Magnetism and MatterNEET Physics · Class 12 · NCERT Chapter 5

You can expect 1 question from this chapter in NEET 2027. The chapter has medium PYQ frequency. Bar magnet field and torque, Curie's law, classification of magnetic materials and basics of Earth's magnetism are favourites.

A bar magnet with pole strength m_pole and length 2 L has dipole moment vector p_m equals m_pole times 2 L, directed from south to north pole. Unit: A m squared (or J per T). For a current loop of N turns, area A and current I, the moment is m equals N I A.

On the axis at distance r far from the magnet, B equals 2 mu_0 m over (4 pi r cubed). On the equator (perpendicular bisector), B equals mu_0 m over (4 pi r cubed). Axial is twice equatorial. Both fall as 1 over r cubed.

Three numbers describe Earth's field at any place: declination (angle between true geographic north and magnetic north on a map), dip or inclination (angle B makes with the horizontal plane), and horizontal component B_H. Total field B equals B_H over cos(dip).

Diamagnetic: weakly repelled by a magnet. chi is small and NEGATIVE (around -10⁻⁵). Examples: bismuth, copper, water. Paramagnetic: weakly attracted. chi is small and POSITIVE (around +10⁻⁴). Examples: aluminium, oxygen. Ferromagnetic: strongly attracted, can be permanently magnetised. chi is large positive (10² to 10⁵). Examples: iron, nickel, cobalt.

For a paramagnetic substance, the magnetisation M is proportional to the applied field H and inversely proportional to absolute temperature T: M equals C H over T, where C is the Curie constant. Equivalently, susceptibility chi equals C over T. Heating reduces magnetisation.

For a ferromagnetic material, the M vs H curve traces a loop instead of a straight line. Starting from zero, M rises with H to saturation. When H is reversed, M does not retrace the same path: there is residual magnetism (retentivity) at H equals 0, and M only reaches zero at a reverse field called the coercivity. The area of the loop equals the energy lost per cycle as heat.