Simple Phenomena of Magnetism – Uses of Permanent Magnets and Electromagnets
4.1 Simple Phenomena of Magnetism
Objective
Describe the uses of permanent magnets and electromagnets.
Permanent Magnets
Permanent magnets retain their magnetism without the need for an external power source. Their magnetic field is produced by the alignment of magnetic domains within the material.
Compass needles – indicate direction by aligning with Earth’s magnetic field.
Door latches and cupboard locks – hold doors closed through magnetic attraction.
Refrigerator doors – magnetic strips keep the door sealed.
Speakers and headphones – interact with a coil carrying current to produce sound.
Magnetic separators in recycling plants – attract ferrous waste.
Magnetic levitation (maglev) toys – use repulsion between like poles.
Electromagnets
An electromagnet consists of a coil of wire wound around a ferromagnetic core. When an electric current \$I\$ flows through the coil, a magnetic field \$B\$ is produced, given by
\$B = \mu_0 n I\$
where \$\mu_0\$ is the permeability of free space and \$n\$ is the number of turns per unit length. The field disappears when the current is switched off, allowing the magnetism to be turned on and off as required.
Electric bells – the coil attracts a metal striker when current flows.
Relays – use an electromagnet to open or close contacts in a circuit.
Magnetic cranes in scrap yards – lift heavy pieces of iron or steel.
MRI machines – strong electromagnets create uniform fields for imaging.
Solenoids in valves and actuators – produce linear motion when energized.
Particle accelerators – steer charged particles using controlled magnetic fields.
Comparison of Uses
Feature
Permanent Magnet
Electromagnet
Power requirement
None after manufacture
Requires continuous current
Field strength control
Fixed
Adjustable by changing current
Typical applications
Compass, fridge doors, simple locks
Relays, cranes, MRI, solenoids
On/off capability
Not possible
Easy on/off with switch
Suggested diagram: Cross‑section of an electromagnet showing a coil wound around an iron core, with current direction indicated and magnetic field lines emerging from the poles.
Key Points to Remember
Permanent magnets are useful where a constant magnetic field is needed without power.
Electromagnets are preferred when the magnetic field must be switched on/off or varied in strength.
Both types exploit the attraction between opposite poles and repulsion between like poles.
Safety: Strong electromagnets can attract ferrous objects suddenly; keep loose clothing and metallic objects away.