An electromagnet is a type of magnet whose magnetic field is generated by the flow of electric current. Unlike permanent magnets, an electromagnet must be continuously powered to maintain its magnetic properties. This reliance on an external energy source makes the electromagnet a temporary magnet, allowing its magnetic force to be precisely controlled. The ability to instantly turn the magnetism on and off makes these devices indispensable in applications from scrapyard cranes to doorbells.
Creating the Magnetic Field
The fundamental principle of the electromagnet was first observed in 1820 by physicist Hans Christian Ørsted. He demonstrated that a moving electric charge, or current, produces a magnetic field in the surrounding space. This discovery established the link between electricity and magnetism, forming the basis of electromagnetism.
To concentrate this generated field, a wire is typically wound into a tight coil known as a solenoid. When electric current passes through this coiled wire, the tiny magnetic fields from each segment combine and align along the central axis. The strength of the resulting magnetic field is directly proportional to the current flowing and the number of turns in the coil. When the power supply is disconnected, the current flow stops, and the magnetic field immediately collapses.
The Role of Soft Iron Core Material
While a coil of wire alone acts as an electromagnet, its magnetic field is relatively weak for most practical purposes. To increase the field strength, a core of ferromagnetic material, most commonly soft iron, is placed inside the solenoid. Soft iron is categorized as being “magnetically soft” due to its specific magnetic properties.
Ferromagnetic materials are composed of microscopic regions called magnetic domains, where the magnetic poles of the atoms are aligned. When the external magnetic field from the solenoid is applied, it forces these randomly oriented domains within the soft iron to quickly rotate and align with the field. This collective alignment creates a powerful magnetic field that is significantly stronger than the coil’s field alone.
The temporariness of the electromagnet stems from soft iron’s property of low retentivity and low coercivity. Retentivity is a material’s ability to retain magnetism after the external field is removed. Because soft iron has low retentivity, its magnetic domains instantly lose alignment and return to a random orientation when the electric current is switched off. This ensures the magnetic field vanishes almost entirely, allowing the electromagnet to be de-energized immediately.
How Permanent Magnets Differ
The difference between a temporary electromagnet and a permanent magnet is primarily a matter of material science, focusing on the magnet’s ability to resist demagnetization. Permanent magnets are constructed from “magnetically hard” materials, such as specialized steel alloys, Alnico, or rare-earth materials like Neodymium. These materials possess high retentivity, meaning they are designed to keep their magnetic domains locked in alignment once magnetized.
These hard materials also exhibit high coercivity, which measures how strongly a material resists having its magnetism reversed or removed. Even after the external magnetizing field is removed, the internal structure requires a strong, opposing magnetic field to disrupt the domain alignment. This property ensures that the permanent magnet maintains a stable, continuous magnetic field without any ongoing power input.