What is Self-Inductance

Chapter 5.17 Self-Inductance

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

  • Electromagnetism 175or, self-induced emf, ddvolteLit(5.9)Similarly, when the switch is subsequently opened, the fl ux produced by coil 1 will collapse to zero. The galvo will again indicate that a momentary emf is induced in coil 2, but of the opposite polarity tothe fi rst case. Thus, an emf has been induced into coil 2, by achanging current (and fl ux) in coil 1. This is known as a mutually induced emf. If the changing fl ux can link with coil 2, then it must also link withthe turns of coil 1. Thus, there must also be a momentary emfinduced in this coil. This is known as a self-induced emf. Anyinduced emf obeys Lenz ’ s law. This self-induced emf must therefore be of the opposite polarity to the battery emf. For this reason, it is also referred to as a back emf. Unfortunately, it is extremely diffi cult to demonstrate the existence of this back emf. If a voltmeter was connected across coil 1, it would merely indicate the terminal voltage of the battery. 5.17 Self-Inductance Self-inductance is that property of a circuit or component which causes a self-induced emf to be produced, when the current through it changes. The unit of self-inductance is the henry, which is defi ned as follows: A circuit has a self-inductance of one henry (1 H) if an emf of one volt is induced in it, when the circuit current changes at the rate of one ampere per second (1 A/s). The quantity symbol for self-inductance is L . From the above defi nition, we can state the following equation Leitd/dhenryNotes:1 The minus sign again indicates that Lenz ’ s law applies. 2 The emf symbol is e, because it is only a momentary emf. 3 The current symbol is i, because it is the change of current that is important.4 The term di/dt is the rate of change of current.