Magnetic materials

There exists another mysterious force which has been known since the time of ancient civilisations. Pieces of iron ore, or lode-stone, found near the city of Magnesia, had the property of exerting forces on one another without touching. We call this property magnetisation.

When a piece of lodestone is suspended so that it is free to rotate, it tends to align itself in a north-south direction. As such, lodestone was used for navigation as early as the 13th century. Petrus Perigrinus de Maricourt, in a letter to a friend in 1269, described a magnetic compass as consisting of a north-seeking and a south-seeking pole. He also observed that like poles repel and unlike poles attract, and was probably the first to realise that the earth itself is a magnet with its south pole located at the north geographic pole, and vice versa.

In his autobiography, Albert Einstein refers to his fascination, at the age of four, with the magnetic compass: 'There was this needle completely isolated, with an uncontrollable urge to point north!' Clearly, the force of gravity which gives everything an 'uncontrollable urge' to fall downwards was too commonplace to young Einstein — but he was to return to gravity as an object of wonder some years later!

When a piece of loadstone is brought near to a piece of ordinary iron, the iron becomes magnetised. This can be demonstrated by sprinkling the surrounding area with iron filings.

Figure 10.8 Iron filings around a magnet. Courtesy of James Ellis, UCD School of Physics, Dublin.

The filings tend to arrange themselves 'head to toe' in a pattern of lines, as shown in Figure 10.8.

The pattern of iron filings gives a map of the magnetic field. The field can also be detected using a small compass needle. The lines of force do not have a real existence, any more than the lines of an electric or gravitational field. If we tap the surface, the filings will rearrange themselves in a similar pattern but not necessarily precisely along the same lines.

A feature of magnetism is that it is not possible to isolate a single magnetic pole. If we cut a magnet in two we are left with two magnets, each with a north and a south pole. Magnetic lines of force always form closed loops; unlike electric or gravitational lines, they never originate from a point source. We can imagine that each line continues on inside a bar magnet to complete a loop.

Magnetic poles always one without the other!

Figure 10.9 Magnetic loops.

come in pairs. We cannot have Mathematically, we can express this property by adapting Gauss's theorem to magnetism, stating that no surface can enclose a single magnetic pole.

Gauss's law for magnetism: The net magnetic flux through any imaginary closed surface is always zero.

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