Introduction to Magnetism
Magnetism. The most familiar form of the physical phenomenon called magnetism is the ability of certain objects to attract iron. Such objects are called magnets. Magnetism is also associated with electric currents. Magnets are widely used. All electric motors (and the generators that provide power for the motors) contain magnets, as do telephones, tape recorders, and loudspeakers. The magnetic compass is a device used for finding direction. The earth itself is a huge magnet.
How Magnets Behave
Each magnet has two poles, at which the attractive force seems greatest. The poles are called north-seeking, or north (N), and south-seeking, or south (S). (The poles are so named because, under the influence of the earth's magnetism, a bar-shaped magnet free to rotate will turn so that one pole points northward and the other southward.) When a magnet is cut into two or more pieces, each piece becomes a new magnet.
Like poles repel; unlike poles attract. When the N poles of two magnets are brought together, the magnets will be repelled—that is, they will move away from each other. The same thing happens when the S poles are brought together. When the N pole of one magnet is brought near the S pole of another, however, the two magnets will strongly attract each other, and will move toward each other.
Magnets do not have to come into contact to repel or attract each other because magnetism acts at a distance. The area in which the effect of a magnet can be detected is called its magnetic field. The field is strongest near the magnet; it weakens as the distance from the magnet increases. A magnetic field is usually pictured as a series of lines, called lines of force, extending from the N pole of a magnet to an S pole, either at the other end of the same magnet or in a nearby magnet.
Magnets attract objects made from iron, steel, cobalt, or certain other materials. In the presence of a magnet, an object made from such magnetic materials will itself become a magnet. (This process is called magnetic induction.) The magnet attracts the object because the pole of the magnet closest to the object produces an unlike pole in the nearest part of the object. For example, the N pole of the magnet will produce an S pole in the part of the object closest to it. (In this example, the most distant part of the object would become an N pole.)
Measurements with extremely accurate instruments show that all materials have some reaction to a magnetic field. The materials usually referred to as nonmagnetic, such as copper and water, are either paramagnetic (showing a slight tendency to line up parallel to the lines of force of a field) or diamagnetic (showing a slight tendency to line up at right angles to the lines of force). Magnetic materials, properly called ferromagnetic, have a strong tendency to line up parallel to the lines of force.
Permanent and Temporary Magnets
There are two basic kinds of magnets—permanent and temporary. A permanent magnet retains its magnetic properties for a long time. A temporary magnet acts as a magnet only as long as it is in the magnetic field produced by a permanent magnet or an electric current. Magnetic materials from which permanent magnets are made are called hard magnetic materials and those from which temporary magnets are made are called soft magnetic materials.
A lodestone is a naturally occurring permanent magnet composed of magnetite, an iron-bearing mineral. Such magnets have been known since ancient times. Virtually all magnets used commercially today are made from synthetic magnetic materials. The most common such materials are alnicos—iron alloys containing aluminum, nickel, and cobalt. Magnetic materials containing such rare-earth elements as samarium or neodymium form very strong permanent magnets. Ferrites, which consist of ferric oxide (an oxide of iron) combined with the oxides of one or more other metals, are widely used in electronic devices. Flexible magnets are made by combining magnetic materials with plastics.
Permanent magnets are typically made into U-shaped horseshoe magnets, with the poles side by side; and bar magnets, with the poles at opposite ends.
Every object that is lifted or moved by a magnet acts as a temporary magnet. Such an object ordinarily loses its magnetism when the permanent magnet is removed, although in certain cases it will retain weak magnetic properties.
An electromagnet is a temporary magnet that is magnetized by the magnetic field produced by an electric current in a wire. Electromagnets have magnetic properties only while the current is flowing.
Theory of Magnetism
The effects of magnetism have been known and used for centuries. Yet scientists still do not know exactly what magnetism is. The theory of magnetism that follows is based on one proposed by Pierre Weiss, a French physicist, in the early 20th century.
Every magnetic substance contains domains, groups of molecules that act as magnets. Before a substance is magnetized, these domains are arranged randomly, so that the magnetism of one is cancelled by the magnetism of another. When the substance is brought within a magnetic field, the domains line up parallel to the lines of force, with all the N poles facing in the same direction.
When the magnetic field is removed, the like poles tend to repel each other. In a substance that is easily magnetized, the domains turn easily, and will return to random ordering. In a substance that is difficult to magnetize, the domains will not have enough force to disarrange themselves and the substance will remain magnetized. In modern versions of this theory, the magnetism of the domains is attributed to the spin of electrons.
How Magnets Are Made
There are four main ways to magnetize a magnetic substance: (1) bringing the substance near a magnet; (2) using electric current; (3) stroking the substance with a magnet; and (4) striking a blow to the substance while it is in a magnetic field. The first two methods were discussed under the subtitle Permanent and Temporary Magnets: Temporary Magnets.
A permanent magnet can be made by stroking a magnetic substance with either the N or the S pole of a magnet. Stroking lines up the domains in the material.
A piece of iron can be magnetized by holding it parallel to a compass needle (along the lines of force in the earth's field) and hitting the piece of iron with a hammer. The blow will overcome the resistance of the domains to movement, and they will line up parallel to the earth's field.
To demagnetize an object, a strong magnetic field is used. In one method, the magnetic field is made to fluctuate very rapidly. In another method, the magnetized object is placed so that a line drawn between its poles would be at right angles to the field. The object is then tapped or hit until its domains are no longer lined up magnetically.