Question

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

Answer 1

Hint: If the raw iron material and steel are kept in the same magnetic field it is seen that they occupy almost the same magnetism. After some time, if these rods are picked up from the magnetic field it is seen that the raw iron and the steel still carry almost the same magnetism.

If we draw the degree of magnetization vs strength of magnetic field for these ferromagnetic substances(iron and steel), we get a close loop. This is called the magnetization cycle. The magnetization dimension is backward of the magnetic intensity – this is called Hysteresis.
iron has more retentivity than steel. But if the iron is used roughly then it destroys easily than the steel.

Complete step by step answer:

If the raw iron material and steel are kept in the same magnetic field it is seen that they occupy almost the same magnetism. After some time, if these rods are picked up from the magnetic field it is seen that the raw iron and the steel still retain almost the same magnetism. But the iron has more retentivity than steel. But if the iron is used roughly then it destroys easily than the steel.

If we draw the degree of magnetization $(M)$ vs strength of magnetic field $(H)$ for these ferromagnetic substances(iron and steel), we get a close loop. This is called the magnetization cycle.
The degree of magnetization is backward of the strength of the magnetic field. This backward characteristic of the degree of magnetization is called Hysteresis.

The figure shows the hysteresis loop of steel.
Coercivity: The coercivity is the property for which the magnetic material can hold its magnetism even after used roughly.
Retentivity: The retentivity is the properties of the magnetic material for which it can retain its magnetism even after disappearing from the magnetic field.
If we consider two different ferromagnetic materials – one is raw iron(soft iron) and another is steel, are occupied magnetism, it is seen that they can hold the almost same amount of magnetism even after removing them from the magnetic field.
But raw iron has more retentivity than steel. And it is seen that if we use the iron roughly it destroys easily than the steel. So steel has more coercivity than iron.

Note: To make a permanent magnet the retentivity should be high so that it can hold some magnetism even after removing from the magnetic field.
To make an electromagnet the retentivity and coercivity should be low. And also the hysteresis loss should be low so that the material would not be hot during demagnetizing.