Question

Write two characteristics of a material to construct a permanent magnet. Give two examples of such materials.

Answer 1

A permanent magnet is a magnetic material that holds its magnetic properties even when the magnetic field (that magnetises it) is removed. Coercivity of a magnetic material tells us how much external magnetic field is required to make it demagnetised. Retentivity tells us how much is the material magnetised even in absence of external magnetic fields.

Complete answer:
Let us first understand what is a permanent magnet. A permanent magnet is a magnetic material that holds its magnetic properties even when the magnetic field (that magnetises it) is removed. There are two terms with which we understand a magnetic material and they are coercivity and retentivity of the magnetic material.

When a magnetic material is subjected to an external magnetic field, the magnetisation of the material increases and saturates at a certain value. Then if the external magnetic field is decreased, the magnetisation of the material also decreases. However, when there is no external magnetic field, there is some magnetisation in the material. This is called rentivity of the material.

Later, if we apply the external magnetic field in the opposite direction, the magnetisation of the material decreases and at a certain value of the magnetic field, the magnetisation becomes zero. This value of the magnetic field is called coercivity. For a good permanent magnet, both the coercivity and the retentivity of the material should be large. The examples of such materials are steel and alnico.

Note: The materials that are used to make permanent magnets are called ferromagnetic materials. Ferromagnetic materials are those materials which remain magnetised even after removing the external magnetic field. Other materials are paramagnetic and diamagnetic materials, which show magnetic properties only in the presence of external magnetic fields.