Magnetism – Learn


Magnetism creates a field that attracts or repels other magnetic materials. Like magnetic poles repel and unlike magnetic poles attract. Magnetic poles exist only as dipoles – this means that any magnet will have a north and a south pole.

All materials experiences magnetism to some extent due to the electric charges that naturally exist in all atoms (electric charges are affected by magnetic fields). Some materials will experience magnetism more strongly than others.

Materials that can be magnetised are called ferromagnetic materials. Ferromagnetic materials such as iron, cobalt and nickel are very easily magnetised. Magnetic materials have magnetic domains within them. A magnetic domain is a region in the material where the magnetic field is aligned. For ferromagnetic materials to become magnets, the majority of the magnetic domains in the material must be aligned. The idea of magnetic domains is illustrated below:

  • A material that is unmagnetised has separate magnetic domains that point in different directions.
  • A material that is magnetised has separate magnetic domains that point in the same direction.

Magnetisation

Magnetic materials may not act as a magnet on other magnetic materials but can become magnetised when exposed to a magnetic field. A permanent magnet is a magnetic material that will maintain its magnetic field after being magnetised. Permanent magnets are made by taking a ferromagnetic material and exposing it to a large magnetic field.

Most magnetic materials are alloys of iron, cobalt or nickel. The alloy is made into the desired shape and exposed to a very strong magnetic field created by a solenoid. A solenoid uses current electricity to create the strong magnetic fields.

A ferromagnetic material that has not been magnetised will not create its own magnetic field. This is due to the domains being randomly aligned and cancelling each other out, resulting in a zero net magnetic field. When exposed to strong magnetic fields, these domains are forced to line up. The more the domains line up, the stronger the resulting magnetic field. Over time these magnets will lose their magnetism. The rate at which they lose their magnetism depends on the material they are made of and how strong the magnetising field was. Higher temperatures will also result in the magnet losing its magnetism.


Modelling magnetic fields

Magnetic fields can be modelled a few ways:

  • A conceptual model which uses field lines to demonstrate the strength and direction of a magnetic field.
  • A mathematical model which uses equations to explain and predict the strength of magnetic fields around current-carrying conductors and solenoids.
  • A computational model can be used to model magnetic fields based on numerical calculations and computer software. This can be very useful for illustrating a magnetic field in 3-D
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