Magnetism

General properties of magnetism and magnets

Like the electric field, which we know to be attractive or repulsive, magnetic effects can be either attractive or repulsive. Nevertheless, in general, the lines of the magnetic field, which indicate the strength and direction of the magnetic field, are closed. This means that magnetic effects are never only attractive or only repulsive, they are always both depending on where we are with respect to the source of the magnetic field.

Before understanding this concept with magnets, take a moment to analyse the magnetic field lines generated by a bar magnet:

In the previous image, the lines of the magnetic field can be easily traced due to the orientation of the iron shards affected by the magnet. It can be seen from the figure above that the lines form closed loops if we close them inside the magnet itself. Although the image is cropped, all lines that appear in the picture will eventually close.

What does this mean for magnets? In general, to each magnet, we assign different poles which are called north and south in reference to the poles of the Earth. The convention we follow is to first assign a direction to the closed loops of the magnetic field and then to call the north pole the site of the magnet where the magnetic field lines leave the magnet and the south pole the site where the magnetic field lines enter it. See the image below for an example of this:

The fundamental laws of magnetism are too complex to be studied here. But, we can still study the description of the behaviour of magnets and the types of magnetism they can produce. The interactions between magnets are governed by the following two laws:

• Poles of the same type repel each other
• Poles of different types attract each other

Basic principles of magnetism and electromagnetism

Magnets are the simplest objects that display magnetic properties, but the number of phenomena that are magnetic in nature is not confined to the bar magnets or horseshoe magnets we are familiar with. However, we can test the properties of general magnetic fields by using these permanent magnets, which are simple and accessible resources.

A key principle of magnetic fields is that they are produced by the movement of charged particles. The magnetic field produced by current-carrying wires is produced by the relative movement of the electrons inside the wire making up the current. The magnetic field produced by a permanent magnet can be explained by considering the angular motion of atomic electrons and the resulting magnetic field that the current caused by this motion produces.

Another key principle of magnetic fields is that their strength decreases with increasing distances from the source of the magnetic field. For example, the magnetic field produced by bar magnets surrounded by iron filings can be seen to decrease in strength with distance from the magnet. This is by virtue of the fact that the magnet seems to have little influence over the iron filings furthest from the magnet. They do not align with the magnetic field produced by the magnet and so appear to be scattered in random directions.

After observing the images of magnetic field lines in this article, you may have noticed a few other key properties of magnetic field lines, a few of which have been mentioned previously. Magnetic field lines always consist of closed loops. This means that if we follow a magnetic field line, we will always eventually end up back where we started. The magnetic field loops cannot cross over each other and have a particular direction associated with them which points in the direction of the magnetic field located at each point around the loop.

Types of Magnetism

The two main types of magnetism that you should be familiar with are permanent magnetism and induced magnetism. Here we will outline the differences between the two types of magnetism and provide some real-life examples of both so that you can contextualise your knowledge of magnetism.

Permanent magnets

Permanent magnets produce their own magnetic field. This permanent magnetic field cannot be 'turned on or off', it is simply always there. A quick and easy way of testing whether an object is a permanent magnet is to hold it close to another known permanent magnet. If the two objects repel or attract each other as soon as they are held close together then we can be sure that the object in question is a permanent magnet too.

Fig. 5. Permanent magnets, as the name suggests, are permanently magnetic. They always have a north pole and a south pole. Wikimedia Commons.

Permanent magnets, like the bar magnet in the figure above, always have a north pole and a south pole. You can determine which end of a permanent is the north pole and which end is the south pole by holding one end of the magnet within a close distance of the north pole of another known permanent magnet; if the magnets attract each other than the end held to the north pole of the known magnet is a south pole and they repel then the end held to the known magnet is the north pole.

Induced magnets

Induced magnets are objects or materials that become magnetic when they are placed in a magnetic field. When induced magnets are removed from the magnetic field, they quickly lose all or most of their magnetism, so we call induced magnets temporary magnets. The direction of the magnetic field of an induced magnet will always point in the direction of the permanent magnet's magnetic field. This means that the induced magnet will be attracted towards the magnet producing it.