13. Magnets — Basic Properties

A magnet is a material or object that produces a magnetic field and has the property of attracting ferromagnetic materials such as iron, nickel, and cobalt, as well as attracting or repelling other magnets. The word 'magnet' is believed to derive from Magnesia, a region in ancient Greece where naturally magnetic ore (magnetite, also called lodestone) was found.

Natural and Artificial Magnets

Natural magnets are magnetic materials found in nature in their magnetised state, of which the primary example is the mineral magnetite (lodestone) — an oxide of iron that occurs naturally with magnetic properties, and which was used in the earliest known compasses by ancient Chinese navigators. Natural magnets are generally weak and irregular in shape. Artificial magnets, by contrast, are magnets made by humans by magnetising suitable materials (typically iron, steel, or special alloys such as alnico) using methods such as stroking with an existing magnet, placing the material in a strong magnetic field, or — most commonly today — by passing electric current through a coil wound around the material (this last method, which produces an electromagnet, is discussed in detail later in this chapter). Artificial magnets can be manufactured in a wide range of controlled, useful shapes, including bar magnets, horseshoe (U-shaped) magnets, and magnetic needles used in compasses.

Magnetic Poles and the Laws of Magnetic Poles

Every magnet, regardless of its shape, has two poles — regions near its ends where the magnetic strength (attractive/repulsive effect) is concentrated and strongest. These are called the north pole (or north-seeking pole) and the south pole (or south-seeking pole), named for the fact that a freely suspended bar magnet will always come to rest with one particular pole pointing roughly toward geographic north (hence 'north pole') and the other pointing toward geographic south. This directional property is precisely what makes the magnetic compass useful for navigation.

A crucially important and frequently examined property is that magnetic poles always occur in pairs — it is impossible to obtain an isolated single magnetic pole (called a 'magnetic monopole'). If a bar magnet is broken into two pieces, each resulting piece does not yield a single isolated north pole and a single isolated south pole separately; instead, each broken piece immediately becomes a new, complete magnet with its own north pole and south pole. This process can, in principle, be repeated indefinitely, down to the smallest magnetised particles, and it reflects the fact that magnetism at the atomic level arises from the alignment of countless tiny atomic magnetic dipoles, each of which already has two poles.

The Laws of Magnetic Poles (also called the laws of magnetism) describe the force between poles, in close analogy to the laws for electric charges: like poles repel each other (a north pole repels another north pole; a south pole repels another south pole), while unlike poles attract each other (a north pole attracts a south pole). The force between two magnetic poles also follows an inverse-square law with distance, closely analogous in form to both Coulomb's law for electric charges and Newton's law of gravitation.