Why do some materials become magnetised when exposed to a current?

Some materials become magnetised when exposed to a current due to the alignment of their atomic magnetic moments.

When an electric current is passed through certain materials, it can cause the material to become magnetised. This is due to a property of the atoms within the material known as the atomic magnetic moment. Each atom has a tiny magnetic field associated with it, and in most materials, these fields are randomly oriented, cancelling each other out and resulting in no overall magnetic field. However, when an external magnetic field is applied, such as from an electric current, these atomic magnetic moments can align with the field, creating a net magnetic field and causing the material to become magnetised.

The ability of a material to become magnetised in this way is known as its magnetic permeability. Materials with high magnetic permeability, such as iron, nickel, and cobalt, are easily magnetised when exposed to a current. These are known as ferromagnetic materials. On the other hand, materials with low magnetic permeability, such as copper and aluminium, are not easily magnetised, and are known as diamagnetic materials.

The process of magnetisation is not permanent in all materials. In some, once the external magnetic field is removed, the atomic magnetic moments return to their random orientations, and the material loses its magnetisation. These are known as soft magnetic materials. In others, the atomic magnetic moments remain aligned even after the external field is removed, and the material retains its magnetisation. These are known as hard magnetic materials.

The phenomenon of magnetisation by electric current is the basis for many technologies, including electric motors, transformers, and data storage devices. Understanding this process is therefore crucial for the design and operation of these technologies.