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52C11A03-TATS TRAINING SUPPORT PACKAGE FOR LESSON TITLE: MAGNETISM THIS PACKAGE HAS BEEN DEVELOPED FOR: TOTAL ARMY TRAINING SYSTEM COURSEWARE (TATSC) PROPONENT FOR THIS TSP IS: U.S. Army Ordnance Center and School, Aberdeen Proving Ground, Maryland. Send comments, recommendations or changes to U.S. Army Combined Arms Support Command (CASCOM) Training Directorate (Ordnance) Ft. Lee, Va. 23801-1713. FOREIGN DISCLOSURE RESTRICTIONS: The materials contained in this course have been reviewed by the course instructors in coordination with the installation/activity foreign disclosure authority. This course is releasable to military students from all authorized requesting countries without restriction. Magnetism 52C11A03 1. Introduction: a. You will now continue your study of the logistic phase with Distance Learning Lesson 52C11A03, Magnetism. The basic two fundamentals of electricity are electronic force and magnetic force, which will be discussed in this lesson. These are the forces that allow the operation of electrical motors, generators, meters, relays, and countless other electrical devices. You have just studied the electric force as explained by the electron theory. In this lesson you will learn the following: The relationship between magnetism and electricity. The characteristics of magnets. The characteristics of a magnetic field around a current carrying conductor. The factors that effect the strength of electromagnets. The three requirements needed to develop an electromotive force (EMF). b. As you can see, this is the third distance learning lesson in annex A: 52C11A01 Course Introduction 52C11A02 Elements of Electricity 52C11A03 Magnetism 52C11A04 Ohm’s Law 52C11A05 Series Circuits 52C11A06 Parallel-Circuits 52C11A07 Series Parallel Circuits 52C11A08 Direct Current Generators 52C11A09 Direct Current Motors 52C11A10 Direct Current Diagrams 52C11A11 Alternating Current Circuits II Magnetism 52C11A03 52C11A12 Rectifier Circuits 52C11A13 Alternating Current Generators 52C11A14 Alternating Current Motors 52C11A15 Three-Phase Circuits 52C11A16 Alternating Current Diagrams 52C11A17 Heat and Heat Transfer 52C11A18 Pressure and Pressure Measurement 52C11A19 Basic Refrigeration Cycle 52C11A20 Refrigerant and Refrigerant Usage 52C11A21 Small Appliances High Pressure Systems 52C11A22 Written Test (IDT) 2. Instructions: a. Read through this lesson, 52C11A03, Magnetism. b. Answer the questions or solve the problems in the Self Check Test at the end of this lesson. c. Check your answers with the solutions provided on the Self Check Solution Sheet. a. You will be required to pass a written test in lesson A22, covering material presented in the distance learning portion, annex A. Save this lesson booklet. You will be allowed to use it during the tests. III Magnetism 52C11A03 1.0. General. In order to properly understand the principles of electricity, it is necessary to study magnetism and the effects of magnetism on electrical equipment. Magnetism and electricity are so closely related that the study of either subject would be incomplete without at least mentioning the basics of the other. Much of today’s modern electrical and electronic equipment could not function without magnetism. Modern computers, tape recorders, and video reproduction equipment use magnetized tape. High-fidelity speakers use magnets to convert amplifier outputs into audible sound. Electrical motors use magnets to convert electrical energy into mechanical motion; generators use magnets to convert mechanical motion into electrical energy. It is magnetic force that attracts small bits of iron and steel to the end of a common horseshoe magnet. It is magnetic force that swings a compass needle toward the north. Magnets may be conveniently divided into two groups, natural magnets and artificial magnets. A natural magnet is an iron ore called magnetite that possesses magnetic qualities. An artificial magnet is produced from magnetic materials. This lesson will help you to develop an understanding of the artificial magnets used in electric equipment. 2.0. Nature of Magnetism. To understand the artificial magnets used in electric equipment, it is necessary to know the nature of magnetism. Magnetism is the power to attract as possessed by a magnet. 2.1. Magnetic Substance. Those materials that are attracted to a magnet are called magnetic substances. Two common magnetic substances are iron and steel. There are other magnetic substances that are attracted by a magnet but not as strong as iron and steel. Some of these magnetic substances are cobalt, nickel, and manganese. It is interesting to note that some of the best permanent magnets are made from alloys. For example, alnico is made of aluminum, nickel, and cobalt. Alnico is derived from the first two letters of the three elements of which it is composed. 2.2. Nonmagnetic Substance. Most other substances are not attracted by a magnet and are nonmagnetic. Examples of nonmagnetic substances are air, wood, paper, glass, copper, aluminum, lead, tin, and silver. Magnetic force acts through any nonmagnetic substance, which can be demonstrated by moving a magnet beneath a piece of paper (nonmagnetic substance) on which there are iron filings. As the magnet is moved, movement of the iron filings can be observed. 2.3. Theory of Magnetism. One common theory of magnetism is that a piece of iron or steel consists of millions of tiny magnets. This is based on the assumption that each of the molecules of a magnetic substance is a tiny magnet. 1 Magnetism 52C11A03 2.3.1. Unmagnetized. The molecular magnets that compose an unmagnetized bar of iron or steel are arranged at random as shown in Figure 1(A). With this random arrangement, the magnetism of each of the molecules is neutralized by that of adjacent molecules, and no external magnetic force is produced. 2.3.2. Magnetized. The molecular magnets that compose a magnetized bar of iron or steel are aligned so that the north poles point one direction and the south poles point the other direction as shown in Figure 1(B). With this aligned arrangement, the magnetism of the molecules is, added and a north pole is set up at one end of the bar and a south pole at the other end. Figure 1. Theory of Magnetism 2.3.3. Broken Magnet. If a bar magnet is broken into several parts as shown in Figure 2, each part constitutes a magnet. The north and south poles of these small magnets are in the same respective directions as those of the original magnet. If each of these parts are again broken, the resulting parts are likewise magnets. If this breaking process continued, smaller pieces would retain their magnetism until each part was reduced to a molecule. It is therefore logical to assume that each of these molecules is a magnet. Figure 2. Magnetic Poles of a Broken Magnet N S N S N S 2 N S N S N S Magnetism 52C11A03 2.4. Magnetic Poles. The magnetic poles of the magnetized bar are the areas where the magnetic force is the greatest. If the magnetized bar were to be suspended from a string, one end would point to the north and the other end would point to the south. Long ago, when this fact was first discovered, it was decided to call the north-seeking pole of the magnet the north pole. Likewise, the south-seeking pole was called the south pole. These designations for poles of a magnet are still used. 2.5. Magnetic Field. A magnetic field is the space surrounding a magnet in which the magnet force acts. A visual representation of the magnetic field around a magnet can be obtained by placing a plate of glass over a magnet and sprinkling iron filings onto the glass. The filings arrange themselves in definite paths between the poles. This arrangement of the filings shows the pattern of the magnetic field around the magnet as shown in Figure 3. Actually, this pattern of the magnetic field is only one of number of possible patterns that can be produced by using one or more magnets. Figure 3. Magnetic Field Pattern S N 3 Magnetism 52C11A03 2.5.1. Unlike Poles. If the area surrounding the north pole of one bar magnet and the south pole of a second bar magnet is sprinkled with iron filings, the pattern shown in Figure 4 will be obtained. A study of this pattern shows one of the fundamental laws of magnetism —unlike magnetic poles attract each other. Figure 4. Unlike Poles (Attraction) 2.5.2. Like Poles. If the area surrounding the north pole of one bar magnet and the north pole of a second bar magnet is sprinkled with iron filings, the pattern shown in figure 5 will be obtained. (Two south poles will give the same effect). A study of this pattern shows another of the fundamental laws of magnetism –like magnetic poles repel each other. Figure 5. Like Poles (Repulsion) 4 Magnetism 52C11A03 2.6. Lines of Force. To represent the direction and intensity of the magnet field about a magnet a drawing is used. This is done by representing the forces in a magnetic field with a few lines called lines of force as shown in figure 6. Note that arrowheads have been placed on each of the lines of force. The arrowheads indicate that lines of force leave the magnet at the north pole and enter the magnet at the south pole. Figure 6. Lines of Force NORTH END N FIELD OF FORCE ABOUT NORTH POLE LINES LEAVE POLE IN ALL DIRECTIONS SIDE VIEW N SOUTH END S FIELD OF FORCE ABOUT A BAR MAGNET LINES OF FORCE SHOWN IN ONE PLANE ONLY, ACTUALLY, LINES EXTEND IN ALL DIRECTIONS AS SHOWN IN END VIEWS AND, THEORETICALLY, THROUGHOUT ALL SPACE 5 S FIELD OF FORCE ABOUT SOUTH POLE LINES ENTER POLE FROM ALL DIRECTIONS Magnetism 52C11A03 2.7. Magnetic Shielding. There is not a known insulator for a magnetic field. If a nonmagnetic material is placed in a magnetic field, the magnetic field penetrates the nonmagnetic material. However, if a magnetic material such as soft iron is placed in a magnetic field, the magnetic field may be redirected. This property is used to protect the sensitive mechanism of electric instruments and meters that can be influenced by stray magnetic fields and will cause errors in their readings. This is accomplished by placing a soft iron case called a magnetic shield about the instrument as shown in Figure 7. Figure 7. Magnetic Shield 11 12 1 10 2 9 3 8 4 5 7 SOFT IRON 6 Magnetism 52C11A03 3.0. Artificial Magnets. Inserting the bar into a coil of insulated wire and passing a direct current through the coil as shown in Figure 8(A) can magnetize an iron, steel, or alloy bar. The same bar may also be magnetized if it is stroked with a bar magnet as shown in Figure 8(B). The best method of making artificial magnets is by electrical means and is described in detail. Figure 8. Methods of Producing Artificial Magnets COIL METHOD (B) STROKING METHOD 7 Magnetism 52C11A03 3.1. Electromagnetism. Electrons moving in a conductor cause a magnetic field to build up around the conductor. Placing a compass needle near a current carrying wire can prove this. The compass will point to the wire’s magnetic field instead of to the earth’s magnetic north as shown in Figure 9. When an electric current flowing through a conductor causes magnetism, it is called electromagnetism. Figure 9. Electromagnetism N S A N COMPASS NEEDLE S B N S C 8 Magnetism 52C11A03 3.1.1. Magnetic Field about a Conductor. The magnetic field produced by electron flow always lies at right angles to the current that produces it. The truth of this statement can be proven by the experiment shown in Figure 10. Lines of force as shown in Figure 11 normally represent the magnetic field. Figure 10. Magnetic Field CARDBOARD - COMPASS IRON FILINGS A LINES OF FORCE DIRECTION OF CURRENT FLOW Figure 11. Lines of Force about a Conductor 9 B Magnetism 52C11A03 3.1.2. Left Hand Rule for a Conductor. A simple rule has been set up for finding the direction of the magnetic field when the direction of current is known. Encircle the conductor with the left hand and point the thumb in the direction of the current flow. The fingers will point in the direction of the magnetic field as shown in Figure 12. Figure 12. Left Hand Rule for a Conductor 3.1.3. Magnetic Field about A Loop. If a current-carrying conductor is bent in the form of a loop, the lines of force surround the wire as when it was straight as shown in Figure 13(A). When the conductor is bent into a loop, all the lines of force will enter on one side of the loop and leave on the other side of the loop as shown in Figure 13(B). Thus, a North Pole is created on one side of the loop and a South Pole on the other side of the loop. Figure 13. Lines of Force about a Loop 10 Magnetism 52C11A03 3.1.4. Magnetic Field about a Coil. The magnetic field resulting under the conditions shown in Figure 14 is relatively weak. But if several loops of wire are wound to form a coil, the strength of the magnetic field within the coil is increased. This is caused by the concentration of magnetic lines of force in a smaller area. We can also say that the density of the field is increased. The combined magnetic field results in a North Pole on one end of the coil and a South Pole on the other end of the coil as shown in Figure 14. Figure 14. Lines of Force about a Coil 11 Magnetism 52C11A03 3.1.5. Left Hand Rule for a Coil. When the direction of current flow through a coil is known, then the north and south poles can be determined by the use of the left-hand rule for coils. Grasp the coil in the left hand, with the fingers wrapped around in the direction of current flow. The thumb will point toward the North Pole as shown in Figure 15. Figure 15. Left Hand Rule for a Coil 3.2. Effects of Cores. An electromagnet is normally designed to make use of a soft core. Iron has the ability to gather and convey lines of force. This characteristic is called permeability. 3.2.1. Permeability. Permeability is the ease with which a material conducts magnetic lines of force. A steel core has a higher permeability than air. An iron core has a higher permeability than steel. Examples of the effect of permeability on a coil are shown in Figure 16. 12 Magnetism 52C11A03 Figure 16. Permeability AIR CORE STEEL CORE IRON CORE AIR CORE STEEL CORE IRON CORE Figure 17. Lines of Force about a Loop 3.2.2. Retentivity. Retentivity is the ability of a material to retain an amount of residual magnetism after current flow has stopped. What happens to an electromagnet after the current flow has been stopped? An air core, of course, retains no magnetism. A soft iron core loses most of its magnetism after current flow is stopped. A hard steel core retains most of its magnetism after current flow is stopped and is said to be a permanent magnet. Examples of retentivity of air, steel, and iron cores, are shown in figure 17. 13 Magnetism 52C11A03 3.2.3. Residual Magnetism. Residual magnetism is the amount of magnetism that remains in a temporary magnet. The type of material used for the temporary magnet will determine the amount of residual magnetism that will be left over after the magnetic field is removed. 4.0. Relationship between Magnetism and Electricity. Electricity current flow (movement of free electrons) through a conductor will always produce some form of magnetic field around that conductor. Magnetism has the reverse affect on current flow. Magnetism induced in a conductor will produce current flow. 5.0. Characteristics of Magnets. All magnets (permanent or electro) have a field of force. All magnets (permanent or electro) have two poles (north and south). Lines of force are directed from the North Pole to the South Pole. Like poles repel and unlike poles attract. Magnetism can be induced in all magnetic materials. 6.0. Strength of Electromagnets. The strength of an electromagnet depends on three things. The number of turns in the coil, the permeability of the core, and the amount of current flow through the conductor. Number of turns in the coil — increasing the number of turns in the coil will increase the strength of the electromagnet by more magnetic lines of force. The permeability of the core — increasing the permeability of the core will increase the strength of the electromagnet by increasing the ease in which the material conducts magnet lines of force. Air has zero permeability. Steel has good retentivity (the ability of a substance to retain magnetism after the current flow has stopped), but low permeability. Soft iron has good permeability (the ease with which a material conducts magnetic lines of force), but low retentivity. 14 Magnetism 52C11A03 The current flows through the conductor by increasing the strength of the electromagnet and increasing the strength of the magnetic lines of force. 7.0. Three Things needed for an Induced Voltage. Conductor — a coil of wire Magnetic Field — without the magnetic field, we will have zero lines of force. Relative Motion — the conductor must be moving inside the magnetic field cutting the lines of force, or the conductor can be stationary and the magnetic field can rotate around the conductor. 8.0. Inductance and Transformers. Inductance —The property of a circuit which tends to oppose a change in the existing current. Transformer — An electrical device consisting of two coils very close together, but electrically insulated yet are not touching. Refer to figure 18. Figure 18. Transformer Construction 15 Magnetism 52C11A03 9.0. Summary. In this lesson you have learned that an electric current (flow of electrons) always produces a magnetic field. In a wire, the current flow causes the magnetic lines of force to circle the wire. It is thought that these lines of force result from the movement of the electrons along the wire. As they move, the electrons “send out” the lines of force. When many electrons move, there are many lines of force (the magnetic field is strong). Few electrons in motion mean a weak magnetic field or few magnetic lines of force. The magnetic field produced by current flowing in a loop of wire must follow the curve of the wire. If two loops are made in the conductor, the magnetic lines of force will circle the two loops almost as though they were a single loop. However, the magnetic field will be twice as strong since the lines of force of the two loops combine. Increasing the number of loops further increases the magnetic field accordingly. Many loops of wire are normally referred to as a coil. The magnetic strength of a coil can be greatly increased by wrapping the loops of wire around an iron core. The iron core passes the lines of force with much greater ease than air. With this great increase in the number of lines of force, the magnetic strength of the electromagnet is greatly increased, even though no more current flows through it. Practically, all electromagnets use an iron core of some kind. Electromagnets can be in many shapes and have many uses. The coils in electric gauges and meters, the field coils in motors and generators, solenoids, relays, and magnetic circuit breakers are just a few of the uses of an electromagnet. 16 Magnetism 52C11A03 SELF CHECK TEST Instructions: Given lesson 52C11A03, Magnetism, answer each question or solve each problem to the best of your ability, then check your answers with the solution sheet. 1. What do you call materials that are attracted to a magnet? 2. What are some of the magnetic substances? 3. What would each part constitute if a bar magnet were broken into several parts? 4. What would happen if a magnetic material such as soft iron were placed in a magnetic field? 5. What can be determined when the direction of current flowing through a coil is known? 6. Where will all the lines of force enter when the conductor is bent into a loop? 7. What do you call the ability to retain magnetism after the current flow has stopped? 8. What has a reverse affect on current flow? 9. What three things affect the strength of an electromagnet? 10. What three things are needed to cause an induced voltage? 17 Magnetism 52C11A03 SELF CHECK SOLUTION SHEET 1. Magnetic Substances. (page. 1, para 2.1) 2. Cobalt, Nickel, and Manganese. (page. 1, para 2.1) 3. Magnet. (page. 2, para 2.3.3) 4. The magnetic field may be redirected. (page. 6, para 2.7) 5. That the north and south poles can be determined by use of the left-hand rule for coils. (page. 12, para 3.1.5) 6. Lines of force will enter on one side of the loop and leave on the other side of the loop. (page. 10, para 3.1.3) 7. Retentivity. (page. 13, para 3.2.2) 8. Magnetism. (page. 14, para 4.0.) 9. The number of turns in the coil, the permeability of the core, and the amount of current flow through the conductor. (page. 14, para 6.0) 10. Conductor, magnetic field, and relative motion. (page. 15, para 7.0) 18