Describe induced magnetism
Simple Phenomena of Magnetism
Objective
To describe induced magnetism – the temporary magnetisation of a soft ferromagnetic material when it is placed in an external magnetic field, as required by the Cambridge IGCSE Physics 0625 syllabus.
1. What is a Magnetic Field?
- A magnetic field is a region of space in which a magnetic pole experiences a force.
- Field‑lines are a useful visual aid:
- They emerge from the north pole of a magnet and enter the south pole.
- The density of lines indicates the strength of the field (B, measured in tesla, T).
2. Definition of Induced Magnetism
When a piece of ferromagnetic material (iron, nickel, cobalt or their alloys) is placed in an external magnetic field, the field forces the magnetic domains inside the material to align. The piece then behaves as a temporary magnet with a north and a south pole that correspond to the direction of the applied field. When the external field is removed the domains randomise and the magnetism disappears.
3. How Induced Magnetism Occurs
- Unmagnetised ferromagnetic material – Domains are randomly oriented, giving a net magnetic field of zero.
- External field applied – The magnetic field B exerts a torque on each domain, rotating the magnetic moments so that they line up with the field direction.
- Temporary magnetisation – The collective alignment creates a net magnetic moment. The end toward which the field lines enter becomes the induced south pole; the end where the lines leave becomes the induced north pole.
- Removal of the field – Thermal agitation randomises the domains again, and the induced magnetism vanishes.
4. Forces Between Magnetic Poles and Materials
- Like poles repel – N ↔ N and S ↔ S experience a repulsive force.
- Opposite poles attract – N ↔ S attract each other.
- Magnet ↔ un‑magnetised ferromagnetic material – The material becomes an induced magnet with the opposite pole nearest the magnet, so the force is always attractive.
- Magnet ↔ non‑ferromagnetic material – No significant attraction or repulsion (e.g., aluminium, copper).
5. Temporary vs. Permanent Magnets
| Feature | Permanent Magnet (hard ferromagnetic alloy) | Induced (temporary) Magnet (soft ferromagnetic material) |
|---|---|---|
| Typical material | Hard ferromagnetic alloys (e.g., alnico, steel) | Soft ferromagnetic material (e.g., soft iron, soft steel) |
| Domain structure | Domains are permanently aligned during manufacture | Domains align only while an external field is present |
| Duration of magnetism | Indefinite (until deliberately demagnetised) | Only while the external field remains; disappears on removal |
| Strength control | Fixed (depends on material and processing) | Proportional to the strength of the external field and the material’s magnetic susceptibility |
| Typical uses | Compass needles, fridge magnets, permanent‑field motors | Temporary holding of metal objects, magnetic shielding, simple electromagnets |
6. Key Characteristics of Induced Magnetism
- It is temporary – the magnetism disappears when the external field is removed.
- Only ferromagnetic materials (iron, nickel, cobalt and their alloys) exhibit induced magnetism; other metals do not.
- The induced north pole forms at the end where the external field lines exit; the induced south pole forms where the lines enter.
- The magnitude of the induced magnet depends on:
- Strength of the external magnetic field (B).
- Magnetic susceptibility of the material (soft iron > soft steel > other alloys).
7. Everyday Examples
- A steel paper‑clip is attracted to a bar magnet because the clip becomes an induced magnet.
- Placing an iron nail inside a solenoid carrying current causes the solenoid’s field to induce a north and a south pole in the nail.
- Ferromagnetic minerals in rocks are weakly magnetised by Earth’s field; the record is used in paleomagnetism.
8. Relevant Formulae
Although induced magnetism is explained by domain alignment, the force on a current‑carrying conductor in a magnetic field is often useful in related experiments (e.g., electromagnets):
\( \displaystyle \mathbf{F}=BIL\sin\theta \)
- B – magnetic flux density (tesla, T)
- I – current (ampere, A)
- L – length of conductor within the field (metre, m)
- \(\theta\) – angle between the conductor and the field direction
9. Suggested Diagram for Revision
Summary
Induced magnetism is the temporary magnetisation of a soft ferromagnetic material when it is exposed to an external magnetic field. The process involves the rotation and alignment of magnetic domains, producing a north and a south pole that correspond to the direction of the applied field. This explains everyday observations such as a paper‑clip being attracted to a magnet, underpins the operation of simple electromagnets, and satisfies the Cambridge IGCSE physics syllabus requirements for forces between magnetic poles, the nature of magnetic fields, and the distinction between temporary and permanent magnets.