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chapter 07

# chapter 07 - PART 3 MAGNETOSTATICS Chapter 7 MAGNETOSTATIC...

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PART 3 MAGNETOSTATICS

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Chapter 7 MAGNETOSTATIC FIELDS No honest man can be all things to all people. —ABRAHAM LINCOLN 7.1 INTRODUCTION In Chapters 4 to 6, we limited our discussions to static electric fields characterized by E or D. We now focus our attention on static magnetic fields, which are characterized by H or B. There are similarities and dissimilarities between electric and magnetic fields. As E and D are related according to D = eE for linear material space, H and B are related according to B = pR. Table 7.1 further shows the analogy between electric and magnetic field quantities. Some of the magnetic field quantities will be introduced later in this chapter, and others will be presented in the next. The analogy is presented here to show that most of the equations we have derived for the electric fields may be readily used to obtain corresponding equations for magnetic fields if the equivalent analo- gous quantities are substituted. This way it does not appear as if we are learning new concepts. A definite link between electric and magnetic fields was established by Oersted 1 in 1820. As we have noticed, an electrostatic field is produced by static or stationary charges. If the charges are moving with constant velocity, a static magnetic (or magnetostatic) field is produced. A magnetostatic field is produced by a constant current flow (or direct current). This current flow may be due to magnetization currents as in permanent magnets, electron-beam currents as in vacuum tubes, or conduction currents as in current-carrying wires. In this chapter, we consider magnetic fields in free space due to direct current. Mag- netostatic fields in material space are covered in Chapter 8. Our study of magnetostatics is not a dispensable luxury but an indispensable necessity. r The development of the motors, transformers, microphones, compasses, telephone bell ringers, television focusing controls, advertising displays, magnetically levitated high- speed vehicles, memory stores, magnetic separators, and so on, involve magnetic phenom- ena and play an important role in our everyday life. 2 Hans Christian Oersted (1777-1851), a Danish professor of physics, after 13 years of frustrating efforts discovered that electricity could produce magnetism. 2 Various applications of magnetism can be found in J. K. Watson, Applications of Magnetism. New York: John Wiley & Sons, 1980. ^ : ,'.-."•• 2 6 1
262 Magnetostatic Fields TABLE 7.1 Analogy between Electric and Magnetic Fields* Term Basic laws Force law Source element Field intensity Flux density Relationship between fields Potentials \ - • , , * Flux Energy density Poisson's equation F f F dQ E D D E v •• y y / = w E V 2 Electric 2,22 4ire 2 ' D • dS = g e n c = gE i = |(V/m) y = -(C/m 2 ) = sE = -W f Pidl J Airsr = / D dS = Q = CV - I . . .

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chapter 07 - PART 3 MAGNETOSTATICS Chapter 7 MAGNETOSTATIC...

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