CBSE Class 12 Physics Chapter 5: Magnetism and Matter NCERT Solutions

(a) At higher temperatures, the random thermal motion of molecules in a paramagnetic material disrupts the alignment of magnetic dipoles with the external magnetising field. When the sample is cooled, this thermal agitation is reduced, leading to a more ordered alignment of dipoles and consequently, a greater magnetisation for the same applied field.

(b) Diamagnetism arises from the orbital motion of electrons, which induces a magnetic dipole moment that always opposes the applied external magnetic field. This induced moment is a consequence of Lenz's law and is not significantly dependent on the thermal motion of atoms or molecules. Therefore, diamagnetism is largely independent of temperature.

(c) Bismuth is a diamagnetic substance, meaning it weakly repels magnetic fields. When used as a core in a toroid, it will slightly reduce the magnetic field strength within the core compared to an empty core. 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. It is generally much higher for lower applied magnetic fields and tends to decrease as the magnetic field strength increases, especially as the material approaches saturation.

(e) Magnetic field lines are nearly normal to the surface of a ferromagnet because the high magnetic permeability of ferromagnetic materials causes them to concentrate magnetic flux lines. Similar to how electric field lines are normal to the surface of a conductor (where the electric field inside is zero), magnetic field lines tend to be normal to the surface of a ferromagnet to minimise the magnetic field within the material itself and to ensure flux continuity. The high permeability means the material readily supports a strong internal magnetic field, which aligns with the external field, pushing field lines towards being perpendicular to the surface.

(f) Yes, the maximum possible magnetisation (saturation magnetisation) of a paramagnetic sample can be of the same order of magnitude as that of a ferromagnet. However, achieving this saturation in a paramagnetic material typically requires very strong external magnetising fields and very low temperatures, conditions not usually encountered in everyday ferromagnets.

(a) In ferromagnetic materials, magnetic dipoles are aligned in small regions called domains. When an external magnetic field is applied, these domains align with the field. The magnetisation curve is irreversible because the domain walls do not move back perfectly to their original positions when the field is reduced or reversed. This is due to factors like pinning of domain walls by impurities or crystal defects, and the energy required to move these walls. This 'lag' in response to the changing magnetic field is what causes the hysteresis loop.

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

(c) The statement means that ferromagnetic materials can 'remember' their magnetic state. Due to hysteresis, a ferromagnet retains a significant amount of magnetisation (remanence) even after the external magnetising field is removed. This retained magnetisation can be in one of two directions, representing binary states (like '0' or '1'). By controlling the applied magnetic field, this state can be changed and then retained, allowing the material to store information, much like a computer memory.

(d) Ferromagnetic materials used for coating magnetic tapes and building memory stores are typically hard ferromagnetic materials. These materials have wide hysteresis loops, meaning they have high retentivity and coercivity. This ensures that the magnetisation is stable and can be reliably read and written, allowing for effective data storage.

(e) To shield a certain region of space from magnetic fields, it should be enclosed by a material with very high magnetic permeability. Ferromagnetic materials, such as soft iron, are excellent for this purpose. The high permeability allows the material to channel the magnetic field lines through itself, effectively diverting them away from the region to be shielded. This creates a 'magnetic shield' that significantly reduces the magnetic field within the enclosed space.