Lenz ’ s Law

Chapter 5.2 Lenz ’ s Law

Fundamental Electrical and Electronic Principles Third Edition Book
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Fundamental Electrical and Electronic Principles Third Edition Book

  • 144Fundamental Electrical and Electronic Principles4 Equation (5.1) forms the basis for the defi nition of the unit of magnetic fl ux, the weber, thus: The weber is that magnetic fl ux which, linking a circuit of one turn, induces in it an emf of one volt when the fl ux is reduced to zero at a uniform rate in one second. In other words, 1 volt 1 weber/second or 1 weber 1 volt second. 5.2 Lenz ’ s Law This law states that the polarity of an induced emf is always such that it opposes the change which produced it. This is similar to the statement in mechanics, that for every force there is an opposite reaction. 5.3 Fleming ’ s Righthand Rule This is a convenient means of determining the polarity of an induced emf in a conductor. Also, provided that the conductor forms part of a complete circuit, it will indicate the direction of the resulting current fl ow. The fi rst fi nger, the second fi nger and the thumb of the right hand are held out mutually at right angles to each other (like the three edges of a cube as shown in Fig. 5.3 ). The F irst fi nger indicates the direction of the F lux, the thu M b the direction of M otion of the conductor relative to the fl ux, and the s EC ond fi nger indicates the polarity of the induced E mf, and direction of Current fl ow. This process is illustrated in Fig. 5.4 , which shows the cross-section of a conductor being thuMbseCond fingerfirst finger Fig. 5.3 vx Fig. 5.4