Chapter 5: Magnetism and MatterClass 12 Physics — summary, notes, extra questions & MCQ quiz

Summary

A bar magnet behaves as a magnetic dipole with two poles that cannot be isolated; its field resembles that of a solenoid, and it has a magnetic moment \(\vec{m}\). In a uniform field it experiences a torque \(\vec{\tau}=\vec{m}\times\vec{B}\) and has potential energy \(U=-\vec{m}\cdot\vec{B}\), oscillating like a torsional pendulum. The Earth itself acts as a giant magnet; its field at a place is described by declination, dip (inclination) and the horizontal component. Magnetic properties of materials are captured by the magnetising field \(\vec{H}\), magnetisation \(\vec{M}\), and permeability, with \(\vec{B}=\mu_0(\vec{H}+\vec{M})\) and susceptibility \(\chi=M/H\). Materials are classified as diamagnetic (weakly repelled, \(\chi\) small and negative), paramagnetic (weakly attracted, small positive \(\chi\), obeying Curie's law \(\chi\propto 1/T\)), and ferromagnetic (strongly attracted, large \(\chi\), showing domains and hysteresis). Ferromagnets lose their magnetism above the Curie temperature, becoming paramagnetic. Hysteresis, the lag of magnetisation behind the field, and the area of the loop indicate energy loss, guiding the choice of materials for permanent magnets and electromagnets.

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