of the applied field.

2. Magnetic Susceptibility ($\chi$):

   • Diamagnetic: Susceptibility is small and negative (e.g., $-10^{-5}$). It is independent of temperature (except for metals).
   • Paramagnetic: Susceptibility is small and positive (e.g., $+10^{-5}$). It varies inversely with absolute temperature (Curie's Law).
   • Ferromagnetic: Susceptibility is very large and positive (ranges from hundreds to thousands). It decreases with temperature and above the Curie temperature, they become paramagnetic.

3. Relative Permeability ($\mu_r$):

   • Diamagnetic: Slightly less than 1 ($0 < \mu_r < 1$).
   • Paramagnetic: Slightly greater than 1 ($\mu_r > 1$).
   • Ferromagnetic: Much greater than 1 ($\mu_r \gg 1$).

4. Effect of Temperature:

   • Diamagnetic: Their magnetic susceptibility is practically independent of temperature.
   • Paramagnetic: Susceptibility follows Curie's Law, given by $\chi = C / T$, meaning susceptibility decreases as temperature increases.
   • Ferromagnetic: Susceptibility decreases with temperature according to the Curie-Weiss law: $\chi = C / (T - T_c)$, where $T_c$ is the Curie temperature.

5. Examples:

   • Diamagnetic: Bismuth, Copper, Water, Gold.
   • Paramagnetic: Aluminum, Platinum, Oxygen, Sodium.
   • Ferromagnetic: Iron, Cobalt, Nickel, Alnico.