ANS:A - coercive force.
a) Coercive force (Hc): Coercive force is a measure of the ability of a magnetic material to resist demagnetization. It represents the strength of an external magnetic field required to reduce the magnetization of the material to zero after the material has been previously magnetized. In hard magnetic materials, the coercive force is high, meaning that these materials require a significant amount of external magnetic field strength to demagnetize them once they have been magnetized. This property makes hard magnetic materials useful for applications where strong, stable magnetic fields are required, such as in permanent magnets.
b) Residual magnetic induction (Br): Residual magnetic induction, also known as remanence or magnetic flux density, refers to the magnetic induction that remains in a magnetic material after the removal of an external magnetic field. In other words, it is a measure of the material's magnetization when the external magnetic field is removed. In hard magnetic materials, the residual magnetic induction is high, indicating that these materials retain a significant amount of magnetization even after the external magnetic field is removed. This property is essential for the functionality of permanent magnets, as it ensures that they maintain their magnetic properties over time and in various operating conditions.
In summary, hard magnetic materials possess both high coercive force (resistance to demagnetization) and high residual magnetic induction (ability to retain magnetization), making them well-suited for applications where strong, stable magnetic fields are required, such as in permanent magnets.