Courtesy of tech america it should be pointed out

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Unformatted text preview: e-Text Main Menu (b) Variable resistors: (a) composition type, (b) slider pot. (Courtesy of Tech America.) It should be pointed out that not all resistors obey Ohm’s law. A resistor that obeys Ohm’s law is known as a linear resistor. It has a constant resistance and thus its current-voltage characteristic is as illustrated in Fig. 2.7(a): its i -v graph is a straight line passing through the origin. A nonlinear resistor does not obey Ohm’s law. Its resistance varies with current and its i -v characteristic is typically shown in Fig. 2.7(b). | Textbook Table of Contents | Problem Solving Workbook Contents CHAPTER 2 Basic Laws Examples of devices with nonlinear resistance are the lightbulb and the diode. Although all practical resistors may exhibit nonlinear behavior under certain conditions, we will assume in this book that all elements actually designated as resistors are linear. A useful quantity in circuit analysis is the reciprocal of resistance R , known as conductance and denoted by G: G= 31 v Slope = R i 1 i = R v (a) (2.7) v The conductance is a measure of how well an element will conduct electric current. The unit of conductance is the mho (ohm spelled backward) or reciprocal ohm, with symbol , the inverted omega. Although engineers often use the mhos, in this book we prefer to use the siemens (S), the SI unit of conductance: 1S=1 = 1 A/V Slope = R i (2.8) Thus, (b) Figure 2.7 Conductance is the ability of an element to conduct electric current; it is measured in mhos ( ) or siemens (S). The i -v characteristic of: (a) a linear resistor, (b) a nonlinear resistor. The same resistance can be expressed in ohms or siemens. For example, 10 is the same as 0.1 S. From Eq. (2.7), we may write i = Gv (2.9) The power dissipated by a resistor can be expressed in terms of R . Using Eqs. (1.7) and (2.3), p = vi = i 2 R = v2 R (2.10) The power dissipated by a resistor may also be expressed in terms of G as p = vi = v 2 G = i2 G (2.11) We should note two things from Eqs. (2.10) and (2.11): 1. The power dissipated in a resis...
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This note was uploaded on 07/16/2012 for the course KA KA 2000 taught by Professor Bkav during the Spring '12 term at Cambridge.

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