CBSE Class 12 Physics Chapter 9: Magnetism and Matter NCERT Solutions

(a) In a paramagnetic material, the magnetic dipoles (atomic magnetic moments) are randomly oriented due to thermal agitation. When an external magnetic field is applied, these dipoles tend to align with the field, causing magnetization. At higher temperatures, thermal motion disrupts this alignment more effectively. Upon cooling, the thermal disruption is reduced, allowing the dipoles to align more completely with the external field, resulting in greater magnetization for the same applied field.

(b) Diamagnetism arises from the orbital motion of electrons within atoms. When an external magnetic field is applied, it induces a change in the orbital motion of electrons, creating an induced magnetic dipole moment that opposes the applied field. This induced moment is primarily determined by the applied field and the electron orbits, and it is not significantly affected by the random thermal motion of the atoms. Therefore, diamagnetism is largely independent of temperature.

(c) Bismuth is a diamagnetic substance. Diamagnetic materials weakly oppose the applied magnetic field. When bismuth is used as the core of a toroid, it will slightly reduce the magnetic field inside the toroid compared to when the core is empty (which would have a permeability close to that of vacuum). Thus, the field in the core will be slightly less than when the core is empty.

(d) No, the permeability of a ferromagnetic material is not independent of the magnetic field. Initially, as the magnetic field strength increases from zero, the permeability of a ferromagnetic material increases. It reaches a maximum value and then decreases as the material approaches magnetic saturation. Therefore, the permeability is generally higher for lower magnetic fields.

(e) Ferromagnetic materials have a magnetic permeability significantly greater than one. This high permeability means they strongly concentrate magnetic field lines within themselves. When magnetic field lines enter or leave the material, they tend to do so in a direction that maximizes this concentration. Consequently, the field lines are nearly normal (perpendicular) to the surface at every point, analogous to how electric field lines are normal to the surface of a conductor in electrostatic equilibrium.

(f) The maximum possible magnetization of a paramagnetic sample, which occurs at saturation, can be of the same order of magnitude as the magnetization of a ferromagnet. However, achieving this saturation in a paramagnetic sample typically requires very strong external magnetic fields and/or very low temperatures, conditions under which ferromagnetic materials are already saturated.

(a) Ferromagnetic materials consist of small regions called magnetic domains, where the atomic magnetic moments are aligned parallel to each other. In an unmagnetized state, these domains are randomly oriented. When an external magnetic field is applied, the domains tend to align with the field. This alignment occurs through two processes: the rotation of domain magnetization vectors and the growth of domains aligned with the field at the expense of others. Crucially, once aligned, the domain walls do not easily return to their original random positions when the external field is reduced or reversed. This 'stickiness' or resistance to change in domain alignment causes the magnetization to lag behind the applied magnetic field, leading to irreversibility and the formation of a hysteresis loop.

(b) The area enclosed by the hysteresis loop of a ferromagnetic material represents the energy dissipated as heat per unit volume during each cycle of magnetization and demagnetization. Since the hysteresis loop of carbon steel has a much larger area than that of soft iron, the carbon steel piece will dissipate greater heat energy when subjected to repeated cycles of magnetization.

(c) A ferromagnet displaying a hysteresis loop acts as a memory device because it can retain a certain level of magnetization even after the external magnetizing field is removed. The state of magnetization (represented by the B-H curve) depends not only on the current applied field but also on the history of the applied field. Specifically, the remanent magnetization (the magnetization remaining when the field is zero) and the coercivity (the reverse field needed to bring magnetization to zero) allow the material to 'remember' whether it has been magnetized and to what extent. Different levels of magnetization can be stored, analogous to binary '0' and '1' states in digital memory.

(d) Ferromagnetic materials with wide hysteresis loops are used for coating magnetic tapes and building memory stores. These materials, such as certain oxides of iron (like gamma-ferric oxide, $\gamma\text{-Fe}_2\text{O}_3$) or alloys, exhibit high retentivity and coercivity. This means they can be easily magnetized to store information and, once magnetized, retain that state effectively, resisting demagnetization by stray fields.

(e) To shield a certain region of space from magnetic fields, it should be surrounded by a material of high magnetic permeability, such as soft iron. Soft iron has the property of concentrating magnetic field lines within itself. By enclosing the region to be shielded with a soft iron casing, the external magnetic field lines will preferentially pass through the iron rather than entering the shielded region, effectively diverting the field lines around it.