MAGNETISM

There is a story that a Cretan shepherd by the name of Magnes, whilst tending sheep on the slopes of Mount Ida, found that his iron tipped crook was attracted to the ground. To find the source of the attraction he dug up the ground to find stones that we now refer to as lodestones (lode = lead or attract) which contain magnetite, a natural magnetic material Fe₃O₄. The story may be apocryphal but the earliest discovery of the properties of lodestone was either by the Greeks or Chinese.

The unexplained nature of the magnetic attraction was ripe for exploitation by story tellers and it became difficult to separate fact from fancy. There was a belief that there were magnetic islands made of lodestone that could attract ships by virtue of the iron nails used in their construction. Archimedes(287-212 BC Greece) is reputed to have used powerful lodestones to pull the nails out of enemy ships thus sinking them. The term magnetism was coined to explain the phenomenon whereby lodestones attracted iron. Today we now understand the nature of this attractive or repulsive force of magnetism and we even know of magnetic bacteria.

In 1820 Hans Christian Oersted (1777-1851 Danish) demonstrated that magnetism was related to electricity by bringing a wire carrying an electric current close to a magnetic compass which caused a deflection of the compass needle. It is now known that whenever a current flows there will be a magnetic field in the surrounding space, or more generally that the movement of any charged particle will produce a magnetic field. James Clerk Maxwell (1831-1879 Scotland) established beyond doubt the inter-relationships between electricity and magnetism and promulgated a series of Laws that are the basis of electromagnetic theory today. What is more remarkable is that Maxwell developed his ideas in 1862 more than thirty years before Sir Joseph John Thomson (1856-1940 England) discovered the electron in 1897.

If magnetism is intimately connected with electricity then where is the electricity is simple magnets? And what is a magnet? The explanation of magnetism very elegantly illustrates how the bulk properties of matter can be explained in terms of the fundamental particles of which it is composed.

Atoms comprise protons and neutrons, which are in the nucleus and need not concern us here, and extranuclear electrons the fundamental negatively charged particles (or waves). Electrons are the fundamental source of magnetism and each electron has a magnetic moment, i.e., behave like a little magnet - this magnetic moment can act in either of two opposing directions.

As matter is made up of atoms containing one or more electrons we might expect all matter to be magnetic but in most matter electrons pair up with opposite magnetic moments and thus there is no nett magnetic effect - the substance is diamagnetic and is actually very weakly repelled by a magnet.

However there are many substances in which it is not possible to pair up all the electrons, e.g., nitric oxide NO which has 7 + 8 = 15 electrons, and a molecule of such a substance must have a magnetic moment due to the unpaired electron. In most substances that have unpaired electrons the individual magnetic moments are randomly orientated and although these materials are attracted to a magnetic field the attraction is only weak -these materials are said to be paramagnetic. The oxygen molecule O₂ is paramagnetic and has two unpaired electrons -blue liquid oxygen is attracted to the poles of a powerful magnet, you may have seen an Open University programme that shows this.

In some substances the individual magnetic moments behavecooperatively so that individual little magnets (10¹⁷to 10²⁰ molecules) align themselves in same direction in tiny chunks (10^-12 to 10^-8 m³) of the material known as a magnetic domains - these substances are strongly attracted to a magnetic field because each domain rotates to align itself with the magnetic field. Such substances are said to be ferromagneticafter iron the most common magnetic material (Latin ferrum = iron). It is still not completely understood to everyones satisfaction why this cooperative behaviour occurs.

Although magnetism is most commonly associated with iron there are other magnetic metals, e.g., cobalt (Co), nickel (Ni), samarium (Sm), dysprosium (Dy) and gadolinium (Gd). Other non-magnetic metals, e.g., copper (Cu), manganese (Mn) and tin (Sn) form magnetic alloys known as Heusler alloys, e.g., 65% copper, 25% manganese and 10% aluminium. Ferromagnetic materials are not always metallic - there are many non-metallicferromagnetic materials including ferrites which are mixture of iron and other metal oxides. Ferrites and related ferromagnetic materials have uses as insulating magnets. Lodestone contains magnetite which is non-metallic ferromagnet. Magnetic alloys include Permalloy (55% iron and 45% nickel), Supermalloy (15.7% iron, 79% nickel, 5% molybdenum and 0.3% manganese) and µ-metal (77% nickel, 16% iron, 5% copper and 2% chromium).

The concept of a magnetic field is used to get round the problem of action-at-a-distance. The force of magnetic attraction behaves like the force of gravitational attraction - but please do not confuse the two effects - you are attracted to the earths surface by gravity NOT magnetism! A piece of magnetizable material or another magnet interacts with the magnetic field of another magnet. The magnetic field of a magnet is easily made visible by means of iron filings which align themselves parallel to the magnetic field lines.

A simple bar magnet, like any magnet, has a north pole and a south pole (the terms are derived by comparison with the poles of the earth). Unlike poles attract each other like poles repel. The convention is that magnetic field lines emerge fromthe north pole of a magnet and converge into a south pole. By definition the pole of a magnet that point towards the north magnetic pole of the earth is known as the north pole of the magnet - thus the north magnetic pole of the earth is in fact a south pole! It is not possible to isolate a single pole or monopole - magnets are always dipoles. If a bar magnet is cut in half midway between the poles then each half of the bar magnet becomes a separate magnet with its own north and south poles.

The strength of a magnetic field is measured in Tesla (T) or in older units gauss (g).

A potentially ferromagnetic material can be magnetized by placing it in a magnetic field or by repeatedly stroking it with the pole of a permanent magnet or by placing it in the centre of a coil of carrying an electric current to give an electromagnet. When magnetized some materials retain the magnetism they are said to be hard ferromagnets because of the remanent magnetism and are used to make permanent magnets. Other materials, e.g., pure iron, do not retain magnetism when removed from magnetic influence are said to be soft ferromagnets which are non-remanent.

Magnetism may be removed by heating because this causes molecular motion which removes the alignment of individual magnetic moments. Repeated hammering of magnetized material can also remove the magnetism. It is also possible to remove magnetism by subjecting a magnetized material to an opposing magnetic field. Television tubes can become magnetized which leads to coloured patches on the screen - the tube has to be demagnetized or degaussed.

If a coil of conducting wire is connected to a volt-meter with no battery then no voltage will be observed but if a magnet is inserted into the coil a voltage is instantaneously recorded and when the magnet is removed from the coil another instantaneous but opposing voltage is recorded. This effect whereby the relative motion of a magnet and an electric coil produced acurrent was simultaneously discovered in 1831 by Michael Faraday (1791-1867 England) and Joseph Henry (1799-1878 America) and is known as electromagnetic induction. Faraday used his discovery to prepare the first dynamo in which the continuous rotation of a conducting copper plate between the poles of a magnet produced a continuous current. In a dynamo mechanical energy is converted into electrical energy via a magnetic field. It was a further fifty years before Faradays discovery was applied to the commercial generation of electricity.

The reverse concept of the dynamo is, of course the electric motor, in which a continuous current is used to set up the continuous circular motion of a magnet or series of magnets. In such a motor electric energy is converted into mechanicalenergy via an intermediate magnetic field. It is difficult to imagine life today without electric motors big or small which are found in sewing machines, car starters, refrigerators, vacuum cleaners, trains, food processors, personal stereos, washing machines, fans, hair dryers, pumps, disc drives, and virtually all industrial machines.

The magnetic storage of data is very important for modern life - magnetic tape was originally used for analogue sound storage but is now used to store both analogue and digital information and this is not limited to tape which has slow access times. Magnetic discs (floppies, hard disks) offer almost instant storage and access of ever increasing amounts of information. We use a wide variety of plastic cards with magnetic strips, e.g., bank cards, debit cards, credit cards, account cards etc., to manage (mis-manage?) our financial affairs, for security access (NEWI access cards), telephone cards, travel tickets (London Underground) etc.

In the television tube the deflection of the electron beams that paint the picture on the television screen is accomplished using magnetic fields. Powerful magnetic fields in the vicinity of a television or monitor can distort the image and in some cases permanently magnetize the tube which leads to coloured patches on the screen - the tube must then be de-magnetized or degausssed.

The magnetic properties of certain atomic nuclei can be used in chemistry in nuclear magnetic resonance (nmr) spectroscopywhich is valuable tool in molecular strcuture determination. A development of this technique is magnetic resonance imagingwhich is rapidly becoming a powerful imaging technique in the medical world for scanning internal organs. Both techniques rely on the fact that some atomic nuclei behave like little magnets and can either align themselves for or against the field of a powerful magnet - applied radio frequency waves can cause the nuclei to flip between the for and against states and the radio frequency required to do this can give information about the atomic nucleis environment

The repulsive force between like poles of magnets can be used for magnetic levitation which engineers are trying to exploit in modern train systems. Conventional train track systems are limited to about 300 km/hour by friction and track and wheel stresses but a train floating on a magnetic levitation field may be able to travel at 500+ km/hour. A prototype was trialled between Birmingham International Station and the Airport and the Japanese National Railway have prototypes.. However the magnetic fields required are very high and to be effective superconducting electromagnets are required in the vehicle with opposing electromagnetic fields set up by the conductor rails below.

Powerful magnets are features of particle accelerators, e.g., cyclotrons and synchrotrons which are widely used by physicists in their quest to understand more about the fundamental nature of matter and its constituent particles.

Nuclear fusion of light elements has the potential to generate vast amounts of energy at low cost. In conventional nuclear fusion the positively charged atomic nuclei must be heated to very high temperatures in order to give them enough kinetic energy to collide by overcoming the repulsion between their positive charges - the very high temperatures (100 000 000^o) are so high that the gaseous matter cannot be stored in any normal material and scientist are looking at ways of storing this very hot matter in magnetic bottles in which it is suspended in a powerful magnetic field without physical contact.

The origin of the earths magnetic field is still not fully understood. We know that the earths core is molten iron and nickel that is too hot to be ferromagnetic. However we also know that electric currents generate magnetic fields and it is believed that convectional flow of the molten core and rotation of the planet give rise to electric currents which set up the magnetic field - the dynamo theory. The magnetic field appears to reverse approximately every million years for reasons which are not understood but which are evident from the magnetic orientation of natural magnetic material (lodestone) in rocks of different ages. The study of earths magnetic past from rocks is paleomagnetism and has led to theory of continental drift and plate tectonics.

When some high energy charged particles or ions, ejected from the sun as the solar wind, become trapped in the earths magnetic field above the poles complex reactions in the ionosphere give rise to aurorae (borealis (N) and australis(S)) visible as flickering and waving arcs, bands curtains and streaks of coloured lights that are visible most nights at the poles. When the sun is most active and powerful electromagnetic storms send particularly large numbers of ions towards the earth the aurorae can be seen from lower latitudes like the U.K. From space the aurorae can be seen as a circle of light surrounding the earths magnetic pole.

In addition to the visual aurorae there are very low frequency(vlf) radio waves which, when picked up on a vlf receiver and amplified, sound not unlike a chorus of birds - called the auroral chorus. Fortunately the earths magnetic field in space or magnetosphere acts as a barrier deflecting most of the solar wind away from the earth.

The sun has a similar, but much more powerful and dynamic, magnetic field that undergoes a perioidic reversal every 11 years corresponding to the well known sunspot cycle. Sunspots are sources of very powerful magnetic fields or magnetic storms on the solar surface. The suns magnetic field is imaged by the suns corona, visible during a total solar eclipse, in a similar way to the image of a magnets field with iron filings.