State the differences between the properties of temporary magnets (made of soft iron) and the properties of permanent magnets (made of steel)
4.1 Simple Phenomena of Magnetism
Objective
State the differences between the properties of temporary magnets (made of soft iron) and permanent magnets (made of hard steel).
Key Definitions (syllabus wording)
Temporary magnet – a piece of soft iron that becomes magnetic only while it is in the presence of an external magnetic field. The magnetism disappears almost immediately when the field is removed.
Permanent magnet – a piece of hard steel that retains a significant amount of magnetisation after the external magnetic field has been removed.
Magnetic and Non‑magnetic Materials
- Magnetic (ferromagnetic) materials: iron, nickel, cobalt and most steels – strongly attracted by a magnet.
- Non‑magnetic materials: wood, plastic, glass, water, etc. – experience no detectable magnetic force because their atoms have all electrons paired, giving no net magnetic moment.
Forces Between Magnetic Poles & Materials
- Like poles (N–N or S–S) repel – if you bring two north poles together they push each other apart.
- Opposite poles (N–S) attract.
- A magnet attracts a piece of ferromagnetic material (e.g., a nail) because the material becomes an induced temporary magnet.
Induced Magnetism (Temporary Magnetisation)
When a piece of soft iron is placed in a magnetic field, the field aligns its magnetic domains, turning the iron into a temporary magnet. Remove the field and the domains randomise, so the magnetism vanishes.
Simple experiment: Hang a soft‑iron nail from a thread, bring a strong bar magnet close, and observe the nail turn to align with the magnet. Move the magnet away – the nail swings back freely.
Magnetic Field Concept
The magnetic field is the region around a magnet where another magnetic pole would experience a force. The field direction is defined as the direction a north pole would move.
- Field‑lines emerge from the north pole and enter the south pole.
- Closer line spacing = stronger field; wider spacing = weaker field.
Plotting Magnetic Field Lines (practical activity)
- Place a bar magnet on a sheet of paper.
- Sprinkle fine iron filings over the magnet.
- Gently tap the paper – the filings arrange themselves along the invisible field lines.
- Use a compass to trace the direction of the field at several points and draw the lines on the paper.
Magnetic Domains – Why Some Materials Keep Their Magnetism
Both soft iron and steel consist of tiny regions called magnetic domains. In an unmagnetised piece the domains point in random directions, giving a net 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.
Comparison of Temporary and Permanent Magnets
| Property | Temporary Magnet (Soft Iron) | Permanent Magnet (Hard Steel) |
|---|---|---|
| Typical material | Soft iron (low carbon) | Hard steel (high carbon or alloyed) |
| How it is magnetised | Induced by an external field (induction) | Magnetised by a strong field and then “locked in” (hysteresis) |
| Retention of magnetism | Very short; disappears as soon as the external field is removed | Long‑term; retains a substantial fraction of its magnetisation for years |
| Magnetic field strength (B) while magnetised | Up to ≈ 0.2 T (only while the external field acts) | Typically ≥ 0.5 T even without an external field |
| Coercivity (resistance to demagnetisation) | Low – small opposing fields or mechanical shock demagnetise it | High – requires a strong opposing field, high temperature, or a hammer blow |
| Retentivity (ability to retain magnetisation) | Low | High |
| Response to an external field | Domains re‑align easily; magnetism appears only while the field acts | Domains are already largely aligned; external field may increase magnetisation only slightly |
| Typical uses |
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Why the Differences Occur
- Soft iron has a crystal structure that allows domain walls to move freely. This gives it high magnetic permeability, low coercivity, and therefore a short‑lived induced magnetism.
- Hard steel contains impurities and alloying elements that “pin” domain walls, making them difficult to move. The result is high coercivity and high retentivity – the essential traits of a permanent magnet.
Magnetic‑Field Equations (useful for A‑Level extension)
Magnetic flux density B and magnetic field intensity H are related by the material’s permeability μ:
\( B = \mu H \)
For a permanent magnet the residual flux density (Br, also called remanence) is a key parameter; for a temporary magnet Br is essentially zero once the external field is removed.
Suggested Diagrams
- Figure 1 – Field‑line pattern of a bar magnet (already shown above).
- Figure 2 – Domain alignment in (a) soft iron before/after induction and (b) hard steel before/after magnetisation.