What is hysterisis? Define the terms 'coercivity' and 'retentivity' of a ferromagnetic material.

Hysteresis refers to the lag between the input and output in a system, particularly in magnetic materials. When a ferromagnetic material is magnetized, it does not return to its original state immediately after the external magnetic field is removed. This behavior is crucial in understanding how materials retain magnetism.

Key Concepts in Hysteresis

Coercivity

Coercivity is the measure of a material's resistance to becoming demagnetized. It indicates the strength of the external magnetic field required to reduce the magnetization of a material to zero after it has been magnetized. A high coercivity means the material retains its magnetization well, making it suitable for permanent magnets.

Retentivity

Retentivity, also known as remanence, is the ability of a ferromagnetic material to retain magnetization after the external magnetic field is removed. It reflects how much magnetic field remains in the material when the magnetizing force is no longer applied. High retentivity is desirable for materials used in applications like magnetic storage devices.

Summary of Terms

• Hysteresis: The lag in magnetization response to an external magnetic field.
• Coercivity: Resistance to demagnetization, indicating the strength needed to reduce magnetization to zero.
• Retentivity: The level of magnetization retained after the external field is removed.