Magnetism

Permanent and induced magnetism

All magnets have a north seeking pole (north pole) at one end and a south seeking pole (south pole) at the other end. These two poles are oppositely charged and allow magnets to repel and attract other magnets depending on their arrangement. Like poles will repel each other and unlike poles will attract each other.

For example, if two magnets were placed so that their north poles were together, the magnets would repel each other. Similarly, if the two south poles were together, the magnets would also repel each other. If the two magnets were arranged so that a north pole and a south pole were together, the magnets would be attracted to each other.

Magnets are made of magnetic materials such as the metals iron, nickel and cobalt. These three metals are the only pure metals which can be turned into permanent magnets. A permanent magnet is one which has two permanent poles, meaning that the magnetism is always present. Steel is an alloy of iron which is also magnetic. An alloy is a mixture of two or more different elements with at least one of them being a metal. The properties of alloys come from the combination of the different properties of each of the elements contained. This makes them much more useful than pure metals alone. For example, alloys are often much harder and stronger than the pure metals contained due to the fact that they contain atoms of different sizes, meaning that the layers in the structure cannot slide over each other as easily as they could in a pure metal.

All permanent magnets produce their own magnetic fields as they always have two oppositely charged poles. A magnetic field is an area in which the non-contact force of magnetism has an effect on other magnets or magnetic materials. For example, cobalt is magnetic. If it was placed close enough to be within the magnetic field of a bar magnet, the cobalt would be attracted to it.

The interaction between the magnetic field and some previously non-magnetic materials can cause them to become magnetic. This process is called induced magnetism.

The strength of the magnetic field produced by a magnet depends on whether the material is a permanent magnet or an induced magnet. Induced magnets produce much weaker magnetic fields than permanent magnets do.

Some of these materials are able to retain their magnetism even after removal from the magnetic field. This means that they have become permanent magnets. Such materials are known as magnetically hard materials. Examples of magnetically hard materials include alloys containing high nickel, iron and cobalt contents.

Some materials can only be temporarily magnetised for as long as they remain in the magnetic field. Once these materials are removed from the magnetic field, they lose their induced magnetism again. These materials are known as induced magnets and are described as magnetically soft materials. Alloys containing lower amounts of iron, nickel and cobalt are magnetically soft and produce very weak magnetic fields.

Magnetic field diagrams

Although the magnetic fields created by magnets cannot be seen, we can see the effects of them using a bar magnet and iron filings. If the bar magnet is placed below a petri dish and iron filings are gently sprinkled into the petri dish and spread around, a pattern is produced. The iron filings will line up to show the positions of the magnetic field lines. The results of this experiment can be difficult to draw so most of the time we represent magnetic fields around a permanent magnet using a magnetic field line diagram as shown in the diagram below:

This diagram shows a bar magnet, and several curved lines going from the north pole to the south pole. The lines appear more concentrated as they get closer to the poles.

The curved lines are the magnetic field lines which represent the strength, direction and shape of the magnetic field. Each magnetic field line has an arrowhead to represent the direction of the magnetic field. The field lines always go from north to south. They are drawn coming out of the magnet at the north pole and going into the magnet at the south pole. This shows that the magnetic field goes from the north pole of the magnet to the south pole of the magnet.

The distance between the lines indicates the strength of the magnetic field. The closer the lines are to each other, the stronger the magnetic field is at that point.

As the field lines are drawn closer together at the two poles, this shows that the magnetic field and force of magnetism is strongest at the poles of the magnet.

All bar magnets produce their own magnetic field. If two bar magnets are placed in close proximity of each other, their magnetic fields will interact. The various ways in which they can interact are shown in the diagram below:

As can be seen from the diagram, if two bar magnets are positioned so that their opposite poles are close and attracting each other, their magnetic fields will interact and will overlap each other.

This overlapping of the two magnetic fields produces magnetic field lines which are parallel to each other and therefore have equal distances between each line. This equal distance between the lines shows that the strength of the magnetic field does not vary and is therefore uniform. This is known as a uniform magnetic field pattern as shown in the diagram below: