IGCSE Physics 0625 – Simple Phenomena of Magnetism: Induced Magnetism
Simple Phenomena of Magnetism
Objective: Describe Induced Magnetism
When a piece of ferromagnetic material (such as iron, nickel or cobalt) is placed in the magnetic field of a permanent magnet, the material becomes a temporary magnet. This process is called induced magnetism. The induced magnet has a north and a south pole that align with the external magnetic field.
How Induced Magnetism Occurs
Domain Alignment: In an unmagnetised ferromagnetic material, magnetic domains are randomly oriented, so the net magnetic field is zero.
External Field Applied: When the material enters an external magnetic field \$ \mathbf{B} \$, the domains tend to rotate so that their magnetic moments align with \$ \mathbf{B} \$.
Temporary Magnetisation: The alignment creates a net magnetic moment, turning the piece into a temporary magnet. When the external field is removed, thermal agitation causes the domains to randomise again, and the material loses its magnetism.
Key Characteristics
Induced magnetism is temporary; the material loses its magnetism when the external field is removed.
The induced poles are opposite to those of the magnet that creates the field (i.e., the side of the material nearest the north pole of the external magnet becomes a south pole, and vice‑versa).
The strength of the induced magnet depends on the strength of the external field and the material’s magnetic susceptibility.
Examples of Induced Magnetism
Bringing a steel paperclip close to a bar magnet – the paperclip becomes a temporary magnet and is attracted to the magnet.
Placing an iron nail inside a solenoid carrying current – the magnetic field inside the solenoid induces magnetism in the nail.
The Earth’s magnetic field induces a weak magnetisation in ferromagnetic rocks, which is the principle behind paleomagnetism.
Comparison of Magnet Types
Feature
Permanent Magnet
Induced Magnet
Electromagnet
Source of Magnetism
Aligned domains after manufacturing
External magnetic field aligns domains temporarily
Electric current through a coil
Duration of Magnetism
Indefinite (until demagnetised)
Only while external field is present
Only while current flows
Strength Control
Fixed
Depends on external field strength
Adjustable by changing current
Typical Uses
Compass needles, fridge magnets
Temporary holding of metal objects
Electric motors, relays, cranes
Relevant Formulae
The force on a current‑carrying conductor in a magnetic field is given by
\$\mathbf{F}=BIL\sin\theta\$
where \$B\$ is the magnetic flux density, \$I\$ is the current, \$L\$ is the length of the conductor within the field, and \$\theta\$ is the angle between the field and the conductor.
Suggested Diagram
Suggested diagram: An iron nail placed inside a solenoid. The external magnetic field lines from the solenoid are shown entering the north pole of the nail and exiting the south pole, illustrating induced magnetism.
Summary
Induced magnetism is a temporary magnetisation that occurs when a ferromagnetic material is exposed to an external magnetic field. The process involves the alignment of magnetic domains, resulting in a temporary north and south pole that mirror the external field. Understanding induced magnetism helps explain everyday phenomena such as the attraction of a paperclip to a magnet and the operation of simple electromagnets.