Tuesday, January 6, 2015

Magnetism and electromagnetism : electricity and magnetism, simple magnets, the magnetic field, ferromagnetic materials and the magnetizing process and magnetic materials and the atomic theory

Magnetism and Electromagnetism
15–1 ELECTRICITY AND MAGNETISM

One of the most familiar and most frequently used effects of electric current is its ability to produce the force we call magnetism. This force is responsible for the operation of motors, generators, electrical measuring instruments, communication equipment, transformers, and a great variety of electrical control devices.

All magnetism is essentially electromagnetic in origin. Electromagnetism results from the energy of motion of electrons. In fact, every time a current flows through a wire, there are magnetic forces at work. Electrical current and its associated magnetic forces are inseparable.

Because of this close relationship between electricity and magnetism, and due to certain similarities, some students tend to confuse one with the other. Some of these pitfalls will be pointed out to you as you progress with this chapter.

To begin our study of magnetism, we will investigate some of the earliest-known properties of magnetism and then explore how these properties can be explained by the action of electrons.

15–2 SIMPLE MAGNETS

A magnet is a piece of material that attracts a number of other materials such as iron, steel, nickel, cobalt, and a few minerals and alloys. Magnets do not attract copper, aluminum, wood, or paper. In fact, magnets have no effect on most substances. Magnetic attraction is quite unlike electrical attraction, which affects all materials.

The force of the magnet is strongest at two areas on the magnet called the poles. If a magnet is supported in the center by a string or is on a pivot, one of its poles turns toward the north and the other pole turns toward the south. Thus the end of the magnet pointing to the north is called the north pole, and the other pole is called the south pole. The needle of a compass is just a lightweight magnet (strip of magnetized steel) mounted on a pivot.

If a compass or magnet is brought near another compass or magnet, the north end of one compass repels the north end of the other compass and attracts its south end, as shown in Figure 15–1. Similarly, the south pole of one magnet (or compass) will repel the south pole of another magnet and attract the north pole. This effect is summarized in

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the magnetic attraction and repulsion law: Like poles repel and unlike poles attract. (Even though this law appears to be similar to the electrical attraction and repulsion law, remember that magnets and electrical charges are different.)

The term poles means points where opposite properties exist, such as in the positive and negative poles of a battery or the north and south geographic poles of the Earth. The poles of a magnet could have been assigned names other than north and south. In fact, it would be less confusing if the poles had been given a pair of opposite names such as black and white, or right and left. The geographic poles of the Earth are the ends of the axis on which the Earth turns; they are not areas of magnetic attraction. The Earth does have magnetic poles, however. There is a place in northern Canada that has the same kind of magnetic force as the south pole of a steel magnet; similarly, there is a place in the Antarctic that has the same kind of magnetic force as the north pole of a steel magnet.

15–3 THE MAGNETIC FIELD

You should recall from our discussion of electrical charges that the attraction and repulsion of electrical charges was explained by the existence of an invisible field of force between the charges. The pattern of an electrostatic field was shown in Section 3–8, Figure 3–13.

Similarly, force existing in the space around a magnet is shown by the pattern resulting when iron filings are sprinkled on a card placed over a magnet, as shown in Figure 15–2. Compare the similarities of these phenomena, but keep in mind that we are dealing with two entirely different forces. Magnetism is not the same force as the attraction and repulsion forces caused by static electrical charges.

These lines of force, often referred to as flux lines, have specific characteristics attributed to them.

• Flux lines are directional. They are said to exit from the north pole and enter into the south pole, forming a closed loop through the magnet.

• Flux lines do not cross each other.

• There is no insulator for magnetic flux. It passes through all materials.

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• Flux lines act like stretched rubber bands; they tend to contract.

• The flux density, or concentration of flux lines at a point, determines the amount of magnetic force. The greater the concentration of flux lines, the stronger the magnetic field. Flux lines are most densely concentrated at the poles.

• Flux lines facing the same direction attract each other, but flux lines facing opposite directions repel each other.

• The concentration of flux lines, and therefore the strength of a given magnet, is limited. When a magnet achieves maximum flux density, it is said to be saturated.

15–4 FERROMAGNETIC MATERIALS AND THE MAGNETIZING PROCESS

Iron, nickel, cobalt, and some oxides and alloys are called ferromagnetic materials. A magnet is a piece of ferromagnetic material that has magnetic poles developed in it by placing it inside a current-carrying coil of wire or by placing it near another magnet.

Early experimenters found that heating or hammering a magnet causes the mag- net to lose some of its strength. Both of these processes disturb the atoms of the metal. Furthermore, it was found that if an ordinary steel bar magnet (or any magnet) is cut into fragments, each fragment has a north pole and a south pole, as shown in Figure 15–3. If we can continue to cut this material into smaller pieces, eventually we will reach the smallest possible fragment of iron, an atom. Thus, scientists stated that all atoms of magnetic materials are themselves permanent magnets.

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In an unmagnetized piece of iron, the atoms of iron are arranged in a disorganized fashion; that is, the north and south poles of these atom-sized permanent magnets point in all directions, as shown in Figure 15–4A. When the iron is magnetized, the atoms are rotated and aligned so that the north pole of each atom faces in the same direction, as shown in Figure 15–4B.

If a magnet is cut without disturbing the atom arrangement, the atomic south poles are exposed on one side of the break and the north poles are exposed on the other side. Before the magnet is cut, these poles exert their attractive forces on each other so that there is no force reaching out into space around them.

Some of the previous conclusions about magnets have changed slightly over the years because of the discovery of a degree of order in an unmagnetized piece of iron. Within a crystal grain of iron, several thousand atoms form a group called a magnetic domain. Within one domain, the atoms are lined up with the north poles all facing in one direction. This group of atoms acts like a minute permanent magnet.

15–5 MAGNETIC MATERIALS AND THE ATOMIC THEORY

Why do atoms of a magnetic material behave like iron magnets? The answer to this question is the result of a long series of complex scientific investigations of the behavior of electrons in atoms. All electrons are constantly spinning on their own axes within an atom. This spin is the reason that each electron is a tiny permanent magnet, as illustrated in Figure 15–5. In most atoms, electrons spinning in opposite directions form pairs. In other words, their north and south poles are so close together that their magnetic effects cancel out, as far as any distant effect is concerned. (Compare this situation with two permanent bar magnets placed together with their north and south poles adjacent to each other.)

An atom of iron contains 26 electrons. Twenty-two of these electrons are paired. Each electron of a pair spins in a direction opposite to that of the other electron so that the external magnetic effect is canceled. In the next-to-the-outermost ring of electrons, 4 electrons are uncanceled. These 4 electrons, because they are spinning in the same direction, are responsible for the magnetic character of the atoms of iron.

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There is still a great deal more to discover about electrons in atoms. Electron spin directions in an atom are affected by temperature and by the presence of other atoms. At 1,420°F, iron loses its magnetism due to a rearrangement of electron spin patterns. Strongly magnetic alloys and compounds have been made from elements that are either weakly magnetic or not magnetic at all in their uncombined form.