A magnet attracts iron but it does not attract plastic. Why?

Materials that are not attracted to a magnet like plastic have a permeability of, essentially, 1.  There is no magnetism induced in them by an external magnetic field, and therefore, they are not attracted by a magnet. 
 Iron has a high permeability (μ), and thereby, magnetic fields can be induced in them when exposed to an external magnetic field.

The degree of the induced magnetism (by which Magnet attracts other materials) is related to a material’s magnetic permeability, and it is expressed as a unit-less value designated by, mu (μ). 
The higher the material’s permeability, the greater the magnetic induction and the resulting force of attraction. 


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