Magnetism

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Magnetism is the property of attracting iron. It is exhibited in varying degree by metals such as iron, nickel, and cobalt. When these metals are magnetic, they not only attract iron, but also each other. Lodestone, a form of magnetite (an ore of iron, an iron oxide), is naturally magnetic, and pieces of lodestone are called natural magnets. It is likely that magnetism was first observed by the ancients in lodestone.

Some substances that are not naturally magnetic can be magnetized artificially either by induction, i.e., by bringing them in contact with an existing magnet, or by placing them in a coil of wire (a solenoid) through which a direct electric current is running. Some substances can be magnetized more easily than others. Soft iron, for example, is easily magnetized when inside a solenoid, but quickly loses its magnetism when taken out again. Steel is more difficult to magnetize, but retains its magnetism for a long time. Magnets that retain their magnetism are called permanent. Magnetic permeability is a measure for the ease by which a material can be magnetized. Substances differ greatly in their permeability, those with a high permeability can be highly magnetized, which means that they can become strong magnets.

The first scientific study of magnets was performed by William Gilbert in the late sixteenth century. He found magnets to have their strength concentrated in small regions, called poles. If a bar of iron is magnetized, magnetic poles occur at the two ends of the bar. These are called north and south pole. If one hangs the bar magnet from a rope, the magnet will direct itself with one pole pointing to the North. The pole that points to the North is the magnetic north pole, the other end of the bar contains the south pole. It is a remarkable phenomenon that when a magnet with two poles is cut in half, each half will then itself have two poles. In other words, it is not possible to obtain a single magnetic pole: magnetic monopoles (charges) do not exist, or, at least have never been found, neither in nature nor in the laboratory.[1]

Like poles repel each other, whereas opposite poles attract. The force by which they repel or attract is given by Coulomb's law named after the French physicist Charles-Augustin de Coulomb, who established clearly the non-existence of magnetic monopoles (ca. 1790).

Physicists find it convenient to picture the magnetic force as being transmitted through a magnetic field. That is, instead of saying that one magnet exerts at a distance a force on the other, they say that the first magnet creates a magnetic field in the space around it and that the second magnet feels a force because it finds itself in a magnetic field. The magnetic field of a magnet consists of lines of force running from its north pole to its south pole. It is found that small magnets placed in a magnetic field tend to align themselves along the lines of force. By means of this effect, a good visual picture of a magnetic field can easily be obtained by mapping it with iron filings: cover the magnet with a cardboard and sprinkle the filings on the cardboard. The iron filings become magnetized by induction through the cardboard and align themselves with the field, forming visible lines running from north pole to south pole.

Different substances vary in their reaction to magnetic fields, Those that are strongly attracted, such as iron and nickel, are called ferromagnetic; those less strongly attracted, paramagnetic. Some substances, such as antimony and bismuth are repelled by magnetic fields; these are called diamagnetic. It turns out that this behavior is described by the magnetic permeability of the substance, mentioned above. One can assign a real number (μ) to the permeability. When μ is much larger than one, say a thousand, then the substance is ferromagnetic. When μ is a little bit larger than one, the substance is paramagnetic, and when μ is just under one, we have a diamagnetic substance.

The magnetic properties of matter can be understood by picturing it as made of many very small magnets. These small magnets are individual atoms or molecules or, for ferromagnetic metals, microscopic regions of the metal, called magnetic domains. In an unmagnetized metal the small magnets are randomly oriented, so that their individual effects tend to cancel one another. When a magnetic field is applied to the metal, the small magnets line up parallel to the field, so that their effects are cumulative, and the metal is magnetized.

Magnetism is intimately related to electricity. This was first discovered by the Danish physicist Hans-Christian Oersted in 1820. In 1825 the French physicist André-Marie Ampère discussed his earlier discovery that magnets exert forces on wires through which a direct current is running. An interesting property of a changing magnetic field is that it produces an voltage difference in wires that are within the (changing) field. This effect, called electromagnetic induction, was discovered by the English scientist Michael Faraday in 1832.

It is now well established that all magnetic fields are due to moving electric charges or electric currents. This explains why metals can be magnetized by current-bearing coils. The magnetism of individual atoms and molecules can be explained in terms of the motion of the charged particles, electrons and nuclei, that make up the atoms and molecules. Since the laws of quantum mechanics dictate the motion of these particles, a knowledge of quantum mechanics (a theory dating from ca. 1925) is indispensable for the understanding of the microscopic origin of magnetism. More globally, magnetism is described by Maxwell's equations first written down by the Scottish mathematical physicist James Clerk Maxwell in the early 1860s. These equations describe mathematically the interconnection between electricity and magnetism and the forces exerted by these phenomena, and thus form the basis of a field of study called classical electrodynamics. The theory based on quantum mechanics and Maxwell equations is called quantum electrodynamics.

Note

  1. Recent discoveries indicate that magnetic monopoles may exist in "spin ice". See S. Sondhi, Wien route to monopoles, Nature, Vol. 461, 15 October 2009. p. 888 DOI and M. J. P. Gingras Observing Monopoles in a Magnetic Analog of Ice, Science, Vol. 326 16 October 2009, p. 375 DOI