A bar magnet is a permanent magnet, generating its own magnetic field without the need for any external power source. This rectangular block of ferromagnetic material, typically made from iron, steel, or alloys like Alnico or Ferrite, possesses two distinct poles, conventionally labeled North and South.
The ability of a bar magnet to sustain this magnetic field makes it the classic example used in classrooms to demonstrate magnetic properties. Its inherent magnetism distinguishes it from other types of magnets that rely on external influence to function.
What Makes a Magnet Permanent
A permanent magnet retains its magnetic properties long after exposure to an external magnetizing field. Unlike materials that lose their magnetism quickly, a permanent magnet holds onto the alignment of its internal structure, producing a stable and continuous magnetic field. The material itself is the source of the magnetic field, requiring no ongoing electrical current or nearby magnet to maintain its strength. The degree to which a material resists demagnetization is known as its coercivity, and permanent magnets are made from “hard” magnetic materials with high coercivity, such as rare-earth magnets like Neodymium, Ferrite, and Alnico compounds.
The magnetic field is a self-sustaining phenomenon arising from its internal structure. This field remains constant unless the magnet is subjected to extreme conditions, such as being dropped repeatedly or heated above the Curie temperature. Heating the magnet disrupts the organized internal structure, causing the magnetic properties to weaken or disappear entirely.
How Magnetic Domains Create Polarity
A bar magnet’s lasting field involves microscopic regions called magnetic domains. Within these domains, the magnetic moments of countless atoms are aligned in a uniform direction. In a non-magnetized piece of ferromagnetic material, these domains are oriented randomly, causing their magnetic effects to cancel each other out, resulting in no net external magnetism.
To create a permanent magnet, the material is exposed to a powerful external magnetic field. This external field forces the boundaries of the magnetic domains to shift, causing the domains that are already aligned with the field to grow at the expense of others. Eventually, the majority of the domains are locked into a single, uniform direction that runs the length of the bar. This collective alignment generates the macroscopic North and South poles observed at the ends of the bar magnet. The material’s inherent resistance to demagnetization keeps these domains from reverting to a random orientation once the external field is removed.
Permanent Versus Temporary Magnets
The distinction between permanent and temporary magnets lies in their ability to retain magnetism once the initial magnetizing force is gone. Permanent magnets hold their magnetic field indefinitely because their internal domain alignment is stable. Temporary magnets, in contrast, only exhibit a strong magnetic field while under the influence of an external force.
A common type of temporary magnet is “soft” ferromagnetic material, such as soft iron, which quickly becomes magnetized when placed near a permanent magnet but loses its magnetic properties almost immediately when the magnet is removed. Another category is the electromagnet, which requires a constant flow of electric current through a coil of wire to produce a magnetic field.
When the electric current is switched off, the magnetic field disappears instantly, allowing for precise control over the magnetism. This makes electromagnets useful for applications like scrap yard cranes and industrial machinery.