More on magnetic materials and magnets
Physics Narrative for 11-14
Read more about the ways in which materials interact with other magnets
Permanent magnets retain their magnetism, while other materials are attracted to magnets (they are magnetic) but do not themselves become permanent magnets. How can we explain this?
The origin of magnetism lies in circulating electric currents. In atoms these circulating currents arise from the distribution of electric charge around the nucleus. How that charge reacts when it is placed in a magnetic field defines what sort of magnetic behaviour the material it forms will have.
Any material placed in a magnetic field becomes magnetised. Clearly some materials will become more strongly magnetised than others.
There are not only differences in how much materials become magnetised but also in how they behave when they are placed in the field. There are three main types of behaviour.
In diamagnetic materials the magnetisation is proportional to the magnetic field and in the opposite direction. This means that diamagnetic materials are always repelled by a magnet whose field they are in. Water is an example of a diamagnetic material.
In paramagnetic materials the magnetisation is proportional to the magnetic field and in the same direction. This means that paramagnetic materials are always attracted by a magnet whose field they are in. For example, paperclips are made of a steel alloy which is paramagnetic and a magnet can be used to pick up the clips. The paperclips are attracted to the magnet but do not themselves become magnets.
Ferromagnetic materials behave differently. Their susceptibilities are large, producing high magnetisations in relatively weak magnetic fields. Most significantly, when the field is removed, some ferromagnetic materials retain their magnetisation. These are the permanent magnets that we are familiar with.
In 11–14 physics, when we refer to a magnet, it is shorthand for a permanent magnet, made from ferromagnetic material (the common one is iron or some alloy containing iron). The magnetic materials we meet in science lessons (and around the home) are essentially paramagnetic – they are always attracted and never repelled by a magnet. It is worth remembering though that, to some extent, all materials respond to a magnetic field when they are placed in one; the manner and magnitude of the reaction depends on the material.