Everyone is familiar with magnets and magnetism. A bar magnet has a north pole at one end and a south pole at the other so called because when the magnet is suspended from its centre, these poles always point toward the north and south poles of the earth respectively. This is due to the fact that the earth itself behaves like a very large magnet and exerts a magnetic force on the suspended magnet, always twisting it into the same direction. This, of course, is the mechanism of a magnetic compass. The magnetic force and its nature can easily be investigated by bringing two bar magnets close to each other. If one is suspended (see figure 1.1) it will be found that, when brought close to it, the south ( S ) pole of the other will attract (pull towards it) the north (N) pole of the suspended magnet whilst its north pole will repel it (i.e. push it away). It will also be found that the effect becomes smaller the further apart are the two magnets, The magnetic force can thus be attractive or repulsive and dies away with distance. Here it must be noted that

Figure 1.1: (a) Attraction and (b) repulsion between magnets.

as far as magnetism is concerned the geographical north pole of the earth is actually near to the earth’s magnetic south pole; that is why it attracts the north pole of a magnet towards it. Also familiar to many is the electric force. For example, an inflated balloon is rubbed against a piece of material and then held close to small pieces of tissue paper which will attach themselves to the balloon. This attachment is due to an attractive electric force between the balloon and the paper. We speak of the balloon as being electrically charged. There are two types of electric charge which are referred to as positive (+) and negative (-) and the unit of charge is called the coulomb after an 18th-century French physicist. Normally the net electric charge of an article is zero; the amount of positive charge it contains is exactly balanced by the negative charge. Here, anticipating discussion in the next chapter, it should be made clear that each atom in a substance contains equal amounts of negative and positive charge; the negative charge is carried by very light particles called electrons which surround the massive and positively charged atomic nucleus. When the balloon is rubbed against material some electrons are transferred from one to the other so that the balloon and the material each have a net charge one because it has lost electrons (and so its net charge is positive) and the other because it has gained electrons (and so its net charge is negative). When the balloon is held near to the tissue paper it then attracts the oppositely charged particles in the tissue paper towards it and repels the like charges and the net effect of these two (unequal) forces is the observed attraction between the balloon and the tissue paper. Analogously with magnetic poles and their associated forces, like charges repel each other and unlike charges attract. This force of repulsion or attraction depends on the sizes of the electric charges involved and it dies away as the distance between two interacting charges increases. This dependence on distance obeys an inverse square law. That is to say if the distance between the charges doubles, the force decreases by a factor of two squared, namely four; if it trebles, the force decreases by a factor of nine and so on. Here the reader is reminded that, the fields of influence surrounding electric charges and magnets are referred to respectively as electric and magnetic fields. Fields are defined by their strength and their direction (i.e. they are vector quantities) at every point. For example, with an electric field the strength and direction are those of the force experienced by a positive unit electric charge (one coulomb) placed at a point. Another important feature of electric charge is that it is conserved. This means that in an isolated system the electric charge of the system remains constant: it can neither be created nor destroyed. We shall see later on that charged elementary particles can be created or destroyed but always in association with another particle having an exactly equal but opposite charge so that the net charge created is zero. It must be stressed that the above analogy between the electric and magnetic force is highly superficial. Whilst electric charges can exist in isolation, north and south poles do not; they always occur together, as what is called a dipole, for example at each end of a magnet. However, in passing, it must be said that some possible fundamental theories do suggest that isolated poles (referred to as monopoles) might exist. However, they would be immensely heavy and, so far, none have been detected. Certainly for our purposes at this juncture, and as we shall magnetism should be regarded as a phenomenon which is not intrinsic but derives from the actual motion of electric charges.