Magnetism

Magnetism is the property of attracting certain metals, most notably iron. It is exhibited in varying degree by metals such as iron, nickel, and cobalt. Lodestone, a form of magnetite (an ore of iron), is naturally magnetic, and pieces of lodestone are called natural magnets. It is likely that magnetism was first observed by the ancients in lodestone.

Other substances 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 (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 magnetisme 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, i.e., 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 (e.g., in an electric coil), 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.

Like poles repel each other, whereas opposite poles attract. The force by which they repel orr attract is given by Coulomb's law named after the French physicist Charles-Augustin de Coulomb.

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 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, are called ferromagnetic; those less strongly attracted, paramagnetic. Some substances, such as antimony and bismuth are repelled by magnetic fields; these are called diamagnetic.

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 discovered 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 electromotive force on wires within the changing field. This effect, called electromagnetic induction, was discovered by Michael Faraday in 1832. It is now well established that all magnetic fields are due to moving electric charges or 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. The laws of quantum mechanics dictate the motion of these particles.