Magnetism, Permeability, Reluctance, Remanence, and Magnetic Shielding

  • The flow of current through a metallic conductor creates a magnetic field around the conductor.
  • The magnetic field is oriented perpendicularly to the direction of the current flow. The strength of this field is inversely proportional to the distance away from the conductor.
  • If the conductor is looped, magnetic field is concentrated in the looped core. If the conductor is turned into a coil, then the magnetic field is concentrated around the coil, and if the number of coils is sufficiently high, then an electromagnet is produced.
  • The magnetic fields in the electromagnet is held more closer to the coil if the electromagnet has an air core. However, if an iron core is introduced, the flux density of the magnetic fields increases as the iron allows the lines of force to be distributed throughout its volume. This ability to concentrate the lines of force of the magnetic field in a material other than the core is called permeability. Conversely, the property of a material to not be affected by lines of magnetic force is called reluctance.
  • Permeability is represented using the symbol μ. Air has a permeability value of 1.
  • According to the atomic theory of magnetism, atoms in nickel, iron, and cobalt are arranged into magnetic entities called domains. Each domain has one of three directions of magnetization: hard magnetization, easy magnetization, and semi-hard magnetization. When this material is placed in a weak field of magnetic force, the domains align themselves in the easy magnetization direction, which means that they will lose their aligned orientation when the force is removed. If the force is increased, then the domains are aligned in the semi-hard direction, which implies that some domains are aligned in the hard direction while others are in the easy direction, and when the force is removed, those domains aligned in the easy direction lose their orientation, while those aligned in the hard direction keep their orientation. The degree of magnetism retained after the external magnetizing force is removed is called residual magnetism. When enough force is applied to align all the domains in the hard direction, then the iron is described as being saturated. This ability to retain magnetism after the external magnetic field is withdrawn is called remanence.
  • Any element that has, or can have, domains is called a ferromagnetic element. Non-ferromagnetic materials are categorized into two broad groups depending on how they behave when they are placed in a magnetic field:
  1. Paramagnetic Material: Its atoms line up with the magnetic field.
  2. Diamagnetic Material: Its atoms are not aligned to the magnetic field, with most atoms aligned perpendicularly to this field.
  • Ferromagnetic material with a high remanence value, e.g Alnico, is used to create a permanent magnet. Relatedly, the ferromagnetic material with a low remanence value such as permalloy or pure iron, is used to create a temporary magnet.
  • A ferrite core is made up of finely powdered iron held together by a non-conductive binder.
  • The permanent magnet can lose its magnetism when heated, and the temperature at which this loss occurs is called the Curie temperature.
  • If a permanent magnet is placed inside a soft iron box, then all its magnetic lines of force are contained within the box and no external field is produced. This is called magnetic shielding.
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