Published by Patrick Mutisya · 14 days ago
State the differences between the properties of temporary magnets (made of soft iron) and the properties of permanent magnets (made of steel).
Both soft iron and steel consist of tiny regions called magnetic domains. In an unmagnetised piece of material the domains are oriented randomly, giving a net magnetic field of zero.
When a magnetic field is applied, the domains tend to align with the field. The ease with which they can be re‑aligned determines whether the material behaves as a temporary or a permanent magnet.
| Property | Temporary Magnet (Soft Iron) | Permanent Magnet (Steel) |
|---|---|---|
| Typical material | Soft iron, low carbon content | Hard steel, high carbon or alloyed steel |
| Magnetisation method | Induced by an external magnetic field (induction) | Magnetised by strong field and then “locked in” (hysteresis) |
| Retention of magnetism | Very short; loses magnetism as soon as the external field is removed | Long‑term; retains a substantial fraction of its magnetisation for years |
| Magnetic field strength (\$B\$) | Typically up to \$0.2\ \text{T}\$ while the external field is present | Can reach \$0.5\ \text{T}\$ or more even without an external field |
| Coercivity (resistance to demagnetisation) | Low; small opposing fields or mechanical shock demagnetise it | High; requires a strong opposing field or high temperature to demagnetise |
| Retentivity (ability to retain magnetisation) | Low | High |
| Response to external field | Domains re‑align easily; magnetism appears only while the field acts | Domains are already largely aligned; external field may only slightly increase magnetisation |
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Soft iron has a crystal structure that allows its magnetic domains to move freely. This gives it a low coercivity and high magnetic permeability, making it an excellent material for temporary magnetisation.
Hard steel contains impurities and alloying elements that “pin” the domain walls, preventing them from moving easily. This results in high coercivity and high retentivity, essential for permanent magnets.
Magnetic field intensity \$H\$ and magnetic flux density \$B\$ are related by the material’s permeability \$\mu\$: \$B = \mu H\$
For a permanent magnet, the residual flux density \$Br\$ (also called remanence) is a key parameter, whereas for a temporary magnet \$Br\$ is essentially zero once the external field is removed.